AS Physical Chemistry Flashcards
What is the relative mass and charge of a proton
1 and +1
Heat is the relative mass and charge of a neutron?
1 and 0
What is the relative mass and charge of an electron?
1/2000 and -1
What does the mass number tell you?
The total number of protons and neutrons in the nucleus
What does the atomic number tell you?
- the number of protons in the nucleus
- all atoms of the same element have the same number of protons
How do you work out the number of neutrons?
Mass number - atomic number = neutrons
What is an ion? And how are positive and negative ions formed?
Different number of protons and electrons
Positive ions = fewer electrons than protons
Negative ions = more electrons than protons
What is an isotope?
Atoms of the same element with different numbers of neutrons but the same number of protons
What is the John Dalton theory
Atoms are solid spheres
What is J.J. Thomsons theory?
- Discovered the electron
- Showed that atoms weren’t solid and indivisible
- Plum pudding model
What was the Rutherford, Geiger and Marsden experiment?
- Gold foil experiment, fired positively charge particles at a very thin sheet of gold.
- Most particles passed trough, some deflected backwards.
- New model = tiny positively charged nucleus surrounded by a cloud of negative electrons - most of the atom is empty space
What was Niels Bohr’s theory?
- electrons exist in shells or orbits of fixed energy
- when electrons move between shells, electromagnetic radiation is emitted or absorbed.
What is the relative atomic mass, Ar?
The average mass of an atom of an element on a scale where an atom of carbon-12 is 12
What is the relative isotopic mass?
The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12
What is the relative molecular mass, Mr?
The average mass of a molecule on a scale where an atom of carbon-12 is 12
How can relative mass be measured?
Using a mass spectrometer
Describe the ionisation step of time of flight mass spectrometers?
Electrospray = sample is dissolved and pushed through a small nozzle at high pressure, a high voltage os applied to it, causing the particle to gain an H+ ion, the sample is turned into a gas of positive ions
Electron impact = sample is vaporised and an electron gun fires high energy electrons at it, knocking of one electron so they become 1+
Describe the acceleration step of time of flight mass spectrometers?
- positively charged ions are accelerated by an electric field, so they all have the same kinetic energy (so lighter ions end up moving faster than the heavier ions)
Describe the ion drift step of time of flight mass spectrometers?
- ions enter a region with no electric field, so they just drift through it, lighter ions will drift through faster than heavier ions.
Describe the detection step of time of flight mass spectrometers?
- as lighter ions travel at higher speeds in the drift region, they reach the detector in less time than heavier ions
- the detector detects charged particles and a mass spectrum is produced.
How is a mass spectrum graph plotted?
Y-axis = abundance of ions (%), the height of each peak gives the relative isotopic abundance
X-axis = units are given as a ‘mass/charge’ ratio m/z
How can you work out relative atomic mass from a mass spectrum?
- each peak, read the % relative isotopic abundance from the y-axis and the relative isotopic mass form the x-axis
- multiply them together to get the total mass form each isotopes
- add these up
- divide by 100 or the sum of the relative abundances
(Abundance x mass) + (abundance x mass) + etc
Equation = ———————————————————————
Sum of the relative abundances
How can mass spectrometry be used to identify elements?
Elements with different isotopes produce more than one line in a mass spectrum because the isotopes have different masses.
- producing characteristic patterns which can be used as finger prints to identify elects
How can mass spectrometry be used to identify molecules?
- a molecular ion, M+ is formed in the mass spectrometer when more electron is removed from the molecule
- gives a peak in the spectrum with a mass/charge ratio equal to the relative molecular mass of the molecule
- used to identify an unknown compound
How do you work out the electron configuration of an element?
- electrons fill up the lowest energy sub-shells first
- two electrons in each orbital spin in opposite directions
- electrons fill orbitals singly before they start sharing
- Nobel gases are sometimes used in electron configuration e.g. calcium [Ar]4s2
Why is the electron configuration of transition metals unusual?
- Chromium and Copper donate one of their 4s electrons to the 3d sub-shell, as they are happier with a more stable full or half-full d sub-shell.
