Identify the Relationship Between Displacement of Metal Ions in Solution by Other Metals to the Relative Activity of Metals
identify the relationship between displacement of metal ions in solution by other metals to the relative activity of metals[pic 1]
Identify: Recognise and name
A displacement reaction will occur spontaneously if the metal is more active than the metal that is present as ions in the solution. Thus calcium will displace copper from a solution of copper (II) ions, as calcium is more active than copper and a stronger reductant. The Cu2+ ions have a greater tendency to accept electrons than Ca2+ ions. Additionally, the further apart two metals are in the activity series, the more vigorous the reaction is. [pic 2]
account for changes in the oxidation state of species in terms of their loss or gain of electrons
Account: state reasons for[pic 3]
Oxidations state is a number given to an atom or ion to indicate (theoretically) the number of electrons it has gained or lost. When a species is oxidised, it loses electrons (and thus its charge becomes more positive). When a species is reduced, it gains electrons (and thus its charge becomes more negative). Reduction and oxidation reactions, or ‘redox’ reactions, occur simultaneously rather than independently of one another. As such, the electrons lost by one species are gained by another.[pic 4]
Rules | Examples |
A pure element has an oxidation state of 0 | Na(s) - H2(g) - Br2(l) |
The sum of all oxidation states in a compound is 0 | In Al2O3, the oxidation states of O are -2 and the oxidation state of Al +3. therefore: [3(-2)]+[2(+3)]=0 |
The oxidation states of hydrogen in its compounds is +1 (except for metal hydrides where it is -1) | Oxidation state of H in HCl or CH4 is +1. Oxidation state of H in LiH is -1. |
The oxidation state of oxygen in its compounds is -2 (except for peroxides where it is -1) | Oxidation state of O in H2O is -2. Oxidation state of O in H2O2 is -1 |
describe and explain galvanic cells in terms of oxidation/reduction reactions
Describe: Provide characteristics and features
Explain: Relate cause and effect
A galvanic cell utilises electron transfer reactions to generate electricity by physically separating the oxidation reaction and the reduction reaction. A galvanic cell consists of two-half cells. In one half-cell oxidation occurs while in the other reduction occurs. These are connected by a conducting pathway (such as an external wire or circuit) so that when electrons are released upon oxidation, they flow through the circuit to the reduction half-cell where they are consumed. This electron flow constitutes electrical energy and can be used to do work.
outline the construction of galvanic cells and trace the direction of electron flow
Outline: indicate the main features of
In a galvanic cell, there are two conducting terminals called electrodes, the anode (negative) and the cathode (positive). Oxidation takes place at the anode and reduction takes place at the cathode. Both electrodes are placed in a suitable electrolyte solution.
The two electrodes are connected with external metallic conductor like a copper wire to allow the flow of electrons from one half cell to the other. However, electrons will not flow unless a complete circuit is present. A salt bridge is required to complete the circuit. If it is not present, an imbalance of charge will result in the half cells. As oxidation occurs at the anode, the metal cations enter the solution, resulting in a build-up of positive charge in this half-cell. As reduction is occurring at the anode, the amount of cations in solution decreases, which results in an excess of negative charge in this half-cell. The purpose of the salt bridge is to allow the migration of ions to occur, preserving electrical neutrality.
The salt bridge solution must not react with any of the ions in it to solutions it is connecting. Potassium nitrate is a good choice because NO3- and K+ do not form any precipitates with other ions. Both positive and negative ions migrate through salt bridge. The excess anions from the reduction half-cell move into the salt bridge towards the anode, while the excess cations from the oxidation half-cell move into the salt bridge towards the cathode.
When the cell is operating, the anode is oxidised and the liberated electrons travel to the cathode through the external conductor where reduction of the metal ions electrolyte solution occurs at the surface of the cathode. For example, a galvanic cell can be made using zinc and copper electrodes in zinc sulfate and copper sulfate solutions respectively. Since zinc is more reactive than copper, the zinc becomes oxidised, sending electrons along the wire. The zinc ions formed added to the zinc sulfate solution. The zinc electrode is the anode, where oxidation occurs: Zn(s) → Zn2+ + 2e-. Copper ions in the copper sulfate solution accept these electrons, forming copper metal. This copper is deposited on copper cathode where reduction occurs: Cu2+ + 2e- → Cu(s).