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This question is about reversible reactions and equilibrium - AQA - GCSE Chemistry - Question 10 - 2019 - Paper 2
Question 10
This question is about reversible reactions and equilibrium. Hydrogen is used to produce ammonia in the Haber process. The hydrogen is made in two stages. Stage 1... show full transcript
Worked Solution & Example Answer:This question is about reversible reactions and equilibrium - AQA - GCSE Chemistry - Question 10 - 2019 - Paper 2
Step 1
Calculate the atom economy for the formation of hydrogen in stage 1.
Answer
To calculate the atom economy, we use the formula:
In this case, the desired product is hydrogen (H₂), and the reactants are methane (CH₄) and water (H₂O).
- Molar mass of H₂ = 2 \times 1 = 2 g/mol
- Molar mass of reactants = 12 + 2 + 16 = 30 g/mol.
Thus, the atom economy is:
Therefore, the atom economy for the formation of hydrogen in stage 1 is 6.67%.
Step 2
Explain why a low pressure is used in stage 1.
Answer
A low pressure is used in stage 1 because it favors the formation of products when there are fewer moles of gas on the right-hand side of the equation.
The equation shows that 1 mole of methane and 1 mole of water (total of 2 moles) react to produce 1 mole of carbon monoxide and 3 moles of hydrogen (total of 4 moles). Hence, increasing pressure would not favor the forward reaction, thereby maximizing yield.
Step 3
What is the effect of increasing the pressure on the equilibrium yield of hydrogen in stage 2?
Answer
Increasing the pressure will have no effect on the yield of hydrogen in stage 2 because the reaction involves the same number of moles of gas on both sides.
Both the reactants (1 mole of CO and 1 mole of H₂O) and products (1 mole of CO₂ and 1 mole of H₂) contain 2 moles of gas. Therefore, the position of equilibrium is unaffected by changes in pressure.
Step 4
Determine how many times greater the percentage yield of ammonia obtained would be.
Answer
From Figure 10, we observe that at a temperature of 350 °C and a pressure of 285 atmospheres, the percentage yield of ammonia is approximately 63%.
At 450 °C and 200 atmospheres, the yield is approximately 62%.
To find how many times greater the yield is, we calculate: