Unit 1: KA2 Structure and bonding Flashcards
Elements are held together in different ways and the properties of chemical compounds are determined by the bonding between atoms and the attractive intermolecular forces between molecules.
What do atoms share in covalent bonds?
Pairs of electrons.
Describe COVALENT BONDING.
When two positive nuclei are held together by their common attraction for the shared pair of electrons.
*an example of an intramolecular force
When are POLAR COVALENT bonds formed?
When the attraction of the atoms for the pair of bonding electrons is different.
Delta positive (δ⁺ or 𝛿⁺) and delta negative (δ⁻ or 𝛿⁻) notation can be used to indicate…
The partial charges on atoms, which give rise to a dipole.
How can ionic formulae be written?
By giving the simplest ratio of each type of ion in the substance.
Describe IONIC BONDING.
The electrostatic attraction between positive and negative ions.
What structure do ionic compounds form?
Ionic compounds form lattice structures of oppositely charged ions.
PURE COVALENT bonding (non-polar bonding) and IONIC bonding can be considered as…
Opposite ends of a BONDING CONTINUUM.
What (type of bonding) lies in the middle of the BONDING CONTINUUM?
POLAR COVALENT bonding.
What does the difference in electronegativities between bonded atoms indicate?
The ionic character.
The smaller the difference in electronegativities between bonded atoms, the more polar the bond will be. True or False.
False, the BIGGER the difference in electronegativity, the more polar the bond will be.
If the difference in electronegativities is LARGE, then the movement of bonding electrons from the element of lower electronegativity to the element of higher electronegativity is complete, resulting in the formation of ions. True or False?
*Feel free to rephrase
True
*LARGE DIFFERENCE in electronegativities = bonding electrons from lower electronegativity go to higher electronegativity = forms ions.
Compounds formed between metals and non-metals are often, but not always —–.
IONIC.
Physical properties of a compound should be used to deduce the type of bonding and structure in the compound. Name the physical properties you should refer to. [5 marks]
State at room temperature [1],
Melting point [1],
Boiling point [1],
Solubility [1],
Electrical conductivity [1].
All molecular elements and compounds, and monatomic elements condense and freeze at sufficiently low temperatures. What must exist between the molecules or discrete atoms for this to occur?
At least one form of attractive force.
Intermolecular forces acting between molecules are known as…
van der Waals forces.
There are several different types of van der Waals forces. Name the 3 forces we focus on in Higher Chemistry. [3 marks]
London dispersion forces [1],
AND
permanent dipole-permanent dipole interactions [1] that include hydrogen bonding [1].
London dispersion forces are forces of attraction that can only operate between a few atoms and molecules. True or False?
False. They can operate between ALL atoms and molecules.
London dispersion forces are much STRONGER/WEAKER than all other types of bonding.
What does the strength of London dispersion forces relate to?
WEAKER.
The STRENGTH of London dispersion forces is related to the NUMBER OF ELECTRONS within an atom or molecule.
How are London dispersion forces formed?
As a result of electrostatic attraction between dipoles (both temporary and induced –> caused by the movement of electrons in atoms and molecules.)
A molecule is described as POLAR if it has a ——— ——.
The spatial arrangement of polar covalent bonds can result in a molecule being polar. True or False?
PERMANENT DIPOLE.
True.
Besides London dispersion forces, what are additional electrostatic forces of attraction between polar molecules?
Permanent dipole-permanent dipole interactions.
Which is the stronger force of attraction for molecules with similar numbers of electrons? Permanent dipole-permanent dipole interactions OR London dispersion forces.
Permanent dipole-permanent dipole interactions are stronger.
Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen are HIGHLY —–.
POLAR.
Describe the bond between a hydrogen atom and a strongly electronegative element (fluorine, oxygen or nitrogen).
It’s a HIGHLY POLAR bond.
What’s a HIGHLY POLAR bond?
