AQA A Level Chemistry

Topic Questions

1.5 Molecules: Shapes & Forces

1a3 marks

The structure of the boron trifluoride is shown below in Figure 1.

Figure 1

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i)
State the number of bonding electron pairs in the molecule.

ii)
State the bond angle. 

iii)
Name the shape of the molecule.
1b1 mark

Name the molecular shape shown in Figure 2.

Figure 2

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1c3 marks

The structure of the sulfur hexafluoride is shown below in Figure 3

Figure 3

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i)
State the number of bonding electron pairs in the molecule.

ii)
State the bond angle. 

iii)
Name the shape of the molecule.
1d2 marks

Name the molecular shape shown in Figure 4 and state the angle between the bonding pairs of electrons. 

Figure 4

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2a1 mark

State the strongest type of intermolecular force that exists between molecules of ethane, C2H6.

2b1 mark

Figure 1 shows the structure of a molecule of hydrogen bromide, HBr. State which type of intermolecular force is the strongest between molecules of hydrogen bromide.

Figure 1

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2c1 mark

State the strongest type of intermolecular force that exists in hydrogen fluoride, HF? 

2d4 marks

The boiling point and relative molecular mass data for a range of organic compounds is shown in Table 1. The boiling points decrease as you go down Table 1

Table 1

Compound

Boiling Point / °C

Relative Molecular Mass, Mr

Butane, C4H10

-1

58.0

Chloromethane, CH3Cl

-24

50.5

Ethane, C2H6

-89

30.0

Methane, CH4

-162

16.0

Use the information in Table 1 to explain the difference in boiling points between the following compounds:

i)
Chloromethane and methane

ii)
Butane and ethane

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3a1 mark

What type of intermolecular force is indicated by the dotted line in Figure 1.

Figure 1

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3b2 marks

A water molecule, H2O has two bonding pairs and two lone pairs of electrons. State the shape and bond angle of the water molecule.

3c2 marks

An ammonia molecule contains four electron pairs, one of which is a lone pair. Draw the shape of the ammonia molecule, NH3. On your diagram include the lone pair and the bond angle

3d2 marks

Explain why ammonia molecules can exhibit hydrogen bonding. 

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4a3 marks

The boiling points of the first four hydrogen halide molecules are shown in Figure 1. The molecules are:

  • Hydrogen fluoride, HF
  • Hydrogen chloride, HCl
  • Hydrogen bromide, HBr
  • Hydrogen iodide, HI

Figure 1

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Explain why the boiling point of hydrogen fluoride is the highest.

4b2 marks

Explain why the boiling point of hydrogen iodide is higher than the boiling point of hydrogen chloride and hydrogen bromide. 

4c2 marks

The boiling points of four Group 4 hydrides are shown in Figure 2. The Group 4 hydride compounds are:

  • Methane, CH4
  • Silicon hydride, SiH4
  • Germanium hydride, GeH4
  • Tin hydride, SnH4

Figure 2
8

Explain why methane has the lowest boiling point.

4d5 marks

Table 1 gives information about two molecules, propanone, C3H6O and butane, C4H10.

Table 1

 

Relative Molecular Mass (g mol-1)

Boiling Point (°C)

Propanone

58.0

56

Butane

58.0

0


Even though the relative molecular mass is the same for both compound, explain why the boiling point is different.

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5a3 marks

Figure 1 shows the structure of three different molecules. Bromomethane, CH3Br, tetrachloromethane, CCl4 and water, H2O. 

Figure 1

9

A student suggests that bromomethane is polar, CH3Br. Is the student correct? Justify your answer

5b2 marks

State the name and bond angle of the molecular shape of the tetrachloromethane, CCl4, molecule.

5c1 mark

Water, H2O, will exhibit high surface tension. Give the definition of surface tension.

5d2 marks

Solids are usually more dense than their liquids as the particles are more tightly packed together. 

i)
What happens to the spacing of the molecules when water freezes?

ii)
How does the spacing, in part (i) influence the density of ice compared to water?

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1a6 marks

Draw the shapes of the CH3+ and CH3- ions. Give the names of each ion shape and bond angles for each of the ions. 

1b3 marks

Explain the differences between the bond angles for the CH3- ion and the CH3+ ion. 

1c4 marks

Predict the shape of the PH4+ and the ClF4- ions and give the bond angles in each of these ions.

1d3 marks

Bromine pentafluoride, BrF5, is a toxic substance used as a fluorinating agent to produce fluorocarbons and also as an oxidising agent in rocket propellant systems. Draw the shape of this molecule including any lone pairs that may exist and suggest the strongest type of intermolecular forces BrF5 will exhibit. 

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2a4 marks

Phosphorus can react with chlorine and fluorine separately, to produce PCl3 and PF3

i)
Draw the shapes of one of these molecules. 

ii)
Identify which of these compounds has the highest boiling point and explain your answer.
2b8 marks

Electronegativity of the elements are assigned values on the Pauling scale and these change depending where they lay in the periodic table. 

i)
Give the definition of electronegativity. 

ii)
State and explain the trends down group 2 and across period 3.
2c5 marks

Phosphorus trifluoride, PF3, has a boiling point of -101.8degree, nitrogen trifluoride, NF3 has a boiling point of -129.1 degreeC and NH3 has a boiling point of -33.3degree. Explain the difference in the boiling points of all three molecules making specific reference to the molar mass, Mr.

