The Shapes of Simple Molecules & Ions (OCR A Level Chemistry)

Exam Questions

3 hours39 questions
1a3 marks

This question is about the shapes of a number of common molecules.

2-5_q1a-ocr-a-as--a-level-easy-sq

i)
Which molecule(s) has/ have tetrahedral structures with respect to electrons?
[1]
ii)
What is the shape of CO2?
[1]
iii)
Draw the 3D representation of ammonia, NH3.
[1]
1b3 marks

Estimate the H-O-H bond angle in water, H2O, using electron pair repulsion theory.

Explain your answer.

1c3 marks

Suggest a way in which the bond angle in ammonia, NH3 could become 109.5o and explain your answer.

1d2 marks

Ozone, O3, is another simple molecule which has the following structure:

2-5_q1d-ocr-a-as--a-level-easy-sq

i)
Estimate the O-O-O bond angle in ozone using VSEPR theory.
[1]
ii)
Identify the shape of the molecule with respect to atoms.
[1]

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2a2 marks

Sulfur can form bonds with six fluorine atoms to form sulfur hexafluoride, SF6.

i)
How many electrons are in the outer shell of the sulfur in SF6?

[1]

ii)
State the minimum and maximum numbers of electrons possible in the outer shell of sulfur.

[1]

2b1 mark

Sulfur has no lone pairs when bonded to fluorines in SF6.

Identify the shape of a molecule of sulfur hexafluoride, SF6.

2c1 mark

State the F-S-F bond angles in SF6.

2d3 marks

Sulfur has an electronegativity value of 2.58 and fluorine has an electronegativity value of 3.98. 

i)
Suggest whether the S-F bond is polar or not.

[1]

ii)
Predict if the SF6 molecule is polar and explain your answer.

[2]

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3a2 marks

Chlorofluorocarbons, often known as CFCs, have been banned due to their ability to deplete ozone in the atmosphere when the C-Cl bond breaks with high-energy UV light.

The C-F bond does not readily break and so fluorocarbons are often used as replacement compounds.

Explain why the C-F bond does not readily break in terms of bond strength and electronegativity.

3b1 mark

Identify the strongest type of intermolecular force present between molecules of CHF3.

3c3 marks

The halogens all exist as diatomic molecules in their elemental states.

Describe and explain the trend in the melting points of the halogens as you move down Group 7.

3d2 marks

The Group 7 elements, known as halogens, range from gas to liquid to solid at room temperature. They are soluble in water.

Predict and explain if the halogen molecules are able to conduct electricity in any state.

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4a2 marks

There are a number of different types of intermolecular forces possible between molecules.

Name the three main types and place them in ascending order of strength.

4b2 marks

Methanol, CH3OH, is a small alcohol molecule that forms hydrogen bonds with water.

Sketch two different hydrogen bonding interactions between methanol and water.

4c4 marks

Methanol, CH3OH can be oxidised to methanal, CH2O and then to methanoic acid, HCOOH.

Identify the strongest type of intermolecular force between:

i)
Methanal molecules
[1] 
ii)
Methanoic acid molecules
[1] 
iii)
Water and methanal
[1] 
iv)
Water and methanoic acid
[1]
4d3 marks

Methanoic acid reacts with sodium hydroxide to form sodium methanoate:

HCOOH + NaOH rightwards arrowHCOONa + H2O

Explain why sodium methanoate is a solid at room temperature and methanoic acid is a liquid.

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5a3 marks

The boiling points of the Group 6 hydrides are shown in the graph below:

2-5_q5a-ocr-a-as--a-level-easy-sq

i)
Explain why the boiling points of the hydrides rise from H2S to H2Te

[1]

ii)
Explain why the boiling point of H2O does not follow this trend

[2]

5b3 marks

Predict which of the following hydride pairs has the higher boiling point: 

i)
CH4 vs SiH4
[1]
ii)
NH3 vs PH3
[1]
iii)
HF vs HCl
[1]

5c2 marks

Explain how the physical properties of ammonia, NH3, allow it to easily be separated in the Haber process from hydrogen and nitrogen gas, which can then be recycled.

5d4 marks

The intermolecular forces between water molecules, H2O, are stronger than those between ammonia molecules, NH3.

Explain this difference.

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1a
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2 marks

The shapes of the molecules BH3 and NH3 are shown below.

aYE5CKrA_1

Explain why each bond angle in BH3 is 120°.

1b
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1 mark

Predict whether the bond angle in NH3 is greater or smaller than the bond angle in BH3.

1c
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2 marks

Explain why the bond angle in NH3 is different from the bond angle in BH3.

