A-Level CIE Chemistry 1.5 Molecules: Shapes & Forces: 1. Hydrazine, N2H4, can be used

Question

1. Hydrazine, N2H4, can be used as a rocket fuel and is stored as a liquid. It reacts exothermically with oxygen to give only gaseous products.

The enthalpy change of a reaction such as that between hydrazine and oxygen may be calculated by using standard enthalpy changes of formation.

(a) Define the term standard enthalpy change of formation, ΔHf⁰:

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(b) Hydrazine reacts with oxygen according to the following equation.

N2H4(l) + O2(g) → N2(g) + 2H2O(g)

(i) Use the data in the table to calculate the standard enthalpy change of this reaction.

compound         ΔHf⁰/kJ mol⁻¹
N2H4(l)         50.6
H2O(g)       −241.8

ΔH⁰ = .................................................. kJ mol⁻¹

(ii) Although the above reaction is highly exothermic, hydrazine does not burn spontaneously in oxygen.
Suggest a reason for this.

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(iii) Suggest why using hydrazine as a rocket fuel could be regarded as being ‘environmentally friendly’.

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(c) The bonding in hydrazine is similar to that in ammonia.

(i) Showing outer-shell electrons only, draw a ‘dot-and-cross’ diagram of an ammonia molecule.

(ii) Draw a diagram to show the three-dimensional shape of an ammonia molecule.

(iii) Draw a diagram to show the shape of a hydrazine molecule.
Show clearly which atom is joined to which and show clearly the value of one bond angle.

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1. Hydrazine, N2H4, can be used as a rocket fuel and is stored as a liquid - CIE - A-Level Chemistry - Question 1 - 2010 - Paper 1

Worked Solution & Example Answer:1. Hydrazine, N2H4, can be used as a rocket fuel and is stored as a liquid - CIE - A-Level Chemistry - Question 1 - 2010 - Paper 1

Define the term standard enthalpy change of formation, ΔHf⁰:

Use the data in the table to calculate the standard enthalpy change of this reaction.

Suggest a reason for this.

Suggest why using hydrazine as a rocket fuel could be regarded as being ‘environmentally friendly’.

Showing outer-shell electrons only, draw a ‘dot-and-cross’ diagram of an ammonia molecule.

Draw a diagram to show the three-dimensional shape of an ammonia molecule.

Draw a diagram to show the shape of a hydrazine molecule.

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