Describes the bond between a hydrogen atom (an atom with very few protons = low electronegativity) and a strongly electronegative element such as fluorine, oxygen or nitrogen.
Describe HYDROGEN BONDS.
The electrostatic forces of attraction between molecules that contain highly polar bonds.
Are hydrogen bonds weaker than forms of permanent dipole-permanent dipole interactions?
No, hydrogen bonds are STRONGER than other forms of permanent dipole-permanent dipole interaction but WEAKER than a covalent bond.
What can be rationalised in terms of the nature and strength of the intermolecular forces that exist between molecules? [3 marks]
Melting points,
Boiling points,
Viscosity
By considering the POLARITY and NUMBER OF ELECTRONS present in molecules, it’s possible to make ———– predictions of the ——– of the intermolecular forces.
qualitative,
strength.
The melting and boiling points of POLAR substances are HIGHER/LOWER than the melting and boiling points of NON-POLAR substances with similar numbers of electrons.
HIGHER
What are properties of substances that are affected by HYDROGEN BONDING?
Boiling points,
Melting points,
Viscosity,
Solubility/Miscibility in water
The anomalous boiling points of AMMONIA, WATER and HYDROGEN FLUORIDE are a result of…
HYDROGEN BONDING
What does hydrogen bonding between molecules in ice result in?
An expanded structure that causes the density of ice to be less than that of water at low temperatures.
Which compounds tend to be SOLUBLE in polar solvents such as water, and INSOLUBLE in non-polar solvents?
IONIC compounds and POLAR MOLECULAR compounds.
Which type of molecular substances tend to be INSOLUBLE in polar solvents such as water, and SOLUBLE in non-polar solvents?
NON-POLAR molecular substances.
To predict the solubility of a compound, key features to be considered are the:-
[2 marks]
presence of O-H or N-H bonds, which implies hydrogen bonding. [1]
OR
spatial arrangement of polar covalent bonds, which could result in a molecule possessing a permanent dipole. [1]
What is a COVALENT BOND?
A pair of shared electrons between two atoms.
Which type of atoms do covalent bonds usually form between?
Between non-metal atoms.
How are the atoms in a covalent bond held together?
The two positive nuclei and their common attraction for the shared pair of electrons, hold the atoms together.
What type of bond is formed when the ATTRACTION of the ATOMS for the pair of bonding electrons is DIFFERENT?
POLAR COVALENT BONDS
How can you distinguish between PURE covalent and POLAR covalent bonds?
By using the ELECTRONEGATIVITY VALUES
The POLARITY of a covalent bond depends on…
The difference in ELECTRONEGATIVITY within the bond
When there is a SMALL or NO difference in Electronegativity, what is the bond called?
PURE COVALENT bond
Give examples of where PURE COVALENT bonds can be found.
In ELEMENTS and HYDROCARBONS
When there is a MEDIUM difference in Electronegativity, what is the bond called?
POLAR COVALENT bond
Give examples of where POLAR COVALENT bonds can be found.
In:
H-Cl,
H₂O,
NH₃
When there is a LARGE difference in Electronegativity, what is the bond called?
IONIC bonding
Give examples of where IONIC bonds can be found.
In:
Na⁺Cl⁻,
Mg²⁺(Cl⁻)₂
What can you use to assign 𝛿⁺ and 𝛿⁻ partial charges on atoms?
The ELECTRONEGATIVITY VALUES in the data book.
*electronegativities of selected elements can be found on page 12 of the Higher Chemistry data booklet (Publication date: 2021)
The atom with the HIGHER ELECTRONEGATIVITY VALUE…
What partial charge will this atom have?
Has a GREATER SHARE of the bonding electrons.
This atom will have a 𝛿⁻ charge.
The atom with the LOWER ELECTRONEGATIVITY VALUE…
What partial charge will this atom have?
Has a SMALLER SHARE of the bonding electrons.
This atom will have a 𝛿⁺ charge.