2d3 marks

The structure of two amino acids, alanine and serine are shown in Figure 1. Amino acids can have a  hydrophilic or hydrophobic side chain and this is an important part of protein structure.

Figure 1

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Suggest which of the amino acids in Figure 1 is hydrophilic and give a reason for your answer.

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3a2 marks

State and explain which of the following molecules is the most polar; boron trifluoride, BF3, chlorine trifluoride, ClF3, sulfur hexafluoride SF6 or carbon tetrafluoride, CF4.

3b3 marks

State which is the strongest type of intermolecular force in boron trichloride, BCl3 and explain how this arises.

3c3 marks

The table gives information about the boiling points of two different hydrocarbons. 

Table 1

Compound

C6H14

CH3C(CH3)2CH2CH3

Mr

   

Boiling Point (°C)

69

49

i)
Complete Table 1.

ii)
Explain why the boiling points of the two compounds are different.
3d8 marks

Table 2 gives information about the boiling points of various substances which contain a halogen atom.

Table 2

Compound

F2

CH3F

HF

Br2

CH3Br

HBr

HCl

Boiling Point (K)

85

195

293

332

277

207

188


Explain the difference between the boiling points of hydrogen fluoride, HF, hydrogen bromide, HBr and hydrogen chloride, HCl.

Identify the strongest type of intermolecular force in bromine, Br2, and bromomethane, CH3Br and explain the difference in the boiling point between the two molecules. 

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4a4 marks

Place the following compounds in order of increasing boiling point and with reference to intermolecular forces, justify your answer.

Ethanol, CH3CH2OH, propane, CH3CH2CH3, dimethyl ether, CH3OCH3

4b2 marks

Pairs of substances can form solutions when combined depending of the intermolecular forces present. Explain why the following pairs of substances will form solutions. 

i)
Bromine, Br2 and carbon tetrachloride CCl4.

ii)
Propanol, CH3CH2CH2OH and water, H2O. 
4c2 marks

Which would be a more suitable solution to dissolve iodine crystals, I2 in, water, H2O or carbon tetrachloride, CCl4. Explain your answer.

4d4 marks

Draw how a molecule of propan-1-ol reacts with a molecule of water. Include the bond angle H-O-H bond in the water molecule and any lone pairs on atoms that may be present.

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5a3 marks

Sulfur tetrafluoride is a colourless gas with a distinct sulfur odour and it can be used as an oil repellent. It has the formula SF4.

i)
Draw the shape of the SF4.

ii)
Suggest why the bond angles in a molecule of SF4 would be slightly less than in a molecule of PF5.

5b1 mark

Xenon difluoride, XeF2, can be used as an antiseptic. Which bond angle would be the most suitable for a molecule of XeF2.

A)
178degree
B)
180degree
C)
118degree
D)
105degree
5c4 marks

Thionyl fluoride, SOF2, is a fleeting intermediate during the decomposition of sulfur hexafluoride SF6 which is used as an insulator for electrical equipment. The basic two dimensional structure of thionyl fluoride, SOF2, is shown below in Figure 1.

Figure 1

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i)
Draw and name the shape of a molecule of thionyl fluoride, SOF2.

ii)
State whether thionyl fluoride, SOF2, is polar and explain your answer.
5d1 mark

A molecule of 4-fluoro cyclohexane-1-carboxylic acid is shown in Figure 2. Deduce the bond angle labelled x in Figure 2

Figure 2

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1a2 marks

The shapes of the molecules BH3 and NH3 are shown in Figure 1 below.

Figure 1

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Explain why each bond angle in BH3 is 120°.

1b1 mark

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

1c2 marks

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

1d2 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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2a4 marks

The electronegativities of some elements are shown in Table 1 below.

Table 1

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.
2b3 marks

State and explain which bond polarities in Figure 1 below are drawn incorrectly.

Figure 1

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2c2 marks

Using Table 1, explain why the bonding in ammonia (NH3) is covalent but the bonding in lithium chloride (LiCl) is ionic.

2d3 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.

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3a4 marks

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

i)
What type of bond is the B-Cl bond?

ii)
How is this bond formed?

iii)
The electronegativities of boron, chlorine and hydrogen are 2.0, 3.2 and 2.2 respectively. Is the B-Cl more or less polar than a B-H bond in borane (BH3)? Explain your answer.
3b4 marks

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

i)
What is 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.
3c6 marks

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

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.

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4a5 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.
4b2 marks

The structures of two hydrocarbons are shown in Figure 1 below.

Figure 1

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

4c2 marks

Which one of the following boron compounds is not planar? Explain your answer.

A)
B(OH)3
B)
BF4-
C)
BH3
4d2 marks

Which one of the following boron compounds has the highest boiling point? Explain your answer.

A)
B(OH)3
B)
BF4-
C)
BH3

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5a2 marks

Solids have higher densities than liquids. An exception to this is ice. Figure 1 below shows the three-dimensional network of bonds in ice.

Figure 1

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

5b3 marks

Both water (H2O) and phosphine (PH3) have the same molecular mass of 18.0 however water has a much higher melting point (0 degreeC) than phosphine (-133 degreeC).

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. 
5c3 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?
5d3 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 in Figure 2 below.

Figure 2

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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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