Electrochemistry Simple cells, formation of metal ions in varying degrees, electric potential, electroplating, electrolysis

Coins on a hand Place a few different coins on someone’s hand so that they are not touching one another. Using a digital voltmeter measure the voltage difference between pairs of coins. Try other metals. Which pair produces the largest voltage?

A nail in water Place a steel nail in a glass of water. Some iron ions (Fe++) are removed from the nail by water molecules. Each of these ions leaves two electrons behind on the nail. As the nail becomes more and more negatively charged it soon begins to attract the positive ions back. A steady state is quickly reached in which the nail is negative and the solution is positive and there is no further net reaction.

Copper wire in water Insert a piece of copper wire into the water. Some copper ions (Cu++) are removed from the wire by water molecules, each leaving two electrons behind. However this effect occurs to a smaller extent with copper than with iron. So the copper is negative but not as negative as the iron. Therefore there is a voltage difference between them. Relative to nail the copper wire is positive. This simple but marvellous effect was discovered in 1800 by Allesandro Volta.

Volta (1745–1827) Alessandro Volta made the first electric battery in 1800. He used a pile of Zn/brine/Ag cells. It was an immediate sensation. Until then there was no way to generate a steady flow of charge. The SI unit of electric potential, the volt (V), is named after him.

Activity series Place a tissue on a tile or plastic sheet and moisten it with salty water or dilute acid. Place a selection of metals on the tissue. Make sure they are not touching. Get a multimeter and set it to read up to 2 V. Hold the negative probe (black) on one of the pieces of metal and then touch the positive probe (red) off each of the others in turn. If any one of them gives a negative reading move the negative probe to it and start again. Record the final values and arrange the metals in the order of increasing voltage.

Gold and silver If you have some gold or silver jewellery you could add them to the selection of metals and check their place in the activity series. (Salt water or dilute acid will not damage them.) You could also try a piece of magnesium ribbon.

Electrolysis of water

Electrolysis apparatus H2 O2 - + + - dilute H2SO4

Electrolysis reactions If the solution contains H+ ions then they will migrate towards the negative electrode where each can pick up an electron and form a neutral hydrogen atom. Pairs of hydrogen atoms bond to form H2. Water molecules near the positive electrode can lose two hydrogen ions (H+) to the solution and two electrons to the electrode, releasing oxygen atoms. The oxygen atoms form O2, and sometimes O3 (ozone). The net effect is the decomposition of water.

Some metals react with acids

Activity series (again) When a piece of metal is placed in water some of its atoms go into solution as positive ions, leaving electrons on the metal. An equilibrium is soon established. If the voltage difference between the metal and the solution is high enough hydrogen ions (H+) may be discharged by it; each H+ ion picks up an electron to form a hydrogen atom. As a result more metal ions go into solution and the process continues.

Zinc and copper in acid solution Zinc reacts with dilute acid; bubbles of hydrogen quickly form on its surface. Copper does not react with dilute acid. However if the copper touches the zinc then bubbles will be seen on the copper also. If the reaction is allowed to continue the zinc metal eventually disappears but the copper is unaffected.

Zinc and copper in acid solution Zinc reacts with dilute acid; bubbles of hydrogen quickly form on its surface. Copper does not react with dilute acid. However if the copper touches the zinc then bubbles will be seen on the copper also. If the reaction is allowed to continue the zinc metal eventually disappears but the copper is unaffected.

Zinc/copper cell Alternatively the zinc and copper may be connected through an external circuit. This is effectively the same as the cells in Volta’s battery. mA

Displacement of metals and electroplating

Displacement of metals If a steel nail is dipped in a solution of copper ions (e.g. in 1M CuSO4) it is quickly coated with copper metal. You should now be able to explain why this happens? Note: If some H2SO4 and ethanol are added to the solution the copper plate will be less likely to fall off.

Electroplating By using an external voltage source it is possible to ‘electroplate’ items. The item to be plated should be negative. For copper plating use a copper anode (+) and a copper sulfate solution. (Note: reduce the voltage if bubbles appear.) + -

Copper electroplating solution Required: 30 g of copper sulphate crystals (bluestone) 5 ml of concentrated sulphuric acid* 12 ml of ethanol (or methylated spirit) Dissolve the above in water and dilute to a total volume of 200 cm3 with distilled water. A PURE copper anode (+) is also required. * NOTE: Add the acid to water slowly with stirring and cooling + -

Nickel electroplating solution Required: 25 g of nickel sulphate (green crystals)* 3g of ammonium chloride (white solid) 3g of boric acid (white solid) Dissolve the above in water and dilute to a total volume of 250 cm3 with distilled water. A PURE nickel anode (+) is also required. * NOTE: The nickel sulphate crystals are slow to dissolve. Objects which have been electroplated should be washed, dried and polished. + -

The terms ‘cathode’ and ‘anode’ In electrolysis or electroplating the cathode is the negative terminal. In the case of an electrochemical cell, such as a zinc/copper cell, the cathode is the positive terminal and the anode is the negative. Rationale… In all cases, cations (positive ions) in the electrolyte move away from the anode or towards the cathode. (Mnemonic: The conventional current carries cations towards the cathode.)