Atoms in a POLAR COVALENT BOND have 𝛿⁺ and 𝛿⁻ partial charges on atoms called…
a PERMANENT DIPOLE
What is a PERMANENT DIPOLE?
When atoms in a covalent bond have 𝛿⁺ and 𝛿⁻ partial charges on the atoms
The type of bonding of a substance is defined by…
the ELECTRONEGATIVITY DIFFERENCE of the elements.
What does the ELECTRONEGATIVITY DIFFERENCE of elements define?
The type of bonding of a substance
What do PURE COVALENT bonds have, in terms of ionic and covalent character?
LOW ionic character,
HIGH covalent character
What do IONIC bonds have, in terms of ionic and covalent character?
HIGH ionic character,
LOW covalent character
Do all ionic compounds have low or high melting points?
So… are all ionic compounds solid or liquid at room temperature?
Ionic compounds have HIGH melting points and so are all solid at room temperature.
Do covalent networks have low or high melting points?
So… are all covalent networks solid or liquid at room temperature?
Covalent networks have HIGH melting points and so are all solid at room temperature.
Do covalent molecules have low or high melting points?
Therefore, are all gases and liquid compounds covalent molecular?
Covalent molecules have LOW melting points, and so ALL GASES and LIQUID compounds are covalent molecular.
Why are IONIC and POLAR COVALENT substances more likely to be soluble in water (a polar solvent)?
As there are charges inside these IONIC and POLAR COVALENT substances.
What do NON-POLAR substances dissolve in?
Remember they are less likely to be soluble in a polar solvent like water.
NON-POLAR SOLVENTS
Do metallic bonds (made up of only metals) conduct electricity?
Yes, they can conduct electricity as a solid and a liquid, but not in solution, as metals don’t dissolve in water.
Do covalent bonds (made up of only non-metals) conduct electricity?
No, they don’t conduct electricity.
Do ionic bonds (consisting of metals + non-metals) conduct electricity?
Yes, they can conduct electricity as a liquid or in solution, but not as a solid (as the electrons aren’t delocalised).
What is the temporary type of van der Waals (intermolecular) force?
London dispersion forces
What are the permanent types of van der Waals (intermolecular) forces?
Permanent dipole-permanent dipole interactions/attractions
AND
Hydrogen bonding
Noble gases are liquefied by lowering the temperature, which indicates that there must be some FORCE OF ATTRACTION between the atoms which is significant at lower temperatures but not at room temperature. Do you know what this force of attraction is called?
LONDON DISPERSION FORCES
What causes LONDON DISPERSION FORCES?
Continual movement of electrons in an atom.
*The temporary uneven distribution of electrons within atoms and molecules.
What happens when electrons continuously move in an atom?
Causes a temporary uneven distribution of charge at opposite sides of an atom.
Known as a TEMPORARY DIPOLE.
What is a TEMPORARY DIPOLE?
Occurs when electrons of an atom are temporarily unevenly distributed on the atom, creating small areas of positive and negative charge.
Describe TEMPORARY DIPOLES (the result of uneven distribution of electrons) in terms of charges.
One side of the atom is temporarily slightly negative (𝛿⁻)…
Meaning the other side is temporarily slightly positive (𝛿⁺). [1]
Resulting in the 𝛿⁻ side of 1 atom attracting the 𝛿⁺ side of a neighbouring atom.
A force of attraction (called the London Dispersion forces) is formed between them.
London dispersion forces are very weak/strong.
weak
The more electrons there are in the atom, the —— the London dispersion force.
As a result, as you go down Group 0, the melting and boiling points of the elements ——–, as —- energy is needed to separate the atoms.
bigger,
increase,
more
The GREATER the number of electrons… (referring to London dispersion forces)
The STRONGER the London dispersion forces.
London dispersion forces only exist in the atoms of the noble gas elements. True or False?
False, they also exist in molecular substances.
When a COVALENT MOLECULAR substance melts or boils, is it the WEAK LONDON DISPERSION FORCES between molecules, or the STRONG COVALENT BONDS holding the atoms together in the molecule, which are broken?
The weak London Dispersion forces
Larger molecules have more ——— than smaller molecules so larger molecules have STRONGER London dispersion forces between them.
electrons
How are PERMANENT DIPOLES formed in a molecule?
Permanent dipoles are formed in a molecule where the ELECTRONEGATIVITY DIFFERENCE is large enough and electrons within the bond are SHARED UNEQUALLY.
What happens to electrons within a polar bond (that has a permanent dipole)?
How does this form a permanent dipole?
Electrons within the bond spend more time at the more electronegative end of the bond.
*electronegativity of hydrogen = 2.2
electronegativity of chlorine = 3.0
H —e-> Cl
(electrons more attracted to Cl because of its higher electronegativity)
Permanent dipole formed over the covalent bond.
(𝛿⁺) H —– Cl (𝛿⁻)
The permanent dipoles formed in each molecule (of a polar covalent bond) are ——— to each other.
attracted
What are PERMANENT DIPOLE-PERMANENT DIPOLE INTERACTIONS?
Additional electrostatic forces of attraction between polar molecules (on top of the London dispersion forces already present).
Describe permanent dipole-permanent dipole interactions, in length.
An intramolecular POLAR COVALENT bond is formed (between two polar molecules).
Permanent dipole-permanent dipole interactions are additional electrostatic forces of attraction between polar molecules (on top of the already present London dispersion forces).
They come about when the difference in electronegativities is large enough and when the electrons within the bond are shared UNEQUALLY between the polar molecules.
The electrons are attracted to the more electronegative molecule, resulting in the more electronegative molecule becoming 𝛿⁻ and the less electronegative molecule becoming 𝛿⁺. This creates a PERMANENT DIPOLE.
The permanent dipoles formed in each molecule are attracted to each other.
This is what we refer to as permanent dipole-permanent dipole interactions/attractions.
What is important in deciding whether a molecule is POLAR or NON-POLAR?
The spatial arrangement of POLAR COVALENT bonds.
Why is Carbon Dioxide CO₂ a non-polar molecule?
It’s non-polar as the polarity cancels out due to its linear shape.
(𝛿⁻) O = C(𝛿⁺) = O (𝛿⁻)
Why is Tetrachloromethane CCl₄ a non-polar molecule?
It’s non-polar as the molecule has 4 𝛿⁻ ends and so the polarity cancels out.
Arrange the four types of bonding (forces of attraction-wise) in order of increasing strength - (1) being the strongest.
(1) Ionic & Covalent Bonding
(2) Hydrogen Bonding
(3) Permanent dipole-permanent dipole forces
(4) London dispersion forces
What does the type and strength of intermolecular bonds affect?
[3 marks]
Melting point [1],
Boiling point and [1],
Viscosity [1].
The higher the melting & boiling point of a substance…
The stronger the intermolecular forces.
What type of bonding do covalent compounds have (in between their molecules) that result in them having higher melting points, boiling points and viscosity than those with London dispersion forces?
Hydrogen bonding
Polar molecules will have WEAKER intermolecular forces than non-polar molecules when comparing molecules with similar number of electrons. True or False?
False, polar molecules will have STRONGER intermolecular forces than non-polar molecules.
Which van der Waals forces cause substances to have higher melting and boiling points and thicker viscosity than London dispersion forces?
Permanent dipole-permanent dipole bonding and Hydrogen bonding.
Which type of van der Waals’ force causes a substance to have super thin viscosity and low melting and boiling points?
London dispersion forces.
HYDROGEN BONDING between molecules and water molecules makes the substance ——- with water. What does this make both molecules?
soluble,
POLAR - Both molecules are POLAR.
