The equation for the reaction between ammonia and oxygen is shown - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 1
Question 3
The equation for the reaction between ammonia and oxygen is shown.
4NH₃(g) + 5O₂(g) ⇌ 4NO(g) + 6H₂O(g)
ΔH = -905 kJ mol⁻¹
Some standard entropies are given in Tabl... show full transcript
Worked Solution & Example Answer:The equation for the reaction between ammonia and oxygen is shown - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 1
Step 1
Calculate the entropy change for the reaction between ammonia and oxygen.
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Answer
To calculate the entropy change (
ΔS
) for the reaction, we use the formula:
ΔS=S°products−S°reactants
For the given reaction:
Products: 4NO(g) + 6H₂O(g)
Reactants: 4NH₃(g) + 5O₂(g)
The standard entropies are:
S°(NO) = 211 J K⁻¹ mol⁻¹
S°(H₂O) = 189 J K⁻¹ mol⁻¹
S°(NH₃) = 193 J K⁻¹ mol⁻¹
S°(O₂) = 205 J K⁻¹ mol⁻¹
Calculating:
ΔS = egin{align*}
& ext{(4 x 211) + (6 x 189)} \
& - ext{(4 x 193) + (5 x 205)}
d \
& = 1978 - 1797 \
& = 181 ext{ J K}^{-1} ext{ mol}^{-1}
egin{align*}$$
Step 2
Calculate a value for the Gibbs free-energy change (ΔG), in kJ mol⁻¹, for the reaction between ammonia and oxygen at 600 °C.
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Explain how this change affects the value of ΔG for the reaction.
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Answer
As the temperature increases, the term
TΔS
grows larger, affecting ΔG. Since
ΔG = ΔH - TΔS
, an increase in temperature may result in a decrease in ΔG, making the reaction more thermodynamically favorable. In summary, ΔG becomes more negative as temperature increases, especially if
ΔS
greater than zero.
Step 4
Describe the stages of this alternative route.
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Answer
Adsorption: Reactants (NH₃ and O₂) are adsorbed onto the platinum surface.
Bond Breaking: Bonds between the nitrogen and hydrogen atoms in NH₃ and the O=O bond in O₂ begin to break.
Reaction: The remaining atoms reorganize on the catalyst surface to form products (NO and H₂O).
Desorption: Finally, the products (NO and H₂O) desorb from the surface, allowing the catalyst to be reused.
Step 5
Deduce the change in oxidation state of nitrogen, when NH₃ is oxidised to NO.
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Answer
In ammonia (NH₃), nitrogen has an oxidation state of -3. In nitrous oxide (NO), nitrogen has an oxidation state of +2. Therefore, the change in oxidation state is +5.
Step 6
Give an equation for this reaction.
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Answer
The balanced equation when ammonia reacts with oxygen to produce nitrous oxide (N₂O) is:
