Bonding (Edexcel A Level Chemistry): Exam Questions

Ammonia reacts with sodium to form sodium amide, NaNH₂ , and hydrogen.

i) Write the equation for this reaction. State symbols are not required.

(1)

ii) Draw diagrams showing the 3-dimensional shape of an ammonia molecule and of an amide ion, NH₂^–. Include any lone pairs of electrons in each species.

(3)

            ammonia molecule                                  amide ion

iii) What is the H–N–H bond angle in an ammonia molecule?

(1)

iv) Explain the difference between the H–N–H bond angle in ammonia and in the amide ion.

(2)

Give a possible reason why samples of sodium amide are stored in oil.

Nitrogen forms several hydrides. In addition to ammonia, NH₃ , it forms hydrazine, N₂H₄ , in which the two nitrogen atoms are covalently bonded together.

i) Explain what is meant by a covalent bond.

(2)

ii) Draw a dot-and-cross diagram for hydrazine, showing the outer electrons only.

Use crosses (×) to represent the electrons from nitrogen and dots (•) to represent the electrons from hydrogen.

(1)

iii) Estimate the H––N––H bond angle in hydrazine.

(1) Bond angle = ................................

Hydrazine is very soluble in water.

Explain, using a labelled diagram and naming the relevant intermolecular interactions, why hydrazine is very soluble in water.

Hydrazine has been used as a rocket fuel.

It is a powerful reducing agent and will react very exothermically with oxidising agents such as hydrogen peroxide.

The equation for the reaction of hydrazine with hydrogen peroxide is

N₂H₄(l) + 2H₂O₂(l) → N₂(g) + 4H₂O(g)

Give two reasons why hydrazine is a good rocket fuel when reacted with hydrogen peroxide.

This question is about ionic bonding.

The elements sodium and fluorine react together to form an ionic compound.

 i) Select the correct equation for this reaction.

(1)

ii) Draw dot-and-cross diagrams of the ions in sodium fluoride, showing all the electrons. 

Use your diagram to explain why the ions are described as isoelectronic.

(3)

iii) Which diagram shows the trend in ionic radius for the isoelectronic ions N³⁻ to Al³⁺? 

(1)

iv) Explain your answer to (a)(iii) in terms of the structure of the ions. 

(2)

The strength of ionic bonding in different compounds can be compared by using the amount of energy required to separate the ions. Some values for this energy are given in the table.

 Using the data provided, explain how changes in the cation affect the bond strength in an ionic compound.