Hydrogen for fuel-cell buses can be made in two ways.
Method A is steam reforming of methane. The overall equation is: CH4(g)+2H2O(g)→CO2(g)+4H2(g)\mathrm{CH_{4}(g)} + \mathrm{2H_{2}O(g)} \rightarrow \mathrm{CO_{2}(g)} + \mathrm{4H_{2}(g)}CH4(g)+2H2O(g)→CO2(g)+4H2(g)
Method B is the electrolysis of water: 2H2O(l)→2H2(g)+O2(g)\mathrm{2H_{2}O(l)} \rightarrow \mathrm{2H_{2}(g)} + \mathrm{O_{2}(g)}2H2O(l)→2H2(g)+O2(g)
The table gives information about the two methods.
| Feature | Method A | Method B |
|---|---|---|
| raw materials | methane from natural gas (finite) and water | water only |
| conditions | 800 °C, 30 atm, nickel catalyst | room temperature, electricity |
| energy source | burning natural gas | electricity, which can come from wind or solar power |
| rate and scale | very fast, very large scale | slower, smaller scale |
| reversible steps | yes, unreacted methane is recycled | no |
| by-product | carbon dioxide | oxygen, which can be sold |
| cost of hydrogen per kg | £1.50 | £5.00 |
Relative formula masses: H2=2,O2=32,CO2=44\mathrm{H_{2}} = 2, \mathrm{O_{2}} = 32, \mathrm{CO_{2}} = 44H2=2,O2=32,CO2=44
Calculate the atom economy of method A for making hydrogen.
Calculate the atom economy of method B for making hydrogen.
Suggest why the atom economy of method B makes it seem less useful than it really is.
Evaluate the two methods for producing hydrogen for fuel-cell buses. Use the information in the table and your answers to (a) and (b).
151 exam-style questions on Edexcel GCSE Chemistry 6.2 Quantitative analysis, covering 6.2.1 Concentration in mol dm⁻³ and titration calculations, 6.2.2 Percentage yield and why actual yield falls short, 6.2.3 Atom economy of a reaction, 6.2.4 Choosing a reaction pathway, and 6.2.5 Molar volume of gases and Avogadro's law. Each one has a worked solution and a mark scheme showing where the marks go.