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Methanol (CH₃OH) is an important chemical in industry - Scottish Highers Chemistry - Question 8 - 2016
Question 8
Methanol (CH₃OH) is an important chemical in industry. (a) Methanol is produced from methane in a two-step process. In step 1, methane is reacted with steam as sh... show full transcript
Worked Solution & Example Answer:Methanol (CH₃OH) is an important chemical in industry - Scottish Highers Chemistry - Question 8 - 2016
Step 1
Step 1: Complete the table to show the most favourable conditions to maximise the yield for step 1.
Answer
For the first step, the reaction is endothermic as indicated by the positive ΔH value (+210 kJ mol⁻¹). To favour the forward reaction, high temperature is needed. Since the reaction produces more molecules of gas (3 moles of H₂ and 1 mole of CO from 1 mole of CH₄ and 1 mole of H₂O), low pressure is favoured to enhance yield.
Therefore:
- Temperature: High
- Pressure: Low
Step 2
Step 2: Complete the table to show the most favourable conditions to maximise the yield for step 2.
Answer
The second step is exothermic, indicated by the negative ΔH value (-91 kJ mol⁻¹). To favour the forward reaction under these conditions, low temperature is preferred. Additionally, since the reaction results in fewer gas molecules (2 moles of reactants to 1 mole of product), high pressure is desirable to maximise the yield.
Thus:
- Temperature: Low
- Pressure: High
Step 3
Step 4
(b)(ii) The atom economy of this reaction is 100%. Explain what this means.
Answer
An atom economy of 100% means that all the atoms of the reactants are converted into the desired product without generating any by-products. In other words, the mass of the desired product is equal to the total mass of the reactants used, ensuring maximum efficiency in material use.
Step 5
(c) Using bond enthalpy and mean bond enthalpy values from the data booklet, calculate the enthalpy change, in kJ mol⁻¹, for the reaction.
Answer
To calculate the enthalpy change, we need to account for bond breaking and bond forming.
The total bond breaking enthalpies are:
- C−H: 4 bonds (412 kJ/mol each)
- C−C: 1 bond (346 kJ/mol)
- C=O: 1 bond (743 kJ/mol)
- H−H: 1 bond (436 kJ/mol)
Total for bond breaking:
The bond formation enthalpies are:
- C=O: 1 bond (743 kJ/mol)
- H−H: 2 bonds (436 kJ/mol each)
Total for bond formation:
Thus, the enthalpy change is: