Chemistry AS Level Flashcards
Where is the mass of an atom concentrated?
Nucleus
What is the atomic mass/nucleon number?
Atomic mass or nucleon no. (A) = no. of Protons + Neutrons
What is an isotopic number
- the number of neutrons minus the number of protons in an atomic nucleus.
- Isotopes are elements in the same spices with the same number of electrons but different neutron number
What is the relative isotopic mass?
Relative isotopic mass the mass of an atom of an isotope as compared to 1/12 of the mass of an atom of carbon-12.
Define Isotopes
Isotopes are atoms of the same element with the same proton number but different number of neutrons
What are features of isotopes?
- Isotopes have similar chemical properties since they have same number of protons and electrons (so chemical interactions are similar)
- Isotopes have different physical properties since they have different number of neutrons, causing them to have different masses and therefore different physical interactions
What is the behaviour of a beam on a subatomic particles?
- Protons: positively charged, therefore they are deflected to negative pole
- Neutrons: no charge, therefore they are not deflected
- Electrons: negatively charged, therefore they deflected to positive pole
- Electrons are lighter than Protons, therefore they are deflected at greater angle
How are electrons arranged?
- Electrons are arranged in energy levels called shells
- Each shell is described by a principle quantum number (PQ)
- As the principle quantum increases, energy of shell increases
- Inside the shell there are subshells: s, p, d and f
- An orbital region in space where there is a maximum probability of finding an electron
What is the structure of an orbital?
- Each orbital can hold 2 electrons in opposite directions
- When electrons are placed in a set of orbital of equal energy, they occupy them singly and then pairing takes place
- Electrons placed in opposite direction: both negatively charged and if placed in same direction, they’d repel. In opposite direction they create a spin to reduce repulsion
- Completely filled or half filled orbitals are more stable (reduced repulsion)
What is Aufbau’s principle?
Aufbau’s principle is a method of showing how atomic orbitals are filled in a definite order to give lowest energy arrangement possible
Which order do you fill subshells?
- Energy difference between 4s and 3d very small therefore an electron from 4s can be promoted to half-fill or full-fill 3d orbital, to make atom more stable
- When filling, fill 4s before 3d and when removing, also remove first from 4s
How many orbitals and maximum orbitals can each subshell hold?
s p d f
Orbitals 1 3 5 7
Max e-s 2 6 10 14
What is the shape of s orbital?
s orbitals are spherical with the nucleus at the centre
What is the shape of p orbital?
p orbitals are dumbell shaped
What is a free radical?
A free radical is a species with one or more unpaired electrons. ions and free radicals are different.
What is the first ionisation energy?
1st ionisation energy (IE) is the energy needed to remove 1 mole of electrons from 1 mole of gaseous atom to form 1 mole of unipositive ions
Why is each successive energy higher than the last?
- Each successive I.E is higher than previous one because as electrons are removed, protons > electrons therefore attraction between protons and remaining electrons increases
- Successive I.Es have large jump in their value when electrons removed from lower energy shell
What factors affect the ionisation energy?
- Nuclear charge
- Shielding effect
- Atomic radius
- Stable configuration
how does nuclear charge affect the ionisation energy?
- positive charge due to protons in nucleus
- Greater nuclear charge greater ionization energy
how does atomic radius affect ionisation energy?
- Distance from the centre of the nucleus to the outermost orbit
- As number of electron shells increases, atomic radius increases
- As number of electrons in outermost shell increases, atomic radius decreases as the electrostatic attraction between nucleus and outer electrons increases
- Greater radius lower I.E. Distance of outermost electrons to nucleus is large therefore less energy needed to remove electrons
How does the shielding effect affect the ionisation energy?
- Inner shells of electrons repel outermost electrons, thus shielding them from positive nucleus. The more electron shells, the greater is the shielding effect
- Greater shielding effect lower I.E because lesser attractive force between nucleus and outer electrons
How does the stable configuration affect the ionisation energy?
High I.E needed to remove electrons from completely or half-filled orbitals
What are the general 1st ionisation energy down a group?
Down a group ionisation decreases:
- New shells added
- Attraction of nucleus to valence e-s decreases
- Shielding effect increases
What are the general 1st ionisation energy across a period?
Across a period ionisation increases:
- Shell no. remains same
- Proton no. increases
- Effective nuclear charge increases
- Atomic radius decreases
What is the trend in 1st I.E across 3rd Period
- I.E of Al lower than Mg: e- removed in Al is from higher energy 3p orbital which is further away from nucleus than 3s electrons being removed from Mg. Nuclear attraction is less for 3p than 3s therefore I.E of Al is lower than Mg
- I.E of S lower than P: electron being removed in P is in a half filled, more stable 3p orbital whereas in S, the pairing of electrons in 3p results in increased repulsion therefore less energy need to remove an electrons
What is an ionic radius?
Ionic radius describes the size of an ion
What is the ionic radius of a positive and negative ion compared to the neutral ion?
