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

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

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 BH_3.

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 the table below. 

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.

Using the table, 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. 

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

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. 

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.  

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.  

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.

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 below.

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

Three examples of boron compounds are B(OH)₃, BF₄^- and BH₃. State which of these compounds is not planar and explain your answer. 

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

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

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 ^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. 

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 below.

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.

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.

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

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

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

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.

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

Tetrafluorohydrazine is a polar molecule but tetrafluoroethene is not.

Explain the difference in molecular polarity.

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.

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

These longer primary alcohols have the following melting points: 

Describe and explain this trend.

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

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.

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ii) Put them in order of increasing volatility and explain your answer.

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

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

i) Draw skeletal formula for each isomer shown above.

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ii) Predict and explain which isomer of pentane would have the greater volatility.

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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.

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

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

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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.

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

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

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Iodine can also form the triiodide ion, I₃^-. 

i) Draw the dot-and-cross diagram for I₃^-.

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ii) Use electron pair repulsion theory to deduce the bond angles in I₃^-.

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iii) Describe and explain the position of the lone pairs on the central iodine.

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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.

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ii) Explain why iodine vaporises easily.

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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.