S-Cool Revision Summary
S-Cool Revision Summary
Redox reactions involve oxidation and reduction reactions occurring simultaneously.
Oxidation is the loss of electrons.
Reduction is the gain of electrons.
Displacement reactions of metals and their ions in solution are an example of redox but also of dynamic equilibria Metal atoms can lose electrons to become ions (oxidation) or metal ions can gain electrons to become atoms (reduction).
An electrochemical cell converts chemical energy into electrical energy. Since metals can be oxidised or reduced depending on their chemical environment.
Electrodes are charged (usually metal or graphite) rods.
Electrolytes are solutions that contain ions.
Salt bridge Completes the circuit and prevents the build up of charge in either half-cell, by allowing the passage of ions.
In order to find the Standard electrode potential of a half cell we use a standard hydrogen electrode (potential zero) and measure the EMF between the two cells.
E = E right-hand half-cell - E left-hand half-cell.
The Standard electrode potential of a metal is the potential acquired when the metal is immersed in a 1 moldm-3 solution of its ions at a temp. of 25°C - symbol - E
We can tabulate the order of oxidising/reducing ability of a system - this we call the electrochemical series. The most +ve E value is at the top - the greatest oxidising agent. The most -ve E value at the bottom - the greatest oxidising agent.
To make a prediction about a reaction, remember that the system which is lower in the series will lose electrons and the one higher in the series will gain electrons.
Different types of cells/batteries are used in everyday life, e.g. lead-acid battery and the dry cell - all carrying out redox reactions in order to convert chemical energy to electrical energy.
Rusting is an example of an electrochemical process.
Electrolysis is the decomposition of a compound using electricity.
An electrolysis cell is illustrated below:
The cell consists of two electrodes (cathode -ve and anode +ve) dipped into a compound in a molten state or in solution - the electrolyte.
When a d.c. current is passed the compound splits up due to the anions (-ve) being attracted to the anode and the cations (+ve being attracted to the cathode.
Reduction occurs at the cathode, oxidation at the anode.
Predicting what products occur at the electrodes is confused if compound is in aqueous solution due to the presence of OH- (anion) and H+(cation) these compete with the other ions present. The electrochemical series is used to predict which cation is most readily reduced, and which anion is most readily oxidised - the other ions remain in solution.
In the Chloro-alkali industry chlorine gas, hydrogen gas and sodium hydroxide are produced from the electrolysis of brine.
The quantity of electricity passed is proportional to the amount of substance discharged at the electrode.
Quantity of electricity (charge) = current x time
One mole of electrons has a charge of 96500C. This is known as the Faraday constant (F).
F = L x e
L = Avagadro's no.
e = charge of an electron
The number of moles of electrons required to discharge 1 mole of ions is equal to the charge on the ion.