Manufacture of Sulfuric Acid
Category: Energy from Chemicals
Define enthalpy change.
Enthalpy change is the amount of energy involved in a reaction.
Write down the anode reaction, cathode reaction, and overall reaction of a hydrogen fuel cell.
Anode: 2H2 (g) + 4OH– (aq) –> 4H2O (l)+ 4e–
Cathode: O2(g) + 2H2O (l) + 4e– –> 4OH– (aq)
Overall: 2H2(g) + O2(g) –> 2H2O (l)
List 3 examples of exothermic reactions, and 3 examples of endothermic reactions.
Examples of exothermic reactions are:
– combustion of fuels
– rusting of iron
– neutralisation reaction
Examples of endothermic reactions are:
– photosynthesis
– thermal decomposition
– dissolving of some ionic compounds in water (eg NH4Cl)
This reaction, CH4 + 2O2 –> CO2 + 2H2O, is exothermic. Describe this exothermic reaction in terms of energy involved in bond breaking and bond making.
The heat energy absorbed to break the bonds in CH4 and 2O2 is less than the heat energy released to form the bonds in CO2 and 2H2O, so overall enthalpy change is negative.
What are the advantages and disadvantages of the hydrogen fuel cell?
Advantages:
1. The hydrogen fuel cell produces water as the only waste products. No harmful gases are released to surroundings when it is used.
2. Hydrogen is an abundant resource, if a cost effective method can be used to extract hydrogen from water.
Disadvantages:
1. There is currently no cost effective method of storing the hydrogen gas.
2. There is currently no cost effective method of extracting hydrogen from seawater.
3. The process to obtain hydrogen, either from electrolysis of water, or from cracking of hydrocarbons, produces waste gases and use up large amount of energy.