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

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).

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

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

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

BCl₃ + Cl^- → BCl₄^-

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.

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

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

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).

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

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. 

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

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 or 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

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. 

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

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

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.

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

178

180

118

105

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

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