ASChemistryEdexcelExam Questions1. Physical Chemistry1.4 Bonding

Bonding (Edexcel AS Chemistry)

Exam Questions

35 mins4 questions
1a2 marks

This question is about covalent bonds.

State what is meant by the term covalent bond.

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1b2 marks

Draw a diagram of the ammonia molecule, clearly showing its shape.

Include any lone pairs of electrons and the value of the bond angle.

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1c1 mark

The dot-and-cross diagram of BF3 is

 
bf3-dot-and-cross
 

What is the bond angle in BF3? 

  ☐ A 90o
  ☐ B 107o
  ☐ C 109.5o
  ☐ D 120o

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1d4 marks
i)
Ammonia and boron trifluoride react to form a compound NH3BF3 which contains a dative covalent bond. Each of the molecules, NH3 and BF3, has a different feature of its electronic structure that allows this to happen.
 
Use these two different features to explain how a dative covalent bond is formed.
 (2)
 
ii)
During this reaction, the bond angles about the nitrogen atom and the boron atom change.
 
State the new H–N–H and F–B–F bond angles.
 (2)

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2a7 marks

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)
  ☐ A 104.5°
  ☐ B 107°
  ☐ C 109.5°
  ☐ D 120°

iv)
Explain the difference between the H–N–H bond angle in ammonia and in the amide ion.
(2)

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2b1 mark

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

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3a4 marks

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 = ................................

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3b3 marks

Hydrazine is very soluble in water.

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

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3c2 marks

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.

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4a7 marks

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)
  ☐ A Na (s) + F (g) → NaF (s)
  ☐ B 2Na (s) + F2 (g) → 2NaF (s)
  ☐ C Na (s) + F2 (g) → NaF2 (s)
  ☐ D 2Na (s) + F (g) → Na2F (s)
 
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)
screenshot-2022-11-22-223039
 
iv)
Explain your answer to (a)(iii) in terms of the structure of the ions.
 
(2)

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4b2 marks

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.

 
Compound

Amount of energy required to separate the ions / kJ mol−1
LiF 1031
KF 817
CaF2 2957
 

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

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