Chemical energy from electrical energy ELECTROLYSIS Chemical energy from electrical energy

AREA OF STUDY 2: Supplying and using energy Key knowledge This knowledge includes • comparison of energy sources: types, uses and sustainability of sources including brown coal, natural gas, nuclear fission and biochemical fuels; • application of calorimetry to measure energy changes in chemical reactions in solution Calorimetry and bomb calorimetry; • use of the electrochemical series in predicting the products of redox reactions and writing half equations; • limitations of predictions made using the electrochemical series; • the construction and operation of simple galvanic primary and secondary cells; • the construction and operation of fuel cells: advantages and disadvantages of fuel cells compared to conventional energy sources; • the construction and operation of simple electrolytic cells: comparison of electrolytic cells; • application of Faraday’s laws in electrochemistry.

The results are hardly surprising: the more charge that passes through the cell, the more metal is formed at the cathode. Even if we perform this experiment in different silver-plating cells and vary conditions such as temperature, voltage, electrolyte concentration and current, the same graph is obtained as long as silver is the only product at the cathode.

These experimental results may be summarised by saying that the mass of metal produced at the cathode is directly proportional to the quantity of electricity passed through the cell. This relationship is known as Faraday’s fi rst law of electrolysis, after Michael Faraday who discovered it.

The second law of electrolysis

The second law of electrolysis

Competition at the electrodes An Oil Rig Cat Oxidation is loss of electrons and occurs at the Anode, this is the Reductant. Reduction occurs at the Cathode and is gain of electrons. The species that undergoes reduction in the oxidant. The highest substance on the left hand side of the electrochemical series is the oxidant, the highest reaction moves in the forward direction. Undergoing reduction as the species accepts electrons.

Remember the electrochemical series applies at 1M concentrations 25 degrees Celcius and 1atm The electrochemical series does not indicate anything about rate of reaction

Competition at electrodes Often just by knowing that oxidation occurs at the Anode and Reduction at the cathode is enough to predict the half reactions occurring in electrolysis. However during electrolysis there may be many chemicals present at the electrode and the most likely reaction must be determined. The electrodes in some instances may also react.

H2O(l) and Cu(s) likely to undergo oxidation H2O(l) and Cu(s) likely to undergo oxidation. The Strongest Reductant will tend to react. The Cu(s) is lower on the ES therefore more likely to react then water Nickel ions and water compete at the Cathode, let the strongest Oxidant win. Nickel ions are higher on the electrochemical series and will therefore have a higher tendency to undergo reduction.

Useful Chemicals from electrolysis