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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 for the formation of hydrogen in stage 1, we need to determine the total mass of the useful product (hydrogen) produced compared to the total mass of the reactants. The balanced equation for the reaction is:
The molar mass of the reactants:
- Methane, CH₄: 12 (C) + 4 (H) = 16 g/mol
- Water, H₂O: 2 (H) + 16 (O) = 18 g/mol
Total mass of reactants = 16 + 18 = 34 g/mol.
The useful product is hydrogen, with a molar mass calculated from the 3 moles of H₂ produced:
- Hydrogen: 2 (H) x 3 = 6 g/mol.
The atom economy is calculated as:
Thus, the atom economy for the formation of hydrogen in stage 1 is approximately 17.65%.
Step 2
Explain why a low pressure is used in stage 1.
Answer
In stage 1 of the Haber process, a low pressure is used to favor the forward reaction where methane and steam form carbon monoxide and hydrogen. This is because at lower pressures, there are fewer moles of gas on the left-hand side (1 mole of CH₄ and 1 mole of H₂O) compared to the right-hand side (1 mole of CO and 3 moles of H₂), leading to a total of 4 moles on the product side. According to Le Chatelier's principle, reducing the pressure shifts the equilibrium towards the side with more moles of gas. Therefore, a low pressure increases the yield of hydrogen produced.
Step 3
What is the effect of increasing the pressure on the equilibrium yield of hydrogen in stage 2?
Answer
Increasing the pressure in stage 2 of the reaction involving carbon monoxide will have no effect on the yield of hydrogen. This is because the equilibrium position is unaffected by pressure when there is an equal number of moles of gas on both sides of the equilibrium equation. In the reaction:
there are 2 moles of gas on the left-hand side (1 mole of CO and 1 mole of H₂O) and 2 moles of gas on the right-hand side (1 mole of CO₂ and 1 mole of H₂). Since both sides have the same number of moles, increasing the pressure has no effect on the yield of hydrogen in this stage.
Step 4
Determine how many times greater the percentage yield of ammonia obtained would be.
Answer
From Figure 10, we note the following yields:
- At 350 °C and 285 atmospheres, the yield is approximately 63%.
- At 450 °C and 200 atmospheres, the yield is approximately 62%.
To find how many times greater the yield is at 350 °C and 285 atmospheres compared to 450 °C and 200 atmospheres, we calculate:
Therefore, the yield at 350 °C and 285 atmospheres is approximately 1.02 times greater than the yield at 450 °C and 200 atmospheres.