Electrochemistry
The study of the relationships between chemical reactions and electricity is known as electrochemistry. There are two basic types of processes involved in this area of study:
1. Electrochemical cells – these generate electric current by using spontaneous chemical reactions. Batteries are this type of cell.
2. Electrolytic cells – these use electricity to cause non-spontaneous chemical reactions to proceed. This process is known as electrolysis and is often used in the refining of metals.
Review of some electrical terms
3 common terms to be familiar with:
1. Voltage (volts, V) – also known as potential difference, it is the work done in moving a unit charge from one point to another in joules per coulomb (known as volts). Or it is the difference in the electrical potential between two points in a circuit or electrical field. Voltage is also referred to as e.m.f. or electromotive force or the driving force of a cell that is ‘pushing’ electrons around the circuit.
2. Current (amperes, A; symbol: I) – this is known as the movement or flow of electrical charge. More specifically, the current is the amount of charge that passes a certain point in a circuit per unit time or coulombs per second (known as amps).
3. Resistance (ohms, Ω ; symbol: R) – in electrical circuits it refers to the ability of a material to oppose the flow of current in a circuit. Resistance depends upon the nature of the material, its dimensions (the greater the length of wire the greater the resistance, the greater the diameter of wire the less the resistance) and temperature (the greater the temperature, the greater the resistance due to greater movement of atoms).
Relationship between V, I and R (Ohm’s Law)
Georg Simon Ohm (1787-1854) experimentally determined that current and voltage are proportional (i.e. as voltage increases, current increases)
current is proportional to voltage
Current is also affected by the resistance of the material in which it is flowing. As the resistance increases, the current decreases or current is inversely proportional to resistance.
current is inversely proportional to resistance
Combining these two relationships, we have:
I = V/R
or as Ohm’s law is normally written:
Oxidation-reduction revisited
Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons from one substance to another. Specifically, one substance loses electrons (is oxidised) and one substance gains electrons (is reduced). An easy way to remember this is OIL RIG:
Oxidation
Is
Loss (of electrons)
Reduction
Is
Gain (of electrons)
It is important to understand that both processes happen simultaneously. However, we often consider the two processes separately as two half-reactions. Consider the following reaction:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
This can be rewritten to the net ionic equation below:
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
Notice chlorine has been removed from the ionic equation as it appears in both sides of the equation and is therefore known as a spectator ion. We can now write this as two separate half-equations, one for the oxidation and one for the reduction.
The oxidation:
Zn(s) → Zn2+(aq) + 2e-
The reduction:
2H+(aq) + 2e- → H2(g)
The electrochemical cell (aka galvanic or voltaic cell)
Some important terms
1. Anode – electrode where oxidation occurs, it is negative because electrons are produced (An-ox)
2. Cathode – electrode where reduction occurs, it is positive because electrons are gained (Red-cat)
3. Salt bridge – consists of an electrolytic solution such as KNO3. The salt bridge may consist of a filter paper saturated with KNO3 solution or a U-tube containing KNO3 solution in an agar jelly. The salt bridge completes the circuit and allows ions to move between each half-cell. Other salts such as KCl can also be used. This is also known as the internal circuit.
4. External circuit – the circuit where electrons flow from the anode to the cathode as a source of electrical energy.
Exercises
1 Explain why a salt bridge or porous barrier is used in the connection of two half-cells.
2 a Draw a diagram of a galvanic cell that will produce electricity using the following reaction:
Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
b On your diagram, clearly:
i identify the anode and cathode
ii indicate the direction of movement of electrons and the migration of each kind of ion in the cell.
c Construct equations representing the reactions occurring at each electrode.
3 The overall equations for two galvanic cells are:
a 3Pb2+(aq) + 2Cr(s) → 3Pb(s) + 2Cr3+(aq)
b Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)
For each, write the relevant half-equations and identify the anode and cathode.