- Positive ion: smaller radius than original neutral atom because shell no. decreases, screening effect decreases but the attraction of nucleus increases.
- Negative ion: larger ionic radius than neutral atom because electrons added while nuclear charge remains same
how does the ionic radius change across a period?
- Proton no. and effective nuclear charge increases
- Ionic radius decreases
What are the number of shells and ion in the groups 1-7 of the periodic table?
Groups 1 to 3 5 to 7
Ion Positive Negative
No. of shells = n − 1, n
What has a higher ionic radius in a period negative ions or positive ions?
Negative ions always larger than positive ions in the same period as they have one more shell
What is the trend of ionic radius down the group?
Ionic radius increases down the group since number of electron shells increases
What is the trend of ionic radius on increasing charge in cations and anions?
- As negative charge on anion increases, ionic radius increases since the number of electrons gained increases such that the number of electrons exceeds the number of protons
- As positive charge on cation increases, number of electrons lost increases, so electrostatic attraction between nucleus and outer electrons increases
What is relative atomic mass?
Relative atomic mass is the weighted average mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12.
What is relative isotopic mass?
Relative isotopic mass is the mass of an atom of an isotope as compared to 1/12 of the mass of an atom of carbon-12.
Isotopic mass is the mass of a particular isotope of an element
What is the relative molecular mass?
Relative molecular mass is the weighted average mass of one molecule of an element or compound compared with 1/12 of the mass of an atom of carbon-12
What is the relative formula mass?
Relative formula mass is the weighted average mass of one unit of a substance compared
with 1/12 of the mass of an atom of carbon-12.
Define the term mole
Mole is the amount of substance that has the same number of particles (atoms, ions, molecules or electrons) as there are atoms in exactly 12g of the carbon-12 isotope. One mole of any substance will always contain exactly the same
number of particles.
What is the avogadros constant?
Avogadro’s constant: number of atoms, ions, molecules or electrons in a mole = 6.02×10^23
Number of particles/atoms/molecules = Avogadro’s constant x moles
How do calculate relative abundance?
Relative abundance = (Total abundance/Peak height) ×100%
AR =∑ (Mass × Relative Abundance) / 100
How do calculate molecular formula?
Molecular Formula = (Empirical Formula)n
Where n = Mass of Empirical Formula / Molecular Mass
How do you calculate empirical formula from masses?
- Divide the mass (or percentage by mass) of each element by its molar mass to calculate the molar ratio.
- Divide each number in the ratio by the smallest number to get the simplest ratio of elements.
- If the ratio contains decimal numbers, multiply it as appropriate to obtain whole numbers
How do calculate empirical formula from combustion data?
If the compound contains only carbon and hydrogen:
- Divide the mass of CO2 produced during combustion by 44 to find the number of moles of
CO2. This is equal to the amount of carbon in the compound. - Divide the mass of water produced during combustion by 18 to find the number of moles of
water. One mole of H2O contains 2 moles of hydrogen atoms so double the number of moles of water to find the number of moles of hydrogen in the compound. - Divide the number of moles of hydrogen and carbon by the smallest value to find the simplest molar ratio. This is the empirical formula
What is empirical formula?
Empirical formula is the simplest whole number ratio of atoms of each element in a compound
What is the molecular formula?
Molecular formula gives actual numbers of each type of atom in a molecule. Which can be calculated using the Mr of a compound and its empirical formula.
How do you calculate the molecular mass from the relative formula mass?
Calculate the relative formula mass of the empirical formula then divide the relative formula mass
of the compound by this calculated value. This will show how many times larger the molecular formula is than the empirical formula.
How do you calculate the molecular mass from the relative formula mass?
Calculate the relative formula mass of the empirical formula then divide the relative formula mass
of the compound by this calculated value. This will show how many times larger the molecular formula is than the empirical formula.
What is the percentage composition?
% Composition = (Atomic Mass × No. of Moles/Molar Mass of Compound) ×100%
How do calculate moles?
Moles = Mass / Molar mass
Calculate is the volume of a gas?
Volume of a Gas = Moles × 24
How do you calculate moles concentration?
Concentration = Moles / Volume
Unit of volume is dm^3
and 1000cm^3 =1dm^3
Concentration unit = mol/dm^-3
How do you calculate molar mass?
Molar mass is the mass per mole of a substance and can be calculated by adding the relative atomic masses of all the atoms in a formula.
When you do calculations involving volumes of gas then ?
- At the same temperature and pressure, one mole of any gas will occupy the same volume. At room temperature and pressure, this is 24 dm^3
- This does not mean that these gases will have the
same mass. - The number of moles of a gas at room temperature and pressure can be calculated, where V is volume: Moles of gas = Volume (dm^3) /24
Define Concentration
Concentration is the amount of solute dissolved in a given volume of solution.
What is a Anhydrous compound?
Anhydrous is a compound in which all water molecules are removed
what is a hydrated compound?
Hydrated is a compound which has a number of water molecules associated with its crystalline structure
What is water of crystallisation?
Water of Crystallisation are water molecules in a hydrated compound are called water of crystallisation
What is mass spectrometry?
- Mass spectrometry is an analytical technique which gives information about the abundance of
different elements on a mass spectra. - Each line represents a different isotope and the relative heights of the peaks show the relative abundance.
How do you calculate empirical formula from combustion data, If the compound contains other elements as well as carbon and hydrogen?
- Use the mass of carbon dioxide to calculate the number of moles of carbon atoms then use
this to calculate the mass of carbon present. - Use the mass of water to calculate the number of moles of hydrogen atoms then use this to
calculate the mass of hydrogen present. - Add the masses of carbon and hydrogen and compare to the initial mass of the compound.
If there is no difference, the compound is a hydrocarbon so continue with step 4. If there is
a difference, assume that this is due to oxygen (unless otherwise told). Calculate the mass of oxygen present and use this to calculate the number of moles of oxygen atoms. - Divide the moles of each element by the smallest value to obtain the empirical formula.
What dose the group number relate on the periodic table?
The group number relates to the number of electrons in the outer shell
How do we fill the sub-shells?
- We fill 1s, 2s,2p 3s,3p 4s 3d, 4p 4d
- We fill orbitals from the lowest energy first
- We fill each shell singular first before adding pairs
Why do we fill 4s before 3d?
This is because 3d has a higher energy then 4s
How do chromium and coppers subshells behave differently?
An electron from the 4s orbital moves into the 3d orbital to create a more stable half full or 3d sub-shell respectively
What is an ionic bond?
Ionic bond is the electrostatic attraction between oppositely charged ions.
How is an ionic bond formed?
An ionic bond is formed when electrons are transferred from a metal to a non-metal, forming an ionic compound. The compound is held together by the electrostatic attraction between the positively charged metal ions and negatively charged non-metal ions.
What is an ionic bonds structure?
It has a giant ionic lattice, crystalline solids. In ionic compounds, ions are surrounded on all sides by oppositely charged ions forming a giant ionic lattice that has high melting and boiling points.
What is a coordination number?
Coordination number is number of oppositely charged ions that surround a particular ion in an ionic solid
E.g: NaCl, MgCl2
What is a dot and cross diagram?
Dot and cross diagrams are often used to represent ionic bonding. For one species electrons are
shown as dots and the other as crosses. Typically only outer shell electrons are shown and charges are shown outside square brackets surrounding the ion.
How to draw a dot and cross diagram?
- For anions, electrons are added to the atom (e.g Chlorine forms Chloride Cl- ions
- For cations, electrons are removed from the atom (e.g Sodium forms Na+ ions
- Atoms form the same number of bonds as the number of electrons required to be added/removed for maximum stability. (e.g: Chlorine will form 1 bond, Sodium will form 1 bond, but Calcium Ca2+ will form 2 bonds
- The central atom may expand its octet to form more bonds if necessary (e.g: S8)
- In ionic compounds, electrons are showed to be completely transferred to each of the ions in the compound.
- In covalent compounds, electrons are showed to be shared between the atoms in the molecule
- Use a legend of different shapes (e.g solid circle, unfilled circle, triangle, cross etc) to show which electrons come from which atom in the same way as colours have been used in the above diagrams
What is electronegativity?
Electronegativity is the ability of an atom to attract a bonding pair of electrons in a covalent bond towards itself
What does electronegativity depend on?
- Radius of atom (atomic size) inversely ∝ electronegativity
- Nuclear attraction directly ∝ electronegativity
What is the electronegativity trend across a period?
Electronegativity increases across a period because atomic radius decreases and nuclear attraction increases, so polarity increases
Electronegativity increases towards the top right of the periodic table
What is the electronegativity trend down a group?
Electronegativity decreases down a group because atomic radius increases and nuclear attraction decreases, so polarity decreases
What is dipole moment?
Dipole moment is slight charges on atoms in a covalent bond due to differences in electronegativity
What determines the overall dipole and overall polarity of a compound?
The difference between the electronegativity of two atoms in a compound determines the overall dipole moment and overall polarity of the compound
What is a polar covalent bond?
- A polar bond is a bond with a permanent charge difference (or permanent dipole).
- Bonds with slight ionic character
- Bond formed with atoms of different electronegativity
- Polar molecules have dipoles; electric charges of equal magnitude and opposite sign
- The greater the difference in electronegativity of the two bonded atoms, the greater is the ionic character
What makes a bond non-polar?
If two atoms in a covalent bond are exactly the same, the electronegativity of both atoms will be the same so the bond will be non-polar.
Non-polar molecules have no overall charge
Why dose a polar bond occur?
Bonding electrons attracted more towards atom with greater electronegativity therefore unequal sharing of electrons therefore molecule develops slight charges = Polar Molecule
Why do polar molecules shape matter?
A polar molecule must contain polar bonds and be non-symmetrical. If a polar molecule is symmetrical, the dipoles will cancel each other out.
How dose oxygen become an oxide that causes hydrolysis?
The oxygen in non-metal oxides is very electronegative. This causes a permanent dipole across the covalent bond so the atom that oxygen is bonded to becomes partially positive. When the oxide is added to water, lone pairs on oxygen in the water are attracted to the partially positive atom in the oxide causing hydrolysis.
Below are examples of this reaction:
P2O5(s) + 3H2O(l) → 2H3PO4(aq)
Cl2O(g) + H2O(l) → 2HClO(aq)
How dose oxygen become an oxide that causes hydrolysis?
The oxygen in non-metal oxides is very electronegative. This causes a permanent dipole across the covalent bond so the atom that oxygen is bonded to becomes partially positive. When the oxide is added to water, lone pairs on oxygen in the water are attracted to the partially positive atom in the oxide causing hydrolysis.
Below are examples of this reaction:
P2O5(s) + 3H2O(l) → 2H3PO4(aq)
Cl2O(g) + H2O(l) → 2HClO(aq)
How dose the difference in electronegativity affect a bound?
A large difference in electronegativity will make the bond more polar (more ionic in nature) but a small difference in electronegativity will make the bond less polar (more covalent in nature)
What is metallic bonding?
Strong electrostatic forces of attraction between metal cations and delocalized mobile electrons
What is the structure of metallic bonding?
In metals, positive metal ions (cations) are fixed in a lattice and surrounded by mobile delocalised
electrons. The strong electrostatic attraction between the positive metal ions and negative electrons hold the metal together.
What dose the strength of metallic bond increase with?
- Increasing positive charge on the ions in the lattice
- Decreasing size of metal ions in the lattice
- Increasing number of mobile electrons per atom
What is covalent bond?
A covalent bond is a chemical bond where electron pairs are shared between the nuclei of two atoms.
Why dose covalent bonding occur?
This is because the negative electrons are attracted to the positive protons in the nuclei and this overcome the repulsion between the two nuclei.
What are non-bounding electrons/ lone pairs?
Non-bonding electrons or lone pair are pair of valence electrons that are not involved in bond formation
What are the forces that hold covalent bonds together?
Covalent compounds are made of molecules which are held together by weak intermolecular forces. They have low melting and boiling points
What is a coordinate bond?
Coordinate bond is a dative covalent bond where both electrons in the bond come from the same atom. Coordinate bond is represented by an “→” drawn from the atom donating to towards the atom accepting. In the dative covalent bond A→B, A donates a pair of electrons to B.
What are the conditions of a coordinate bond?
- An atom should have a lone pair of electrons
- An atom should be in need of a pair of electrons
What is a donor and acceptor in coordinate bonds?
- Donor is the atom that supplies the pair of electrons
- Acceptor isthe atom that accepts the pair of electrons
What is a donor and acceptor in coordinate bonds?
- Donor is the atom that supplies the pair of electrons
- Acceptor isthe atom that accepts the pair of electrons
What is a orbital overlap?
For a covalent bond to form, atomic orbitals containing unpaired valence electrons must overlap each other
What is a sigma bond?
A sigma bond is a single covalent bond formed when two orbitals overlap end-to-end. The pair of electrons are found between the two nuclei.
What are non-polar covalent bonds are formed between?
- Identical atoms: the electronegativity of both atoms is the same so pair of electron shared equally
- Symmetrical polyatomic molecules: dipoles of bond exert equal & opposite effects hence cancel charge
Why dose a sigma bound occur?
The sigma bond forms when two orbitals from different atoms overlap end-to-end.
The process of promoting an electron, hybridisation and formation of the molecular orbitals follows
the same pattern in all covalently-bound molecules.
What is a pi bound?
A pi bond is a covalent bond formed when 2 orbitals overlap sideways. The pi bond is the region above and below a sigma bond where this pair of electrons can be found. Orbitals overlap sideways to form a pi bond
What comparisons can be made between sigma and pi bound?
- Sigma bond has greater overlap then pi bound therefore sigma > pi
- Pi bond cannot exist without a Sigma bond.
Why can some elements in period three bond with more than 4 electrons at once?
Elements in Period 3 can expand their octet by making use of the energetically accessible lower lying d-subshell for bonding. This means that some elements of period 3 can bond with more than 4 electrons at once.
What dose the shape and bound angles of molecules depend on?
- The number of pairs of electrons around central atom
- Whether these pairs are lone pairs or bonded pairs
- The arrangement of electrons around the central atom
How do electron pairs arrange themselves?
Electron pairs are regions of negative charge so they repel each other and arrange themselves as
far apart as possible.
How do valence shell electrons arrange themselves?
Valence shell electrons are arranged in pairs to minimize repulsion between themselves
What has the greatest repulsion pair?
Lone pairs offer more repulsion than bonding pairs
Lone pair - lone pair
Lone pair - bonding pair
What has the lowest repulsion pair?
Bonding pair - bonding pair
How do you draw shapes of molecules?
When drawing the shapes of molecules, a bond in the plane of the paper is a normal line. A bold
wedge shows the bond is coming towards you and a dotted wedge shows the bond is going away
from you. Dots are used to represent electrons in a lone pair
What is the shape of a molecule with 2 bounded pairs?
2 bounded pair
2electrons
Linear
180°
CO2
What is the shape of a molecule with 3 bounded pairs?
3 bounded pair
Trigonal planar
120°
BF3
What is the shape of a molecule with 4 bounded pairs?
4 bounded pair
Tetrahedral
109.5°
CH4
What is the shape of a molecule with 5 bounded pairs?
5 bounded pair
Trigonal bipyramidal
90° and 120°
PF5
What is the shape of a molecule with 6 bounded pairs?
6 bounded pair
Octahedral
90°
SF6
What is the shape of a molecule with 2 bounded pairs and 2 lone pairs?
2 bounded pair, 2 lone pair
Non-linear or V-shaped
104.5°
H2O
What is the shape of a molecule with 3 bounded pairs and 1 lone pairs?
3 bounded pair, 1 lone pair
Pyramidal
107°
NH3
Do the shapes and bond angles of ions differ from that of molecules?
No, Ions have the same shapes and bond angles as molecules
What are the factors that affect the reactivity of covalent compounds?
- Bond energy is the amount of energy needed to break one mole of a given gaseous covalent bond to produce gaseous atoms. Bond energies given in the data book are an average and don’t consider the specific molecule the bond is found in.
- Bond length is the distance between two nuclei in a covalent bond. A longer bond means the shared pair of electrons is further from at least one nucleus so the attraction and bond strength decreases with increasing bond length.
- Bond polarity. If the electronegativities of the bonding atoms are different, the bond will be polar and the bonding atoms will have partial charges.
What typically determines the rate of a reaction?
The strength of a bond rather than polarity typically determines the rate of a reaction. Polarity may mean molecules are attracted to each other which triggers the reaction.
What determines the strength of a bound?
- Strength of the bond depends on the length of the bond, which depends on radii of the two bonded atoms; larger the radius, longer the bond length.
- A stronger bond means the compound is less reactive
Why are double bonds shorter then single bounds?
Double bonds are shorter than single bonds because double bonds have a greater negative charge density between the two atomic nuclei hence greater attraction
What has the strongest type of intermolecular force in covalent bonds?
Hydrogen Bonding
What do we need for hydrogen bonding to occur?
- Molecule having a H atom bonded to F, O or N
- Molecule having F, O or N atom with lone pair of electrons
What do we need for hydrogen bonding to occur?
- Molecule having a H atom bonded to F, O or N
- Molecule having F, O or N atom with lone pair of electrons
Why can molecules containing N-H, O-H or F-H bonds form hydrogen bonds?
This is because oxygen, nitrogen and fluorine are very electronegative meaning they draw the bonding electrons towards them to create a strong dipole (a charge difference across the bond).
.
What is a hydrogen bound?
A hydrogen bond is the attraction between the partially positive hydrogen (Hᵟ+) and a lone pair on Oᵟ-, Nᵟ-, or Fᵟ-.
What is intermolecular forces?
Intermolecular forces are weak forces present between two covalent neighbouring molecules.
What are induced dipole (Van Der Waals’ Forces)?
Very weak forces present between non-polar molecules. There are no permanent dipoles but electrons are mobile and in an instant, they may be unevenly distributed.
How is induced dipole forces made?
Due to constant motion of electrons, at an instant, a non-polar molecule develops poles. This creates a temporary dipole, with the side containing more electrons becoming partially negative. The temporary dipole can induce dipoles in neighbouring molecules as the partial negative charge repels electrons.
These opposite partial charges will remain attracted to each other.
What dose Van der Waals forces increase with?
- increasing number of contact points between molecules; point where molecules come close together
- increasing number of electrons (+ protons) in molecule
What are permeant dipole-dipole forces?
Permanent dipole-dipole forces are another type of van der Waals forces found between polar molecules, they are weak forces.
What dose Permanent dipole-dipole forces in molecules mean?
- The permanent dipole in these molecules means the partial charges are more strongly attracted to one another. Molecules with permanent dipole-dipole forces usually have higher boiling points than those which only have London forces between them
- Molecules always attracted to charged rod, whether positive or negative because molecules have positive and negative charges
What are the forces between elements in group 18 and what are the trends in this group?
- Elements in group 18 exist as single atoms so the only forces between the atoms are London
forces. - These forces are relatively weak so require little energy to break meaning group 18
elements have low boiling points. - Boiling point increases down the group because the number of electrons and atomic radius increases meaning there are stronger temporary dipole and
stronger London forces between the atoms.
What are the properties of a Covalent giant structures only ?
- High melting and boiling points - strong covalent bonds require a large amount of energy to break.
- Mostly non-conductors - don’t contain mobile charged particles (except graphite which contains delocalised electrons).
- Insoluble - covalent bonds in the lattice are too strong to be broken
What are the properties of an Ionic bonded structure?
- High melting and boiling points - strong electrostatic attraction between oppositely
charged ions requires a lot of energy to break. - Electrical conductor - when aqueous or molten, the ions are free to move and conduct electricity. When solid, the ions are fixed in an ionic lattice so can’t conduct electricity.
- Soluble in polar solvent - charged parts of the solvent are attracted to the oppositely
charged ions.
What are the properties of a metallic bonded structure?
- High melting and boiling point - the attraction between the ions and delocalised electrons is strong so a lot of energy is needed to overcome the metallic bonding.
- Good electrical conductor - contains mobile delocalised electrons which can conduct
electricity as a solid. - Malleable and ductile - the regular structure and delocalised electrons allow the uniform layers of ions to slide over one another.
What are the properties of a hydrogen bound?
- High boiling point - The melting and boiling points are greater than those of molecule with only van der Waals forces between them because hydrogen bonds are stronger.
- Soluble in water - strong permanent dipoles allow the formation of hydrogen bonds with water.
- Non-conductors - no mobile charges so are unable to conduct electricity
- The hydrogen bonding in water causes water molecules in ice to align in an open lattice structure. This means water expands as it freezes so ice is less dense than water.
What are the properties of Van der waals forces?
- Low boiling point - forces are weak so require little energy to break. Larger molecules have more van der Waals forces so have higher melting and boiling points.
- Solubility - unless they react with water, most molecular compounds are insoluble in water because they release too little energy when they dissolve. They are often soluble in organic solvents because they both contain van der Waals forces.
- Non-conductors - no mobile charges so are unable to conduct electricity.
What is an exothermic reaction?
An exothermic reaction is a reaction in which energy is released. This is because less energy is required to break bonds than is released when making bonds.
Exothermic reactions are involved in bond making.
ΔH negative
EReactants > EProducts
What is an endothermic reaction?
An endothermic reaction is a reaction which takes in energy. This is because more energy is required to break bonds than is released when making bonds. Endothermic Reactions is a bond breaking reaction that makes the surrounding cooler.
ΔH positive
EReactants < EProducts
What are the state symbols of a reaction?
(s) - solid
(l) - liquid
(aq) - aqueous
(g) - gas
What is a displacement reaction?
these are reactions where a more reactive element takes the place of a less reactive element in a compound
What is a precipitation reaction?
When a solid is produced after two aqueous reactions react
How do you write sulfate?
SO4 -
How do you write carbonate?
CO3 ^2-
How do you write hydrogencarbonate?
HCO3 -
How do you write ammonium?
NH4 +
How do you write nitrate?
NO3 -
How do you write phosphate?
PO4 ^3-
How do you write hydroxide?
OH-
How do you work out concentration in gdm^-3?
Concentration (gdm-3) = mass of substance (g) / volume (dm-3)
What is an ideal gas?
Ideal gas is a gas whose volume varies in proportion to temperature and in inverse proportion to pressure.
When does gas approach ideal behaviour?
- Temperature must be high enough above the boiling point so that there are no
intermolecular forces between molecules. - Pressure must be low enough so that the volume of the individual molecules is negligible
relative to the volume of the container.
What is the ideal gas law assumption?
- Molecules are in constant random motion in straight lines.
- Molecules are rigid spheres.
- Pressure is due to molecules colliding with the walls of the container.
- All collisions are elastic (whether between molecules or between molecules and the walls of the container).
- Temperature is proportional to the average kinetic energy of the molecules
What are the conditions at which gases behave ideally?
- High temperature
- Low pressure
What is the ideal gas laws?
- Gas molecules move rapidly and randomly
- Distance between gas molecules is greater than diameter of molecules therefore volume is negligible
- No forces of attraction/repulsion between molecules
- All collisions between particles are elastic EK
conserved - Temperature of gas related to average EK of molecules
What are the limitations of ideal gas laws?
Real gases do not obey kinetic theory in two ways:
- There is not zero attraction between molecules
- We cannot ignore volume of molecules themselves
What happens to gases at low temperature and high pressure?
- Molecules are close to each other
- Volume of molecules not negligible relative to container
- Van der Waals forces present, pulling molecules to each other
- Pressure is lower than expected from ideal gas
- Effective volume is less than expected from ideal gas
What is the ideal gas equations?
The ideal gas equation is: pV = nRT
p - pressure (Pa)
V - volume (m^3)
n - number of moles (mol)
R - gas constant (8.31441 J K-1 mol-1)
T - temperature (K)
What is the calculation of mole fraction?
Mole Fraction = Mols of One Gas / Total Mols of Gases
What is the partial pressure of a gas?
Partial Pressure of a Gas = Mole Fraction × Total Pressure
What are standard conditions of gas?
Standard Conditions: 101KPa and 273K
What are the pressure of 1 pascals?
1 kPa = 1000 Pa
How do you calculate Kelvin?
Temperature + 273
What is the arrangement of liquids?
Liquids contain randomly arranged particles which are close together with some gaps. The gaps allow the particles to move. The particles in a liquid have enough energy to prevent the intermolecular forces holding them in a fixed arrangement. Most liquids have a slightly lower density than the solid.