- Cr = 1s2 2s2 2p6 3s2 3p6 3d5 4s1
- Cu = 1s2 2s2 2p6 3s2 3p6 3d10 4s1
- transition metals as ions los their 4s electrons before their 3d electrons
What is the exception to the rule for electron configuration?
- 4s sub-shell has a lower energy level than the 3d sub-shell even though its principle quantum number is bigger, so the 4s shell fills up first.
How does electronic structure decide the chemical properties of s block elements?
Group 1 and 2, one or two outer shell electrons are easily lost to form positive ions with an inert gas configuration
How does electronic structure decide the chemical properties of p block elements?
Groups 5,6,7 can gain 1,2 or 3 electrons to form negative ions with an inert gas configurations
Groups 4 and 7 can also share electrons when they form covalent bonds
How does electronic structure decide the chemical properties of group 0 elements?
Have completely filled s and p sub-shells and don’t need to bother gaining, losing or sharing electrons = inert
How does electronic structure decide the chemical properties of d block elements?
Transition metals tend to lose s and d electrons to form positive ions
What is first ionisation energy? And is it endo or exo thermic?
The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to from 1 mole of gaseous 1+ ions.
It is an endothermic reaction
Key points to remember about ionisation energies:
- Must use the gas state symbol, as ionisation energies are measured for gaseous atoms
- Always refer to 1 mole of atoms, as stated in the definition, rather than to a single atom
- Lower the ionisation energy, the easier it is to form an ion
How does nuclear charge affect ionisation energy?
The more protons there are in the nucleus, the more positively charged the nucleons is and the stronger the attraction for the electrons.
How does distance from the nucleus affect ionisation energy?
Attraction falls off very rapidly with distance, an electron close to the nucleus will be much mire strongly attached than one further way.
How does the shielding affect the ionisation energy?
As the numbers of electrons between the outer electrons and the nucleus increases, the out shell electrons feel less electrostatic attraction towards the nuclear charge.
What is meant by a high ionisation energy?
There is a high electrostatic attraction between the electron and the nucleus and so more energy is needed to remove an electron.
What is second ionisation energy?
The energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to from 1 mole of gaseous 2+ ions.
What is the equation for any successive ionisation energy?
X(n-1)+ (g) —> Xn+ (g) + e-
Why does successive ionisation energies increase, within each shell?
- electrons are being removed from an increasingly positive ions so there is less repulsion amongst the remaining electrons, so they are being help more strongly by the nucleus
- the big jumps in ionisation energy happen when a new shell is broken into and an electron is being removed from a closer shell
What are the trends in first ionisation energies?
First ionisation energy:
- decreases down a group
- increases across a period
Why does ionisation energy decrease down group 2?
- if each element down down group 2 has an extra electron shell compared to the one above, the extra inner shells will shield the outer electrons from the attraction of the nucleus
- extra shells also mean the outer electrons are further away and there is a smaller nucleus attraction
Why does ionisation energy increase across a period?
- across a period the number of protons is increasing, which means a stronger nuclear attraction
Why is there a drop in ionisation energy between groups 2 and 3?
Aluminiums outer electron is in a 3p orbital rather than 3s. The 3p orbital has a slightly higher energy so the electron is further than the nucleus
The 3p orbital has additional shielding provided by he 3s2 electrons
Why is there a drop in ionisation energy between groups 5 and 6?
Because of electron repulsion:
- shielding is identical in the phosphorus and sulphur atoms and the electron being removed from an identical orbital.
- for phosphorus the electron is being removed from a singly-occupied orbital
- for sulphur the electron is being removed from an orbital containing two electrons
- the repulsion between two electrons in an orbital means that electrons are easier to remove form shared orbitals.
What is avogadro’s constant?
6.02 x 10 to the power of 23 = one mole
E.g. 1 mole of carbon contains 6.02x1023 atoms, 1 mole of electrons constrains 6.02x1023 electrons
What is the formula to convert between number of particles and number of moles?
Number of particles = number of moles x avogadro’s constant
E.g. how many atoms in 0.45 moles of pure ion?
= number of atoms = 0.45 x (6.02x1023) = 2.71x1023
What is the formula to work out moles?
mass of substance
Moles = —————————
Mr
What is the formula for moles that links concentration and volume?
Moles = concentration (mol dm-3) x volume (dm3)
What is the ideal gas equation?
