Molecules: Shapes & Forces (AQA A Level Chemistry)

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

Figure 1

Explain why each bond angle in BH₃ is 120°.

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

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

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.

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

Table 1

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.

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

Figure 1

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

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.

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.

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.

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

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.

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.

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

Figure 1

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

Explain which of the following boron compounds is not planar.

• B(OH)₃

• BF₄^-

• BH₃

Explain which of the following boron compounds has the highest boiling point.

• B(OH)₃

• BF₄^-

• BH₃

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

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

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

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. 

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?

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

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.

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

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

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

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. 

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.

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.

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

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

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

State and explain which of the following molecules is the most polar:

• Boron trifluoride, BF₃

• Chlorine trifluoride, ClF₃

• Sulfur hexafluoride SF₆

• Carbon tetrafluoride, CF₄.

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

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

Table 1

i) Complete Table 1.

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

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

Table 2

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. 

Determine the order of the following compounds based on increasing boiling point. Justify your answer with reference to intermolecular forces.

• Ethanol, CH₃CH₂OH

• Propane, CH₃CH₂CH₃

• Dimethyl ether, CH₃OCH₃

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. 

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

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.

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.

Xenon difluoride, XeF₂, can be used as an antiseptic.

State the expected bond angle for a molecule of XeF₂.

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

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.

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