The galvanic cell

Introduction

Reaction Types in Grade 11 discussed oxidation , reduction and redox reactions . Oxidation involves a loss of electrons and reduction involves a gain of electrons . A redox reaction is a reaction where both oxidation and reduction take place. What is common to all of these processes is that they involve a transfer of electrons and a change in the oxidation state of the elements that are involved.

Oxidation and reduction

1. Define the terms oxidation and reduction .
2. In each of the following reactions say whether the iron in the reactants is oxidised or reduced.
   1. $Fe\to F{e}^{2+}+2e^{-}$
   2. $F{e}^{3+}+e^{-}\to F{e}^{2+}$
   3. $F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$
   4. $F{e}^{2+}\to F{e}^{3+}+e^{-}$
   5. $F{e}_2{O}_3+2Al\to A{l}_2{O}_3+2Fe$
3. In each of the following equations, say which elements in the reactants are oxidised and which are reduced.
   1. $CuO\left(s\right)+H_2\left(g\right)\to Cu\left(s\right)+H_{2}O\left(g\right)$
   2. $2NO\left(g\right)+2CO\left(g\right)\to N_2\left(g\right)+2C{O}_2\left(g\right)$
   3. $Mg\left(s\right)+FeS{O}_4\left(aq\right)\to MgS{O}_4\left(aq\right)+Fe\left(s\right)$
   4. $Zn\left(s\right)+2AgN{O}_3\left(aq\right)\to 2Ag+Zn{\left(N{O}_3\right)}_2\left(aq\right)$
4. Which one of the substances listed below acts as the oxidising agent in the following reaction? $3S{O}_2+C{r}_2{O}_7^{2-}+2H^{+}\to 3S{O}_4^{2-}+2C{r}^{3+}+H_{2}O$
   1. H ${}^{+}$
   2. Cr ${}^{3+}$
   3. SO ${}_2$
   4. Cr ${}_2$ O ${}_7^{2-}$

In Grade 11, an experiment was carried out to see what happened when zinc granules are added to a solution of copper(II) sulphate. In the experiment, the Cu ${}^{2+}$ ions from the copper(II) sulphate solution were reduced to copper metal, which was then deposited in a layer on the zinc granules. The zinc atoms were oxidised to form Zn ${}^{2+}$ ions in the solution. The half reactions are as follows:

$C{u}^{2+}\left(aq\right)+2e^{-}\to Cu\left(s\right)$ (reduction half reaction)

$Zn\left(s\right)\to Z{n}^{2+}\left(aq\right)+2e^{-}$ (oxidation half reaction)

The overall redox reaction is:

$C{u}^{2+}\left(aq\right)+Zn\to Cu\left(s\right)+Z{n}^{2+}\left(aq\right)$

There was an increase in the temperature of the reaction when you carried out this experiment. Is it possible that this heat energy could be converted into electrical energy? In other words, can we use a chemical reaction where there is an exchange of electrons, to produce electricity? And if this is possible, what would happen if an electrical current was supplied to cause some type of chemical reaction to take place?

An electrochemical reaction is a chemical reaction that produces a voltage , and therefore a flow of electrical current. An electrochemical reaction can also be the reverse of this process, in other words if an electrical current causes a chemical reaction to take place.

Electrochemical reaction

If a chemical reaction is caused by an external voltage, or if a voltage is caused by a chemical reaction, it is an electrochemical reaction.

Electrochemistry is the branch of chemistry that studies these electrochemical reactions. In this chapter, we will be looking more closely at different types of electrochemical reactions, and how these can be used in different ways.

The galvanic cell

Experiment : electrochemical reactions

Aim:

To investigate the reactions that take place in a zinc-copper cell

Apparatus:

zinc plate, copper plate, measuring balance, zinc sulphate (ZnSO ${}_4$ ) solution (1 mol.dm ${}^{-3}$ ), copper sulphate (CuSO ${}_4$ ) solution (1 mol.dm ${}^{-3}$ ), two 250 ml beakers, U-tube, Na ${}_2$ SO ${}_4$ solution, cotton wool, ammeter, connecting wire.

Method:

1. Measure the mass of the copper and zinc plates and record your findings.
2. Pour about 200 ml of the zinc sulphate solution into a beaker and put the zinc plate into it.
3. Pour about 200 ml of the copper sulphate solution into the second beaker and place the copper plate into it.
4. Fill the U-tube with the Na ${}_2$ SO ${}_4$ solution and seal the ends of the tubes with the cotton wool. This will stop the solution from flowing out when the U-tube is turned upside down.
5. Connect the zinc and copper plates to the ammeter and observe whether the ammeter records a reading.
6. Place the U-tube so that one end is in the copper sulphate solution and the other end is in the zinc sulphate solution. Is there a reading on the ammeter? In which direction is the current flowing?
7. Take the ammeter away and connect the copper and zinc plates to each other directly using copper wire. Leave to stand for about one day.
8. After a day, remove the two plates and rinse them first with distilled water, then with alcohol and finally with ether. Dry the plates using a hair dryer.
9. Weigh the zinc and copper plates and record their mass. Has the mass of the plates changed from the original measurements?