- find out the number of moles in a certain volume of gas.
pV = nRT
For the ideal gas equation (pV = nRT) what are the units for each of the components?
Pressure = Pa
Volume = m3
N = moles
R = 8.31 J K-1 mol-1
Temperature = K
K = Degrees Celsius + 273
How do you write ionic equations?
- only the reacting particles
- write out all the ions in the equation
- and cancel out any that appear on both sides
What is a standard solution?
- involves dissolving a known amount of solid in a known amount of water to create a known concentration
How do you make a standard solution?
250 cm3 of 2.00 mol dm-3 solution of sodium hydroxide
1- work out how many moles of the solid you need = moles = conc x vol
2 - week out how many grams of solid you need = mass = moles x mr
3 - place a weighing boat on a digital balance and weigh out mass of solid, put it in a beaker, reweigh boat and subtract mass of boat from mass of boat and solid together = precise mass of solid
4 - add distilled water to beaker and stir till dissolved
5 - add solution to volumetric flask, using a funnel
6 - rinse beaker and stir rod with distilled water and add to the flask
7 - top the flask up to the correct volume
8 - add stopper and invert to homogenise
How do you do a titration?
- use a pipette to measure out a set volume of the solution that you want to know the concentration of and put it in a flask
- add a few drops of indicator
- fill a burette with a standard solution of the acid, using a funnel, below eye level
- open tap till colour change for rough titre
- repeat at least 3 times drop-wise, until concordant results
- work out mean titre
What is the colour change for methyl orange?
Red in acid to yellow in alkali
What is the colour change for phenophthalein`/
Colourless in acid to pink in alkali
What is empirical formula?
The simplest whole number ratio of atoms of each element in a compound
What is molecular formula?
The actual number of atoms of each element in a compound
It is made up of whole number of empirical units so if empirical formula is C4H3O2 and Mr = 166 then the molecular formulas is 2 empirical units so it is C8H6O4
How can you work out the empirical formula from percentages?
- Divide the given percentages of each element by the Mr to get the moles of each one
- Then divide each one by the smallest number to get the ratio
How can empirical formulas be calculated from experiments?
- work out moles of compounds and therefore moles of the atoms
- put into a ratio
How can you calculate theoretical yield?
- divide the mass given my its Mr to get the moles
- use the ration in the equation to work out moles of product
Theoretical yield = moles x mr of product
Why is the actual yield always less than the theoretical yield?
- sometimes not all the starting chemicals react fully, some chemicals are lost
How do you work out percentage yield?
Actual yield
Percentage yield = ———————— x 100
Theoretical yield
What does atom economy measure?
The proportion of reactant atoms that become part of the desired product in the balanced chemical equation
What is the formula for percentage atom economy?
molecular mass of desired product
% atom economy = ————————————————— x 100
Sum of molecular mass of all reactants
What is ionic bonding?
- when electrons are transferred from one atom to another
- ions have either lost or gained electrons so they have a full outer shell
- ions are held together with electrostatic attraction - very strong
- can also have compound ions which are ions made up of several atoms such as OH- or NH4+
- metal and non-metal
What is a giant ionic lattice?
- giant lattices of ions
- giant = made up of the same basic unit repeated over and over again
How does ionic structure determine the behaviour of ionic compounds?
- ionic compounds conduct electricity when they’re molten or dissolved - not when solid = ions can freely move and carry a charge
- ionic compounds have high melting point = held together by strong electrostatic forces
- ionic compounds tend to dissolve in water = water molecules are polar - part of the mole has a small negative charge and other parts have a small negative charge, these charges pull ions apart causing it to dissolve
What is covalent bonding?
- two atoms share electrons so they both have got full outer shells of electrons
- two non-metals
What is a giant covalent structure?
- huge network of covalently bonded atoms
- macromolecular structures
- carbon atoms can form theses structures because they can each form four strong covalent bonds
How does the structure of graphites relate to is properties?
- weak van der waal bonds between the layers = sheets can slide over each other = dry lubricant
- delocalised electrons, free moving = can carry a charge = good electrical conductor
- layers are far apart = low density, used to make lightweight sports equipment
- strong covalent bonds = very high melting point
- insoluble = covalent bonds too strong
What is the structure and properties of diamond?
