8.1

Introduction A potential difference is set up between two half cells which is at a maximum when no current is flowing. This is called the cell potential. The e.m.f. of the cell can be measured using a high-resistance voltmeter. As no current is drawn by the voltmeter, each electrode is in equilibrium. Changing the concentration of the ions in one of the cells will alter the position of equilibrium.

• safety spectacles • voltmeter – range 0 – 1 V • 2 x wire and crocodile clip • copper strip (clean) • carbon rod • beaker (100cm3) • filter paper • scissors • 2 x measuring cylinder (50 cm3) • Emery paper

• Set up the apparatus as shown. Clean the metal strips by rubbing them with emery paper, rinsing in distilled water, and drying them. • Add 50 cm3 of the CuSO4(aq) to the 100 cm3 beaker which contains the copper electrode • In a second beaker put the carbon electrode in 50 cm3 of solution A (1:5 ratio) • Soak a strip of filter paper in saturated potassium nitrate(V) solution, and use it as a salt bridge to connect the solutions in the two beakers. • Measure the cell potential of the cell formed. • Repeat for solutions B–E. Use a fresh salt bridge for each experiment.

• Note the cell potential changes from solutions A to E. Use Le Chateliers Principle to explain the trend in terms of the equilibria involved. • Given the electrode potential of the copper/copper (II) half cell is +0.34V, use your result from beaker C (1:1) to calculate the standard electrode potential for Fe2+/Fe3+. • What is the ionic equation for each half cell? • Write the overall cell reaction for the cell.

No hazard classification at this concentration

Harmful – only handle with forceps.

Harmful

No hazard classification at this concentration

• The salt bridge connects the two halves of the electrochemical cell. It allows the electrons to flow and complete the circuit while keeping the solutions separate.