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 Table 3. Table 3 Gas S° / J K⁻¹ mol⁻¹ NH₃(g) 193 O₂(g) 205 NO(g) 211 H₂O(g) 189 1. Calculate the entropy change for the reaction between ammonia and oxygen. 2 marks 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 (If you were unable to obtain an answer to Question 03.1, you should assume that the entropy change is 211 J K⁻¹ mol⁻¹. This is not the correct answer.) 2 marks 3. The reaction between ammonia and oxygen was carried out at a higher temperature. Explain how this change affects the value of ΔG for the reaction. 2 marks 4. Platinum acts as a heterogeneous catalyst in the reaction between ammonia and oxygen. It provides an alternative reaction route with a lower activation energy. Describe the stages of this alternative route. 3 marks 5. Deduce the change in oxidation state of nitrogen, when NH₃ is oxidised to NO. 1 mark 6. When ammonia reacts with oxygen, nitrous oxide (N₂O) can be produced instead of NO. Give an equation for this reaction. 1 mark

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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 will use the formula:

$ΔS = S°_{products} - S°_{reactants}$

Here, the standard entropies from the table are:

Products: 4 moles of NO and 6 moles of H₂O
Reactants: 4 moles of NH₃ and 5 moles of O₂

Calculating for products:

$S°_{products} = 4 imes S°_{NO} + 6 imes S°_{H₂O} = 4(211) + 6(189) = 844 + 1134 = 1978 \, J K^{-1} mol^{-1}$

Calculating for reactants:

$S°_{reactants} = 4 imes S°_{NH₃} + 5 imes S°_{O₂} = 4(193) + 5(205) = 772 + 1025 = 1797 \, J K^{-1} mol^{-1}$

Therefore, the change in entropy is:

$ΔS = 1978 - 1797 = 181 \, J K^{-1} mol^{-1}$

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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Answer

We use the equation: $ΔG = ΔH - TΔS$ Where:

ΔH = -905 kJ mol⁻¹
Temperature (T) = 600 °C = 873 K
We assume the entropy change (ΔS) = 211 J K⁻¹ mol⁻¹ = 0.211 kJ K⁻¹ mol⁻¹

Now substituting values: $ΔG = -905 - (873)(0.211)$ Calculating: $ΔG ≈ -905 - 184.453 = -1089.453 \, kJ mol^{-1}$ Thus, the Gibbs free-energy change is approximately -1089.5 kJ mol⁻¹.

Step 3

Explain how this change affects the value of ΔG for the reaction.

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Answer

As the temperature increases, the term $TΔS$ becomes larger. If ΔS is positive, this results in a larger negative contribution to ΔG, making ΔG more negative. In contrast, if ΔS is negative, the value of ΔG would become less negative or even positive at higher temperatures, potentially making the reaction less favorable.

Step 4

Describe the stages of this alternative route.

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Answer

Adsorption: The reactants (NH₃ and O₂) are adsorbed onto the platinum surface.
Bond Breaking: The bonds within the reactant molecules are broken, weakening intermolecular interactions.
New Bond Formation: New bonds are formed between the reactants to create products (NO and H₂O).
Desorption: The products are released from the catalyst surface, completing the reaction cycle.

Step 5

Deduce the change in oxidation state of nitrogen, when NH₃ is oxidised to NO.

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Answer

In NH₃, nitrogen has an oxidation state of -3. In NO, nitrogen has an oxidation state of +2. Therefore, the change in oxidation state is: $+2 - (-3) = +2 + 3 = +5$ Thus, the change in oxidation state is +5.

Step 6

Give an equation for this reaction.

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Answer

The equation for the production of nitrous oxide (N₂O) from ammonia and oxygen is:

ightarrow N₂O(g) + 3H₂O(g) $$

The equation for the reaction between ammonia and oxygen is shown - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 1

Worked Solution & Example Answer:The equation for the reaction between ammonia and oxygen is shown - AQA - A-Level Chemistry - Question 3 - 2018 - Paper 1

Calculate the entropy change for the reaction between ammonia and oxygen.

Calculate a value for the Gibbs free-energy change (ΔG), in kJ mol⁻¹, for the reaction between ammonia and oxygen at 600 °C.

Explain how this change affects the value of ΔG for the reaction.

Describe the stages of this alternative route.

Deduce the change in oxidation state of nitrogen, when NH₃ is oxidised to NO.

Give an equation for this reaction.

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