- each carbon atoms share electrons is covalently bonded to four carbs in a tetrahedral shape
- very high melting point
- very hard
- vibrations travel easily through the stiff lattice = good thermal conductor
- cannot conduct electricity
- insoluble
- refract light a lot
What is dative covalent bonding?
- where both electrons come from one atom
- e.g. ammonium ion (NH4+) nitrogen donates a pair of electrons too the hydrogen ion
How do electron charge clouds repel each other?
Electrons are all negatively charged so will repel each other as much as they can
- lone pair/lone pair angles are the biggest
- lone pair/bonding pair angles are the second biggest
- bonding pair/bonding pair angles are the smallest
- valence-shell electron pair repulsion theory
How can you use the number of electron pairs to predict the shape of a molecule?
- use periodic table to work out the number of electrons in the outer shells of the central atoms
- add one to this number from every atom that the central atom is bonded to
- divide by 2 to find the number of electron pairs on the central atom
- compare the number of electron pairs to the number of bonds to didn’t the number of lone pars and the number of bonding pairs
What’s the shape when there is two bonding pairs and 0 lone pairs
Linear
What is the shape when there is 3 bonding pairs and 0 lone pairs?
Trigonal planar
What the shape when there is 4 bonding and zero lone pairs?
Tetrahedral
What’s the shape when there is 3 bonding 1 lone pairs?
Trigonal pyramidal
What’s the shape when there is 2 bonding and 2 lone pairs?
Bent
What’s the shape when there is 5 bonding and 0 lone pairs?
Trigonal bipyramidal
What’s the shape when there is 4 bonding and 1 lone pairs?
Seesaw
What’s the shape when there is 3 bonding and 2 lone pairs?
T-shaped
What’s the shape when there is 6 bonding no lone pairs?
Octahedral
What’s the shape when there is 4 bonding and 2 lone pairs?
Square planar
What is electronegativity?
An atoms ability to attract the electron pairs in covalent bonds
How can covalent bonds be polarises by differences in electronegativity?
In a covalent bond between two different electronegativities, the bonding will be pulled towards the more electronegative atom making the bonding polar
- in a polar bond the difference in electronegativity causes a permanent dipole, a dipole is a difference in charge between the two atoms cause by a shift in electron density.
How can whole molecules be polar too?
- a molecule that has am uneven distribution of charge across it then it is polar
- if the polar bonds are arranged symmetrically in the molecule then the charges cancel out and there is no permanent dipole
What are the three types of intermolecular forces?
- induced dipole-dipole or van der waals
- permanent dipole-dipole
- hydrogen bonding
How do van der waal forces form between atoms?
- electrons in electron clouds are always moving really quickly, when the electrons are on one side more than th other a temporary dipole is created
- this dipole can cause another temporary dipole in the opposite direction on another atom and the two are attracted to each other
- the second dipole can cause a third dipole in another atom
- dipoles are being created and. Destroyed constantly but give the overall effect of atoms being attracted to one another
How can van der walk forces be stronger and therefore have a higher boiling point?
- larger molecules have larger electron clouds = stronger van der waal forces
- longer, straight molecules lie closer together then branched one - the loser the molecules = stronger
- compounds with stronger van der waal forces have a higher boiling point
What is hydrogen bonding and how does it occur?
- only when hydrogen is covalently bonded to F, N, or O
- F,N and O are very electronegative = draw bonding away from hydrogen atoms
- the bond is so polarised and hydrogen has such a high charge density, that the hydrogen atoms for weak bonds with lone pairs of electrons on the F, N or O
- e.g. water and ammonia
How does hydrogen bonding have a huge effect on the properties of substances?
- hydrogen bonding = higher boiling points and melting points
- makes ice less dense then water as the molecules make more hydrogen bonds and arrange themselves into a regular lattice, where the h20 molecules are further apart then in liquid water so ice is less dense then liquid water.
What is metallic bonding?
Metal elements exist as giant metallic lattice structures:
- outermost shell of electrons of metal atom is delocalised, laving a positive metal ion
- positive metal ions are attracted to the delocalised electrons and form a lattice of closely packed positive ions in a sea of delocalised electrons
How does metallic bonding explain the properties of metals?
- high boiling points = strong electrostatic attraction between positive metal ions and sea of delocalised electrons
- the more delocalised electrons per atom the stronger the bonding will be and the higher the melting point
- delocalised electrons can pass kinetic energy = good thermal conductors and electrical conductors
- insoluble because of strength of bonds
How do the physical properties of solids, liquids and gases depend on particles?
Solid = has its particles very close, high density and incompressible, particles vibrate about a fixed point
Liquid = high density, incompressible, particles move about freely and randomly allowing it to flow
Gas = loads more energy, further apart, very compressible, move about freely, diffuse easily
Why don’t covalent bonds break during melting and boiling?
- to melt or boil a simple covalent compound you only need to break the intermolecular forces that hold the molecules together
- relatively low melting point
- for giant you do need to break the covalent bonds
How does the physical properties of a solids depend on the nature of its particles?
- melting and boiling points determined by the strength of the attraction between its particles
- will only conduct electricity if it contains charged particles free to move
- depends on the type of particles it contains
What are the properties of ionic bonding?
- high melting and boiling points determined
- state at room temp = solid
- can’t conduct electricity as a solid but can as a liquid
- is soluble in water
What are the properties of simple covalent bonding?
- melting a boiling points = low
- typical state at room temp = usually liquid or gas, can be solid
- cannot conduct electricity as a solid or liquid
- solubility how polarised a molecule is
What are the properties of giant covalent bonding?
- high melting and boiling points determined
- solid at room temp
- cannot conduct electricity as a solid apart from graphite or as a liquid instead it sublimes
- not soluble
What are the properties of metallic bonding?
- high melting and boiling points
- solid at room temp
- can conduct electricity as a solid and a liquid
- not soluble in water
What is enthalpy change?
The heat energy transferred in a reaction at a constant pressure.
KJ mol -1
What are the standard conditions?
100 kPa pressure
298 K (25’C)
What is an exothermic reaction?
Gives out energy, enthalpy change is negative
- temperature usually increases
- oxidation reactions are usually exothermic
- energy is released when bonds are made
What is an endothermic reaction?
- absorb energy, enthalpy change is positive
- temperature usually falls
- energy is needed to break bonds
What are bond enthalpies?
The energy required to break bonds
- always positive as they are always endothermic
- they are an average of the energy needed to break a certain type of bond
How can enthalpy changes be calculated using mean bond enthalpies?
Enthalpy change of reaction = total energy absorbed - total energy released
What is standard enthalpy of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
How can you find the enthalpy of combustion using calorimetry?
- using a calorimeter burning the fuel where it will heat the water
- you can work out heat energy absorbed by the water if you know the mass of the water, the temp change and the specific heat capacity of water
- all calorimetry experiments lose heat to the surrounding
- sometimes when burning a fuel the combustion may be incomplete
- fuels are often volatile and some heat can be lost to evaporation
How can a calorimeter measure the enthalpy change of solutions?
For a neutralisation reaction
- add a known volume of acid into an insulated container and measure the temperature
- add a known volume of alkali and record the temperature of the mixture at regular intervals over a period of time
- find the temperature change and use it to calculate the enthalpy change for the reaction
How can you use a graph to find an accurate temperature change?
- record the temperature at regular intervals beginning a couple minutes before the reaction starts
- plot your results on a graph, draw two lines of best fit one before the reaction stance and one through the points after it started
- extrapolate both line so the pass each other
- the distance between the two lines is the accurate temperature change
How can you calculate enthalpy change and what equation do you need?
q = mc^T
q = heat lost or gained (joules)
m = mass of water (grams)
c = specific heat capacity
^T = change in temperature (kelvin)
And…
-q
^H = ———
Moles
REMEMBER = covert from Jules to kilojoules
What is Hess’s Law?
The total enthalpy change of a reaction is independent of the route taken
How can enthalpy changes be worked out from enthalpies of formation?
- arrows face outwards
- route 1 = route 2
- if it the rout goes against the arrow the sign on the enthalpy change flips
Enthalpy change of the reaction = the sum of the products - the sum of the reactants
How can enthalpy changes be worked out from enthalpies of combustion?
- arrows face downwards
- reactants - products
What is the equation for rate of reaction?
Amount of reactant used or product formed
Rate of reaction = ——————————————————————
Time
What two conditions need to take place for a reaction to take place?
- particles collide in the right direction
- particles collide with at least a certain minimum amount of kinetic energy
What is activation energy?
The minimum amount of kinetic energy needed for a reaction to start
What are features of the Maxwell-Boltzmann distribution curve?
- y-axis = number of molecules, x-axis = kinetic energy
- area under the curve is equal to the total number of molecules
- peak of the curve represents the most likely energy of any single molecule
- mean energy of all the molecules is to the right of the peal
- the far right of the curve is where the particles that have more than the activation energy and are the only ones to react
- no molecules have 0 energy
How does temperature affect rate of reaction?
Increasing temperature increases rate of reaction
- particles have more kinetic energy
- great proportion of the particles have activation energy to be able to react
- push M-B curve to the right and less of a peak
- more frequent collisions
How does concentration affect rate of reaction?
Increasing concentration increases rate of reaction
- if you increase concentration, particles will be closer together
- if they are closer they will collide more often
How does pressure affect rate of reaction?
Increasing pressure increases the rate of reaction
- pushes particles closer together so they will collide more frequently
How do catalysts affect rate of reaction?
Catalysts increase the rate of reaction
- provides an alternative pathway with a lower activation energy
How can you measure rate of reaction by timing how long a precipitate takes to form?
- watch a marker through a solution and time how long it takes to be obscured
- use the same observer and the same mark each time
- subjective
How can you measure rate of reaction by measuring a decrease in mass?
- when one of the products is a gas, you can measure the rate of formation using a mass balance
- as gas is given off the mass decreases
- accurate and easy to
How can you measure the rate of reaction by measuring the volume of gas given off?
- use a gas syringe to measure the volume of gas being produced
- accurate
What is a dynamic equilibrium?
- as the reactants are used up the forward reaction slows down, so there is more product causing backward reaction to speed up until both reactions are going at the same rate
- can only happen in a closed system
What is Le Chatelier’s principle?
If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change
What is a homogenous equilibrium?
Where all the species are the same physical state
What happens to equilibrium of concentration of increased?
If concentration of reactants is increased = equilibrium will try to oppose the change and make more product, so equilibrium shifts to the right
If concentration of the products is increased = shifts to the left
Decreasing has the opposite effect
What happens to equilibrium if you increase pressure?
- increasing pressure shifts equilibrium to side with fewer particles
- decreasing pressure shifts equilibrium to the side with more particles
What happens to the equilibrium if temperature is increased?
Increasing temperature = equilibrium shifts to the endothermic side
Decreasing temperature = equilibrium shifts to the exothermic side
What happens to the equilibrium of a catalyst is added?
NOTHING! Catalysts have no effect on the position of equilibrium
- they can make the rate of reaction increase though
What is the equilibrium constant?
Kc, it uses the concentration of the substances
[D]d [E]e
aA + bB <=> did + eE, Kc = ——————
[A]a [B]b
How does temperature alter Kc?
- temperature is the only thing affecting Kc
- if the temperature change means there is more product made, Kc will rise
What is a redox reaction/ OIL RIG
- loss of electrons is called oxidation
- gain of electrons is called reduction
- reduction and oxidation simultaneously is redox
- oxidising agents accept electrons and is reduced
- reducing agents donates electrons and get oxidised
What are the rules for assigning oxidation states?
- uncombined elements like He and Ar = 0
- diatomic elements like O2 and H2 = 0
- monoatomic ions = same as its charge
- compound ions = overall oxidation state is the ion charge
- neutral compounds = 0
- combined oxygen almost always = 0 except in peroxides where its -1
- combined hydrogen is +1 except in metal hydrides where it is -1
What is an oxidation state?
- total number of electrons it has donated or accepted
How do roman numerals show oxidation states?
Roman numerals = oxidation number
E.g. iron(II) oxide = +2
E.g. iron(III) oxide = +3
How can you write half equations to combine them into a redox reactions?
1) write reduction half equation
2) write oxidation half equation
3) both equations need the same amount of electrons = so double or triple if needed
4) combine the two equations not including electrons
