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
Question 1
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 ... show full transcript
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
Step 1
Define the term standard enthalpy change of formation, ΔHf⦵:
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Answer
The standard enthalpy change of formation, ΔHf⦵, is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Step 2
Use the data in the table to calculate the standard enthalpy change of this reaction.
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Answer
To determine the standard enthalpy change for the reaction:
ΔHf=ΔHf(products)−ΔHf(reactants)
Substituting the values:
ΔH∘=[2(−241.8)]−[50.6]=−534.2 kJ mol−1
Step 3
Suggest a reason for this.
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Answer
The activation energy (E_a) is too high for the reaction to occur spontaneously under standard conditions despite being exothermic.
Step 4
Suggest why using hydrazine as a rocket fuel could be regarded as being ‘environmentally friendly’.
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Answer
The products of the reaction are water (H2O) and nitrogen (N2), both of which are non-toxic and harmless, and are already present in the atmosphere.
Step 5
Showing outer-shell electrons only, draw a ‘dot-and-cross’ diagram of an ammonia molecule.
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Answer
The dot-and-cross diagram for ammonia (NH3) shows the nitrogen atom with one lone pair and three hydrogen atoms bonded to it, each represented with dots for the nitrogen and crosses for the hydrogens.
Step 6
Draw a diagram to show the three-dimensional shape of an ammonia molecule.
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Answer
The three-dimensional shape of an ammonia molecule is trigonal pyramidal, with the nitrogen atom at the apex and the hydrogen atoms at the base.
Step 7
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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Answer
The hydrazine molecule (N2H4) has a staggered formation with each nitrogen atom bonded to two hydrogen atoms. The bond angle around the nitrogen atom is approximately 109°.