How does a liquid vaporise?
- Energy transferred makes liquid particles move faster
- Forces of attraction weaken
- Highest energy particles escape first
- Liquid starts to evaporate – temp. below b.p.
- Forces weaken further – particles move faster and spread
- Liquid boils – temp. at b.p.
Define vaporisation
Heat energy causes the particles in a liquid to move fast enough to break all the forces of attraction between them and become a gas
Define enthalpy of vaporization
Enthalpy of vaporization is the heat energy required to change 1 mole of liquid into a gas at its boiling point
Define enthalpy of fusion
Enthalpy of fusion is the heat energy required to change 1 mole of solid into a liquid at its melting point
What happens when you heat a solid?
Energy transferred makes solid particles vibrate faster
Forces of attraction weaken & solid changes to liquid
Heat energy causes the particles in a solid to vibrate. Eventually, the particles have enough energy to disrupt the regular arrangement and become a liquid.
What happens during evaporation of a liquid in a closed container?
- Constant evaporation from surface
- Particles continue to break away from surface but are trapped in space above the liquid
- As gaseous particles collide, some of them hit the surface of the liquid again, and become trapped there
- An equilibrium is set up in which number of particles leaving surface is balanced by number rejoining it.
- Liquid water molecules ⇌ Vapor water molecules
- In this equilibrium, there will be a fixed number of the gaseous particles in the space above liquid.
Define vapour pressure
When a liquid evaporates in a closed container, the gaseous particles move around above the liquid. When these particles collide with the walls of the container, they exert a pressure called the vapour pressure.
What is a finite resource?
Finite resource are resources which doesn’t get replaced at the same rate that it is used up. A finite resource is used up faster than it is replaced so it will run out if it is continually used. Examples of finite resources are crude oil, copper and aluminium.
Why is recycling important and what are some advantages?
Recycling is important to reduce the rate at which resources are used up. It may reduce costs and
the environmental impact of materials.
Advantage of Recycling:
- Saves energy
- Reduces environmental issues and what are the conditions for it to occur
- Conserves ore supplies
- Less wastage
- Cheaper than extracting
How does a hybrid sp^3 orbital form and what are the conditions for it to occur?
The 2s and 2p are close in energy to each other. Given the right amount of energy this allows both electrons to move form the 2s orbital to one of the empty 2p orbital fairly easily. For the bonding to occur we must have singly occupied orbital
How dose a hybrid sp^2 orbital form?
When the s-orbital is hybridised with two p-orbitals instead of the 3
What is the solubility of a solid?
For solids, generally solubility increases with increasing temperature as the increase in temperature facilitates the overcoming of intermolecular bonds, making it easier for the solid to dissolve
What is the solubility of gases?
For gases, generally solubility decreases with increasing temperature as the pressure of the gas increases (pressure only affects solubility of gases)
What is the best conducter and worst conducter?
Solids (metals) are generally the best conductors of electricity while gases are the worst conductors
Why are solids better conducter then liquids?
Solids (metals) are generally better conductors of heat than liquids, while liquids are better thermal conductors than gases. This is because of the proximity of molecules in solids allowing heat to be transferred rapidly through vibrations of neighbouring molecules
What is the lattice structure of a giant ionic lattice?
In a giant ionic lattice the positive and negative ions alternate in a three dimensional structure, held by ionic bonds.
- Sodium chloride, Na+ and Cl- ions.
- Magnesium oxide, Mg2+ and O2- ions
What is the lattice structure of a simple covalent molecular lattice?
In a simple molecular (covalent) lattice the molecules held together by van der Waals forces in a cubic structure.
- Iodine, face centred cubic structure.
- C60 has a ball like structure made up of hexagons and pentagons of carbon atoms with van der Waals forces between molecules.
- Nanotubes are a cylinder of graphene (single sheet of carbon atoms covalently bonded together).
What is the lattice structure of a diamond?
Diamond, each carbon shares an electron with 4 other carbon atoms.
What are the physical properties of a dimond?
- High m.p./b.p. - each carbon forms four covalent bonds
- Hard - tetrahedral structure
- Doesn’t conduct heat or electricity – no free electrons
- Used for cutting as is strongest known substance and has sharp edges
What is the lattice structure of graphite?
Graphite, hexagons of carbon atoms in layers with each carbon atom covalently bonded to 3 carbon atoms, one delocalised electron per carbon atom so van der Waals forces between layers. Graphene, single layer of graphite.
What are the physical properties of graphite?
- Three strong (sp^2) covalent bonds
- Fourth electron in p orbital forms a pi bond, forming a cloud of delocalised electron above and below the planes
- Layers kept together by weak Van der Waal’s forces
- High m.p./b.p. - strong covalent bonds throughout
- Soft – forces between layers are weak
- Conducts electricity - has delocalized electrons
What is the lattice structure of silicon(IV) oxide?
- Silicon(IV) oxide, similar 3D structure to diamond with oxygen and silicon atoms covalently bonded together.
- Each Si is bonded to 4 oxygen atoms, but each oxygen is bonded to 2 Si atoms
- Sand is largely SiO2
- Similar properties to diamond
What is the lattice structure of a hydrogen bonded lattice?
A hydrogen-bonded lattice is an ice, open lattice structure held by hydrogen bonds between partially positive hydrogen and a lone pair of electrons on oxygen.
How is a hydrogen bonded lattice formed?
- In ice form, water molecules slow down and come closer together
- Due to polarity, molecules form hydrogen bonds between lone pairs of oxygen and δ+ charge of hydrogens
- Each water molecule has 2 H-bonds
- They arrange themselves into an open crystalline, hexagonal structure
- Due to large spaces, ice is less dense than water
What are the physical properties of hydrogen bonding lattice?
- Relatively high m.p./b.p.: many strong H-bonds
- High viscosity: hydrogen bonding reduces ability of water molecules to slide over each other
- High surface tension: hydrogen bonds in water exert a downward force on surface of liquid
- Ice less dense than water: larger spaces between molecules in hexagonal structure
What is the structure of the simple molecular lattice Iodine?
- Diatomic molecule formed due to covalent bond between individual atoms
- Molecules have weak Van der Waals forces of attraction between them
What is the properties of the simple molecular lattice iodine?
- Dark grey crystalline solid; vaporizes into purple gas
- m.p./b.p. are slightly higher than room temp
- Slightly soluble in water; dissolves in organic solvents
What is the simple molecular lattice structure of the fullerene Buckminsterfullerenes?
- C atoms in pentagonal and hexagonal rings
- Spherical
- C60 molecules held together by Van Der Waals
What is the properties of the fullerene Buckminsterfullerenes?
- Conducts heat and electricity
- Very strong and tough
- Insoluble in water
- High m.p./b.p.
What is the simple molecular lattice structure of the fullerene Nanotubes?
- C atoms in hexagonal rings only
- Cylindrical
- Structure is rod like due to continuing rings
What are the properties of the fullerene Nanotubes?
- Conducts heat and electricity
- Very strong and tough
- Insoluble in water
- High m.p./b.p
How do predict the structure and bonding, from data? 1. The boiling point,
2. The compounds solubility
3. The compounds conductivity
4. The compounds appearance
- A high boiling point indicates a giant structure (ionic, metallic or giant covalent) while a low boiling point indicates simple molecules (or atoms for noble gases).
- Compounds that are soluble in water tend to be ionic. If the compound is soluble and has a low boiling point, it may be a simple molecule that is small and very polar or able to form hydrogen bonds.
- If the solid compound conducts electricity, it is likely to be a metal, graphite or graphene. If the substance only conducts electricity when molten or dissolved, it is an ionic compound.
- The appearance of a substance can be used to distinguish between giant structures. A shiny, malleable and ductile substance is a metal whereas ionic and giant covalent structures tend to be brittle.
What is enthalpy change, ΔH?
Enthalpy change, ΔH, is the thermal energy stored in a chemical system. It can’t be measured directly.
What makes the enthalpy change negative?
If energy is released into the surroundings, the reaction is exothermic and ΔH is negative.
What makes the enthalpy change positive?
If energy is taken in from the surroundings, the reaction is endothermic and ΔH is positive
Define standard molar enthalpy change of combustion
Standard molar enthalpy change of combustion ΔHc is enthalpy change when 1 mole of element or compound is completely combusted under standard conditions in their standard states
What are the standard conditions of enthalpy?
Standard conditions are the temperature of 298 K (25°C), pressure of 100 kPa (1 bar), solution
concentrations of 1 mol dm-3. All products and reactants are in their standard states.
Define standard enthalpy change of ΔHf
Standard enthalpy change of formation, ΔHӨf, is the enthalpy change that takes place when one mole of a given substance is formed from its elements under standard conditions
Define standard enthalpy change of solution, ΔHsol
Standard enthalpy change of solution, ΔHӨsol, is the enthalpy change that occurs when one mole of a solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions in their standard states
Define standard enthalpy change of hydration, ΔHhyd
Standard enthalpy change of hydration, ΔHӨhyd is the enthalpy change that takes place when one mole of gaseous ions dissolves in water under standard conditions in their standard states
Define standard enthalpy change of atomisation, ΔHat
Standard enthalpy change of atomisation, ΔHӨat is the enthalpy change when one mole of gaseous atoms are formed from its element under standard conditions in their standard states.
Define standard enthalpy change of neutralisation ΔHneut
Standard enthalpy change of neutralisation ΔHӨneut is the enthalpy change when one mole of water is formed from a neutralisation reaction under standard conditions
Define standard enthalpy change of reaction, ΔHr
Standard enthalpy change of reaction, ΔHӨr is the enthalpy change that occurs when a reaction takes place in the molar quantities given in a chemical equation, all reactants and products in their standard states under standard conditions.