Hydrogen bonding decreases the viscosity of a substance. True or False?
False, Hydrogen bonding INCREASES the viscosity of a substance by bringing the molecules closer together.
Hydrogen bonding increases the melting & boiling point of a substance. How?
By bringing the molecules closer together.
The thicker/more viscous a substance…
The stronger the intermolecular forces
Describe HYDROGEN BONDING.
[5 marks]
Electrostatic forces of attraction [1] between molecules that contain highly polar [1] -OH, -NH or H-F groups [1].
Molecules with N, O or F directly bonded to H [1] will have hydrogen bonding between molecules [1].
Why are the boiling points of ammonia, water and hydrogen fluoride higher than expected, given its number of electrons?
NH₃, H₂O and HF have hydrogen bonding between their molecules which raises its boiling points.
These boiling points are described as ANOMALOUS.
What does ANOMALOUS mean?
Different from the norm.
Hydrogen bonding causes ice to be denser than water at low temperatures. True or False?
Why? [9 marks]
And so, hydrogen bonding causes ice to sink in water. True or False?
False, ice is less dense than water at low temperatures because of hydrogen bonding.
The HYDROGEN BONDING between H₂O molecules (in solid ice) causes the molecules to be spaced further apart than in liquid water [1].
*Hydrogen bonds at high temperatures break and reform very quickly (hence why water is a gas at high temperatures).
At low temperatures, the molecules move more slowly around [1] = have less (kinetic) energy [1] = more hydrogen bonds exist between the H₂O molecules [1] because these bonds cannot be overcome by the little energy there is at low temperatures [1].
There are many hexagonal empty spaces between the H₂O molecules (due to hydrogen bonding) [1], meaning the ice structure takes up more space than in liquid water [1].
*Remember that ice and water have the same mass. But in ice, the volume has increased, and so the density has decreased [1]. Density=mass/volume [1].
Therefore, ice has an EXPANDED STRUCTURE and is LESS DENSE than water at low temperatures.
And so, hydrogen bonding causes ice to float in water.
What compounds dissolve in polar solvents?
What is insoluble in a polar solvent?
Polar covalent compounds and Ionic compounds.
Non-polar substances are insoluble in polar solvents e.g. water.
What substance dissolves in non-polar solvents?
What is insoluble in a non-polar solvent?
Non-polar covalent substances.
Polar substances are insoluble in non-polar solvents.
What do molecules with O-H and N-H groups have between their molecules?
How does this result in a permanent dipole?
Hydrogen bonding.
The O𝛿⁻ or N𝛿⁻ in one molecule attracts the H𝛿⁺ in a neighbouring molecule. This spatial arrangement of polar covalent bonds results in a permanent dipole.
The stronger the intermolecular forces between the solute molecule and solvent molecule…
The greater the solubility of the solute in the solvent.
“Like dissolves like.”
Polar molecules and non-polar molecules do not mix. True or False?
True
Between two polar molecules, the molecule with the smaller hydrocarbon portion (or the larger polar portion) is more/less soluble in water.
More soluble in water.
Decide if CH₃CH₂OH is soluble in water.
Yes, it’s soluble in water.
The smaller the hydrocarbon portion/the larger the polar portion in the molecule = the more soluble in water.
Decide if CH₃CH₃CH₃CH₃CH₃CH₂OH is soluble in water.
It’s insoluble in water.
Has too many hydrocarbons/too many non-polar bonds.
*The smaller the hydrocarbon portion/the larger the polar portion in the molecule = the more soluble in water.
Decide if CH₂(OH)CH(OH)CH(OH)CH(OH)CH(OH) is soluble in water.
Yes, it’s soluble in water.
It’s a large molecule but there are many OHs that can hydrogen bond with water.
*The smaller the hydrocarbon portion/the larger the polar portion in the molecule = the more soluble in water.
Decide if CH₃CH₃CH₃CH₃CH₃CH₃ is soluble in water.
