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  1. 1. 14. Electrochemical reactions Chemistry Grade 12 Everything Science www.everythingscience.co.za
  2. 2. 2Electrochemical reactionsAn electrochemical reaction is one where either a chemical reaction produces an externalvoltage, or where an external voltage causes a chemical reaction to take place. Everything Science www.everythingscience.co.za
  3. 3. 3Galvanic CellsIn a galvanic cell a chemical reaction produces a current in the external circuit.An example is the zinc-copper cell.A galvanic cell has a number of components. It consists of two electrodes, each of which isplaced in a separate beaker in an electrolyte solution. The two electrolytes are connected bya salt bridge. The electrodes are connected two each other by an external circuit wire. Everything Science www.everythingscience.co.za
  4. 4. 4Galvanic cell continuedOne of the electrodes is the anode, where oxidation takes place. The cathode is theelectrode where reduction takes place.In a galvanic cell, the build up of electrons at the anode sets up a potential differencebetween the two electrodes, and this causes a current to flow in the external circuit.A galvanic cell is therefore an electrochemical cell that uses a chemical reaction betweentwo dissimilar electrodes dipped in an electrolyte to generate an electric current.The standard notation for a galvanic cell such as the zinc-copper cell is as follows:where Zn∣Zn 2 ∥Cu 2 ∣Cu| = a phase boundary (solid/aqueous)|| = the salt bridgeThe galvanic cell is used in batteries and in electroplating. Everything Science www.everythingscience.co.za
  5. 5. 5Electrolytic cellAn electrolytic cell is an electro-chemical cell that uses electricityto drive a non-spontaneousreaction. In an electrolytic cell,electrolysis occurs, which is aprocess of separating elementsand compounds using an electriccurrent.One example of an electrolyticcell is the electrolysis of coppersulphate to produce copper andsulphate ions. Everything Science www.everythingscience.co.za
  6. 6. 6Standard hydrogen electrodeThe standard hydrogen electrode is aredox electrode which forms the basis ofthe scale of oxidation-reduction potentials.The actual electrode potential of thehydrogen electrode is estimated to be at 250°C, but its standardelectrode potential is said to be zero at all4.44±0.02 Vtemperatures so that it can be used as forcomparison with other electrodes. Thehydrogen electrode is based on thefollowing redox half cell:2 H  (aq)2 e −  H2 (g) Everything Science www.everythingscience.co.za
  7. 7. 7Reaction potentialsDifferent metals have different reaction potentials. The reaction potential of metals (inother words, their ability to ionise), is recorded in a standard table of electrode potential.The more negative the value, the greater the tendency of the metal to be oxidised. Themore positive the value, the greater the tendency of the metal to be reduced.The values on the standard table of electrode potentials are measured relative to thestandard hydrogen electrode. Everything Science www.everythingscience.co.za
  8. 8. 8Electromotive force (emf)The emf of a cell is defined as the maximum potential difference between two electrodesor half cells in a voltaic cell. Emf is the electrical driving force of the cell reaction. In otherwords, the higher the emf, the stronger the reaction.Standard emf is the emf of a voltaic cell operating under standard conditions(i.e. 100 kPa, concentration = 1 M and temperature = 298 K). The symbol ° denotesstandard conditions.The emf of a cell can be calculated using one of the following equations: Ecell =E0 −E0  0 right left 0 0 0 Ecell =Ereduction half reaction−Eoxidation half reaction 0 0 0 Ecell =Eoxidising agent−Ereducing agent Ecell=E0 0 0 cathode −Eanode It is possible to predict whether a reaction is spontaneous or not, either by looking at the sign of the cell’s emf or by comparing the electrode potentials of the two half cells. Everything Science www.everythingscience.co.za
  9. 9. 9Balancing redox equationsIt is possible to balance redox equations using the half-reactions that take place.The following method is used: Write down each half-reaction. Balance the number of atoms in each half-reaction. Water can be added to eitherside to balance hydrogens and oxygens. For a reaction in acid you can usehydrogen ions to balance the hydrogens and for a reaction in base you can usehydronium ions to balance the hydrogens. Check that the charges are balanced. Multiply each half-reaction by an appropriate amount to balance the electrons. Write the combined reaction in ionic form. Remember to cross out the electronsand any other common ions. Check that the reaction is balanced. Everything Science www.everythingscience.co.za
  10. 10. 10ApplicationsThere are a number of important applications of electrochemistry. These includeelectroplating, the production of chlorine and the extraction of aluminium. Everything Science www.everythingscience.co.za
  11. 11. 11For more practice or to ask an expert for help on thissection see:www.everythingscience.co.zaShortcode: ESCBH Everything Science www.everythingscience.co.za