1d
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2 marks

Draw a sketch of a molecule with two lone pairs and two bonding pairs of electrons. Indicate the lone pairs of electrons and the bond angles in your sketch.

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2a
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4 marks

The electronegativities of some elements are shown in the table below. 

Element Electronegativity
Li 1.0
H 2.1
C 2.5
N 3.0
Cl 3.0

i)
Explain what is meant by electronegativity. 

ii)
Explain how the carbon-hydrogen bond (such as in CH4) differs from the nitrogen-hydrogen bond (such as in NH3) in terms of the bond polarity.
2b
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2 marks

Using the table, explain why the bonding in ammonia (NH3) is covalent but the bonding in lithium chloride (LiCl) is ionic.

2c
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3 marks

Sketch the shape of a methane (CH4) molecule. In your sketch indicate any lone pairs of electrons, name the shape of the molecule and suggest the bond angle. 

2d
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3 marks

Explain why tetrachloromethane, CCl4, is a non polar molecule, whilst CH3Cl, is polar.

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3a
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4 marks

Chlorine (Cl) reacts with boron (B) to form boron trichloride (BCl3).

i)
State the type of bonding occurring in B-Cl.

ii)
Explain how this bond is formed

iii)
The electronegativities of boron, chlorine and hydrogen are 2.0, 3.2 and 2.2 respectively. Explain whether the B-Cl bond or B-H bond is more polar. 
3b
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4 marks

Molecules and separate atoms (such as noble gases) are attracted to one another by intermolecular forces. 

i)
State the strongest type of intermolecular force that takes place between water (H2O) molecules.

ii)
Draw a diagram that illustrates the interaction between two water molecules. Indicate any lone pairs of electrons and partial charges in your diagram.
 
3c
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6 marks

The boiling point of tetrachloromethane (CCl4) is 77 oC whereas the boiling point of water (H2O) is 100 oC. 

i)
Suggest why the boiling point of H2O is higher than that of CCl4

ii)
Predict whether the boiling point of methane (CH4) is higher or lower than the boiling point of CCl4. Explain your answer.
 
3d
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3 marks

Deduce whether each of the three molecules oxygen difluoride, OF2, phosphorus trifluoride, PF3, and boron trichloride, BCl3, are polar or non-polar.

Give a reason in each case.

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4a
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5 marks

Boron trichloride can react with a chloride ion (Cl-) to form a boron tetrachloride ion, (BCl4-)

BCl3 + Cl- → BCl4-

i)
Draw and name the shape of the BCl4- ion. In your diagram, include any lone pairs of electrons on the boron atom and predict the bond angle.

ii)

The chloride ion can also react with a hydrogen ion (H+) to form hydrogen chloride (HCl)

Cl- + H+ → HCl

What is the strongest type of intermolecular force between HCl molecules? 

iii)
The general trend is that the boiling point of compounds increases with increasing molecular mass. However, the boiling point of hydrogen fluoride (HF) is higher than that of HCl. Explain this anomalous boiling point of HF.
4b
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2 marks

The structures of two hydrocarbons are shown below.

Straight chain / branched chain

The boiling point of hydrocarbon B is higher than that of hydrocarbon A. ExplaIn this difference in boiling points.

4c
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2 marks

Three examples of boron compounds are B(OH)3, BF4and BH3. State which of these compounds is not planar and explain your answer. 

4d
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2 marks

State the compound in part (c) that has the highest boiling point. Explain your answer.

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5a
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2 marks

Solids have higher densities than liquids. An exception to this is ice. The image below shows the three-dimensional network of bonds in ice.

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Using intermolecular forces, explain why ice has a density which is lower than liquid water. 

5b
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3 marks

Both water, H2O, and phosphine, PH3have the same molecular mass of 18.0 however water has a much higher melting point, 0 oC than phosphine, -133 oC

i)
State the strongest type of intermolecular forces between water molecules and between phosphine molecules.

ii)
Explain why the melting point of water is much higher than that of phosphine. 
5c
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3 marks

A PH3 molecule can react with a hydrogen ion (H+) to form a PH4+ ion.

i)

Draw and name the shape of the PH4+ ion. Indicate any lone pairs of electrons.

ii)
What is the bond angle in the PH4+ ion?
5d
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3 marks

Phosphoric acid (H3PO4) is another phosphorus compound that is often used in making fertilisers and detergents. Unlike phosphine (PH3), phosphoric acid can form hydrogen bonds. The shape of phosphoric acid is given below.

5

i)
What is the maximum number of hydrogen bonds one molecule of phosphoric acid can form when dissolved in water?

ii)
Draw a diagram to show one hydrogen bond between one molecule of phosphoric acid and one water molecule. Indicate any lone pairs of electrons.