It’s insoluble in water.
It’s a completely non-polar molecule and so cannot dissolve in polar solvents.
*like dissolves like
The O𝛿⁻ sides of a water molecule are always doing their best to orient themselves toward the H𝛿⁺ sides of other molecules. True or False?
What does this have to do with the strength of water’s surface tension?
True.
The strength of water’s surface tension depends on the intermolecular forces that pull molecules of a liquid together. These attractive (a.k.a. cohesive) forces pull the surface molecules inward.
Describe how ionic solids dissolve in water.
The 𝛿⁻ are attracted to the positive ions and vice versa.
This breaks the structure apart, dissolving them into freely moving ions.
A non-polar molecule is one where the electrons are distributed more symmetrically (than polar molecules). True or False?
What does this mean for the charges in a non-polar molecule?
True.
Non-polar molecules don’t have an abundance of charges at the opposite sides. The charges cancel each other out.
Is I-Br (Iodine monobromide) a non-polar molecule?
Yes, it’s relatively non-polar because there is an electronegativity difference of less than 0.5.
Generally, if the molecule has hydrogen bonding, the molecule will be…
Polar.
Molecules are normally POLAR if they have an electronegativity difference smaller than 0.5, and are symmetrical. True or False?
False, polar molecules normally have an electronegativity difference GREATER than 0.5 and are ASYMMETRICAL.
Explain how the density of metallic bonds increase across a period.
The greater the number of electrons delocalised (also to do with the increasing number of protons), the greater the charge on the metal ion –> the stronger the metallic bond –> leading to an increase in density.
Explain how metallic bonding gets weaker down a group.
Down a group each atom is gaining 1 more electron energy level. This means that the delocalised electrons are further from the metal ion, hence the metallic bond weakens going down a group.
Metallic bonds are generally strong. The strength depends on…
The NUMBER OF ELECTRONS that each atom contributes, and the CHARGE of the METAL IONS.
Name one of the uses of Phosphorus?
Used in chemical warfare as it’s highly poisonous.
The bond angles in Phosphorus are at 60*. Is this unusual?
What does this mean for the bonding in P₄?
Yes, the bond angle P-P-P-P is 60* which is considerably smaller than the normal bond angle.
The bonding in P₄ is strained and weak. This accounts for its weak bonds by stronger.
What’s the difference between permanent dipole-permanent dipole interactions and hydrogen bonds?
Hydrogen is one of the atoms involved in the polar bond.
The elements nitrogen, oxygen, fluorine and neon:
A - can form negative ions
B - are made up of diatomic molecules
C - have single bonds between the atoms
D - are gases at room temperature
D - they’re all covalent molecules.
Which of the following is not an example of a van der Waals’ force?
A - Covalent bond
B - Hydrogen bond
C - London dispersion force
D - Permanent dipole-permanent dipole attraction
A
Which of the following has more than one type of van der Waal’s force operating between its molecules in the liquid state?
A - Br₂
B - CO₂
C - NH₃
D - CH₄
C
*NH₃ is polar due to the large electronegativity difference of the N-H bond, and so would have more than just van der Waals forces –> it would also posses intramolecular forces (covalent bonds)
Particles with the same electron arrangement are said to be isoelectronic.
Which of the following compounds contain ions which are isoelectronic.
A - Na₂S
B - MgCl₂
C - KBr
D - CaCl₂
All molecular elements and compounds, and monatomic elements condense and freeze at sufficiently low temperatures. True or False?
For this to occur, what needs to exist between the molecules/discrete atoms?
True.
For this to occur, some attractive forces must exist between the molecules or discrete atoms.
Arrange the 5 bonds in order of relative strength.
How can this strength be shown?
1) Covalent bonds
2) Hydrogen Bonds
3) Permanent dipole-permanent dipole interactions
4) London Dispersion Forces
This can be shown by comparing two molecules with the same number of electrons.
