Oxidation and Reduction. Chemistry 3B 2014. Oxidation and Reduction. apply the table of Standard Reductions Potentials to determine the relative strength of oxidising and reducing agents to predict reaction tendency
salt bridge and ion migration
electron flow in external circuit
Reduced Oxidising agent (Cl2)
Cl2 + 2Na 2NaCl
(0) (0) (+1)(-1)
Oxidised Reducing agent (Na)
The oxidised species is identified by an increase in its oxidation number while the reduced species shows a decrease in its oxidation number.
Increasing strength of oxidising agents
2Hg Hg22+ + 2e-
Cl2 + 2e- 2Cl-
A useful mnemonic to remember is OIL RIG
2Hg Hg22+ + 2e-
Cl2 + 2e- 2Cl-
2Hg + Cl2 Hg22+ + 2Cl-
Balance the following redox reaction in acidic conditions.
Cr2O7−2(aq)+HNO2(aq)→ Cr3+(aq) + NO3− (aq)
Step 1: Separate the half-reactions.
Step 2: Balance elements other than O and H.
Step 3: Add H2O to balance oxygen.
Step 4: Balance hydrogen by adding H+.
Step 5: Balance the charge of each equation with electrons.
Step 6: Scale the reactions so that the electrons are equal.
3∗[HNO2(aq) + H2O(l)→3H+(aq) +NO3−(aq) +2e−] ⇒ 3HNO2(aq) +3H2O(l)→9H+(aq)+3NO3− (aq)+6e−
Step 7: Add the reactions and cancel out common terms.
3HNO2(aq)+3H2O(l)+6e−+14H+(aq)+Cr2O7−2 (aq)→9H+(aq)+3NO3− (aq)+6e−+2Cr3+(aq)+7H2O(l)
The electrons cancel out as well as 3 water molecules and 9 H+. This leaves the balanced net reaction of:
3HNO2(aq) +5H+(aq)+ Cr2O7−2(aq)→ 3NO3−(aq)+2Cr3+ (aq)+4H2O(l)
The Daniell cell uses copper sulphate (Cu2+) as the oxidising agent and zinc metal (Zn) as the reducing agent.
The zinc electrode becomes oxidised releasing zinc ions and electrons. These electrons flow from zinc electrode, through the external conducting path, to the copper electrode. Here copper ions in the electrolyte move to the copper electrode where they gain electrons from the oxidation of zinc, to be reduced to metallic copper.
A salt bridge completes the circuit allowing ions to move between the cathode and anode.
Is the following redox reaction spontaneous?
2Al + 6H+ 3H2 + 2Al3+
2Al 2Al3+ + 6e- +1.66
6H+ + 6e- 3H2 0.00
2Al + 6H+ 3H2 + 2Al3+ +1.66
Yes the reaction is spontaneous
The overall cell reactions for a dry cell are:
Anode: Zn(s) -> Zn2+(aq) + 2e-
Cathode: 2NH4+(aq) + 2MnO2(s) + 2e- -> Mn2O3(s) + H2O(l) + 2NH3(aq)
The anode grid contains spongy lead, while the cathode grid is packed with powdered lead dioxide. The powdered nature of both these reagents allows for a fast reaction rate because of their high surface area. This allows the cell to produce high currents.
The electrolyte is concentrated sulphuric acid.
Anode: Pb+ SO42-(aq) → PbSO4(s) + 2e-
Cathode: 4H+ + PbO2 + SO42- + 2e- → PbSO4(s) + 2H2O
The overall equation is
Pb(s) + PbO2(s) + 4H+ + 2SO4 2-(aq) → 2PbSO4(s) + 2H2O(l)
Fuel cells do not store the oxidising and reducing agents, instead these reactants are constantly fed into the cell to generate electricity.
Three Mercedes-Benz model O530BZ “Citaro” hydrogen fuel-cell buses were used in Perth between 2004 and 2007 as part of an international trial to study the operation of alternative energy driven buses.
Anode: H2(g) + 2OH-(aq) 2H2O(l) + 2e-
Cathode: O2(g) + 2H2O(l) + 4e- 4OH-(aq)
Overall redox reaction: 2H2(g) + O2(g) 2H2O(l)
The reducing agent H2 diffuses into the nickel electrode and is oxidised forming H+ ions and free electrons.
Oxygen absorbed into the cathode then gains electrons forming OH- ions.
Considering the sketch of a water droplet, the oxidizing iron supplies electrons at the edge of the droplet to reduce oxygen from the air. The iron surface inside the droplet acts as the anode for the process
Fe(s) -> + Fe2+(aq) + 2e-
The electrons can move through the metallic iron to the outside of the droplet where
O2(g) + 2H2O(l) + 4e- -> 4OH-(aq)
Within the droplet, the hydroxide ions can move inward to react with the iron(II) ions moving from the oxidation region. Iron(II) hydroxide is precipitated.
Fe2+(aq) + 2OH-(aq) -> Fe(OH)2(s)
Rust is then quickly produced by the oxidation of the precipitate.
4Fe(OH)2(s) + O2(g) -> 2Fe2O3 •H2O(s) + 2H2O(l)
Sacrificial anodes are usually made from magnesium, zinc or aluminium; magnesium is used on-shore and in freshwater, and zinc and aluminium are used in salt water where resistance is lower.