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1a2 marks

Amides are commonly used in industrial processes as solvents, and there are suggestions that the smallest amide, methanamide, HCONH2, could function in a similar manner to water as a solvent for life on other planets.

Draw the dot-and-cross diagram for methanamide, HCONH2.

1b6 marks

Predict and explain the H-C-N and C-N-H bond angles around the C and N atoms.

1c2 marks

Predict the shape around each of the C and N atoms in HCONH2.

1d3 marks

State, with a reason, whether HCONH2 is a polar molecule.

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2a2 marks

Tetrafluorothene is best known for being the monomer that forms poly(tetrafluoroethene), PTFE, otherwise known by the trade name Teflon and used in non-stick cookware. Tetrafluorohydrazine, however, is a toxic and irritant gas with few significant commercial uses.

Tetrafluoroethene, C2F4, and tetrafluorohydrazine, N2F4, are fluorides of adjacent elements in the Periodic Table.

Draw the dot-and-cross diagrams for C2F4 and N2F4 showing all valance electrons.

2b5 marks

Predict and explain the F-C-F bond angle in tetrafluoroethene and the F-N-F bond angle in tetrafluorohydrazine.

2c4 marks

Tetrafluorohydrazine is a polar molecule but tetrafluoroethene is not.

Explain the difference in molecular polarity.

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3a3 marks

Methanol is often used as a solvent, commonly in the production of inks and dyes. However, it is also highly soluble itself in water.

Explain why methanol is soluble in water.

3b4 marks

Methanol, ethanol and propan-1-ol are all primary alcohols.
Describe and explain the trend in their melting points shown below. 

Alcohol

Methanol

CH3OH

Ethanol

C2H5OH

Propan-1-ol

C3H7OH

Melting point / oC -97 -114 -126

3c4 marks

These longer primary alcohols have the following melting points: 

Alcohol C4H9OH C5H11OH C6H13OH C7H15OH C8H17OH C9H19OH C10H21OH
Melting point / oC -90 -79 -52 -34 -16 -6 6

Describe and explain this trend.

3d2 marks

Predict, with a reason, which of ethanol or ethane-1,2-diol will have the higher melting point?

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4a6 marks

Alkanes are naturally occurring hydrocarbons present as a mixture in crude oil. In order for them to be useful the mixture needs to be separated into fractions, which is done using a temperature gradient as they have varying boiling points.

C2H6, C4H10 and C3H8 are all examples of alkanes. 

i)
Put them in order of increasing boiling point and explain your answer.
[3]
ii)
Put them in order of increasing volatility and explain your answer.

[3]

4b5 marks

Predict, with a reason, whether alkanes are soluble in either water or propanone.

4c5 marks

Pentane can exist as isomers, including pentane, CH3CH2CH2CH2CH3 and 2,2-dimethylpropane, CH3C(CH3)2CH3.

i)
Draw skeletal formula for each isomer shown above.
[2]
ii)
Predict and explain which isomer of pentane would have the greater volatility.

[3]

4d5 marks

There are two isomers possible with the molecular formula C2H6O. One is an alcohol and one is an ether, which has a C-O-C functional group.

i)
Draw the skeletal formulae of both isomers.

[2]

ii)
Identify the strongest type of intermolecular force present in each isomer.

[2]

iii)
Predict which isomer would have the higher melting point.

[1]

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5a4 marks

Halogens can form covalent compounds between themselves, called interhalogen compounds, as well as with other elements.

One interhalogen compound is IF5

i)
Draw the dot-and-cross structure for IF5.

[1]

ii)
Use electron pair repulsion theory to deduce the bond angles in IF5.

[1]

iii)
Predict whether IF5 will be a polar molecule and explain your choice.

[2]

5b4 marks

Iodine can also form the triiodide ion, I3-

i)
Draw the dot-and-cross diagram for I3-.
[1]
ii)
Use electron pair repulsion theory to deduce the bond angles in I3-.
[1]
iii)
Describe and explain the position of the lone pairs on the central iodine.
[2]
5c4 marks

Halogens can also form ionic compounds.
Sodium chloride and iodine are both solids. Sodium chloride does not melt until it reaches a temperature of 1074 K, yet iodine sublimes when heated gently, giving off purple vapours. 

i)
State the type of structure for each of iodine and sodium chloride.

[2]

ii)
Explain why iodine vaporises easily.

[2]

5d3 marks

Sodium chloride will conduct electricity when molten, not solid, but iodine is a very poor conductor of electricity in all states.

Explain the differences in electrical conductivity of sodium chloride and iodine.

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