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 | 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. (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.) 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. 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. Deduce the change in oxidation state of nitrogen, when NH₃ is oxidised to NO. When ammonia reacts with oxygen, nitrous oxide (N₂O) can be produced instead of NO. Give an equation for this reaction.

Photo AI

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.

96%

114 rated

Answer

To calculate the entropy change (ΔS) for the reaction, we use the formula:

$ΔS = S°_{products} - S°_{reactants}$

Applying this formula:

$ΔS = [4 imes S°(NO) + 6 imes S°(H_2O)] - [4 imes S°(NH_3) + 5 imes S°(O_2)]$

Substituting the values:

$ΔS = [4 imes 211 + 6 imes 189] - [4 imes 193 + 5 imes 205]$

Calculating each part:

Products: $4 imes 211 + 6 imes 189 = 844 + 1134 = 1978$
Reactants: $4 imes 193 + 5 imes 205 = 772 + 1025 = 1797$

Thus,

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

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.

99%

104 rated

Answer

To find the Gibbs free energy change (ΔG), we use the formula:

$ΔG = ΔH - TΔS$

Where:

ΔH = -905 kJ mol⁻¹ (given)
T = 600 °C = 873 K (convert to Kelvin)
ΔS = 181 J K⁻¹ mol⁻¹ × (1 kJ / 1000 J) = 0.181 kJ K⁻¹ mol⁻¹

Now substituting values:

$ΔG = -905 - (873 imes 0.181)$

Calculating:

$ΔG = -905 - 158.453 = -1063.453 ext{ kJ mol}^{-1}$

Thus,

$ΔG ≈ -1063.5 ext{ kJ mol}^{-1}$

Step 3

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

96%

101 rated

Answer

As the temperature increases, the term $TΔS$ in the Gibbs free energy equation becomes larger, meaning that the value of $ΔG$ becomes more negative. A more negative value of $ΔG$ indicates a more favorable reaction, potentially leading to an increased reaction rate or extent.

Step 4

Describe the stages of this alternative route.

98%

120 rated

Answer

Adsorption: Reactants (NH₃ and O₂) are absorbed onto the active sites of the platinum catalyst surface.
Reaction: The catalyst provides an alternative pathway with lower activation energy, allowing the reaction to occur more readily.
Desorption: Products (NO and H₂O) are released from the catalyst surface, freeing the active sites for further reactions.

Step 5

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

97%

117 rated

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:

$ΔOxidation~State = +2 - (-3) = +5$

This indicates an increase in oxidation state by 5.

Step 6

Give an equation for this reaction.

97%

121 rated

Answer

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

ightarrow N₂O(g) + 2H₂O(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.

Join the A-Level students using SimpleStudy...

Other A-Level Chemistry topics to explore