Chemistry 2A/B
REDOX Revision
Megan Ngo
Definitions
- Oxidation is the addition of oxygen and the removal of hydrogen. (loss of electrons)
- Reduction is the removal of oxygen and the addition of hydrogen. (gain of electrons)
- Disproportionation is when the same species has undergone BOTH oxidation and reduction.
For example; 2H2O2 → O2 + H2O
- Anode is where oxidation occurs and is connected to the positive terminal. Anions from the solution
would move to the anode.
- Cathode is where reduction occurs and is connected to the negative terminal. Cations from the
solution would move to the cathode.
Explain oxidation and reduction as an electron transfer process
Oxidation
Reduction
- A substance is said to be oxidised or to have
- A substance is said to be reduced or to have
undergone oxidation when it loses one or more
undergone reduction when it gains one or more
electrons in a reaction
electrons in a reaction
Oxidation and reduction MUST occur simultaneously in a reaction.
Calculate oxidation numbers
The oxidation number of an atom is an arbitrary ‘charge’ assigned to the atom according to a set of
rules.
These rules are:
- For atoms of elements in the elemental state, their oxidation number of zero.
- For monatomic ions, the oxidation number equals the charge on the ion.
- In compounds, the oxidation number of oxygen is -2 (except in peroxides 02-2, here it is -1 and in F2O,
where it is +2)
- In compounds, the oxidation number of hydrogen is +1 (except in metal hydrides which H-,it is -1)
- For polyatomic species, the sum of the oxidation number of all the atoms must equal the charge on the
ion or molecule
- The sum of the oxidation numbers of all the atoms in a compound must equal zero.
Chemistry 2A/B
REDOX Revision
Megan Ngo
*Identify and name oxidants and reductants in equations
Oxidation
- An oxidising, or an oxidant, is a substance that
oxidises another substance that is an oxidant
‘removes’ electrons from another substance.
Reduction
- A reducing agent, or a reductant, is a substance
that reduces another substance. That is, a
reductant ‘donates’ electrons to another
substance.
- In a redox reaction, the substance that is oxidised is the reductant. The substance that is reduced is the
oxidant.
Example: 2Na + Cl2 → 2NaCl
- Sodium has been oxidised and has acted as the reductant.
- Chloride has been reduced and has acted as an oxidant.
Identify oxidation-reduction reactions using oxidation numbers
Example: 2Ag+ + Cu → Cu+2 + 2Ag
Oxidation numbers BEFORE reaction:
Ag = +1
Cu = 0
Oxidation numbers AFTER reaction:
Ag = 0
Cu = +2
- Silver has gained an electron, therefore reduction.
- Copper has lost electrons, therefore it is oxidation.
Chemistry 2A/B
REDOX Revision
Megan Ngo
Describe, write equations for and interpret observations for:
METAL DISPLACEMENT REACTIONS
- A more reactive metal will displace a less active metal
- The more active metal will donate electrons more readily and will oxidise
- When a solid sample of a more active metal is placed into a solution of a less active metal a
displacement reaction will take place but if a solid sample of a less active metal is placed into a solution
of a more active metal NO reaction will occur.
- DATA SHEET: on the back from the bottom gives us the activity series of the metals in order.
HALOGEN DISPLACEMENT
- A more active halogen will displace a less active halogen
- The more active halogen will accept electrons more readily and will reduce
- If a more active halogen is bubbled through a solution of a less active halogen a displacement reaction
will occur BUT if a less active halogen is bubbled through a solution of more active halogen NO reaction
will occur
- DATA SHEET: group 17 activity series like above
DISPROPORTIONATION REACTION
- When the same species has undergone BOTH oxidation and reduction
Write balanced simple redox equation
Metal/Metal Ion
2Fe (s) + Sn+4 (aq) → Sn (s) + 2Fe+2 (aq)
Metal/Hydrogen Ion
Mg (s) + 2H+ (aq) → Mg+2 (aq) + H2 (g)
Halogen/Halide Ion
Write and balance oxidation-reduction half-equations in acidic conditions
*Note when doing half equations balance it all out!
i) A piece of Mg ribbon is added to HCl (aq) liberating H2 (g) and forming MgCl2 (aq).
Mg (s) + HCl (aq) → MgCl2 + H2
Oxidation - Mg → Mg+2 + 2eReduction - H+ + e- → H
Overall Equation - 2H+ (aq) + Mg (s)→ H2 (g) + Mg+2(aq)
ii) A sample of Cr metal was added to an HCl (aq) solution. This resulted in the formation of H2 (g) and
CrCl3 (aq)
Cr (s) + HCl (aq) → H2 + CrCl3
Oxidation - Cr → Cr+3 + 3eReduction - 2H+ + 2e- → H2
Overall Equation - 2Cr (s)+ 6H+(aq)→ 3H2(g)+ 2Cr+3 (aq)
Chemistry 2A/B
REDOX Revision
Megan Ngo
Describe and explain how an electric current is conducted in an electrolytic cell
The electric current is converted into chemical energy as this is their purpose, in order to create a
chemical reaction. What this does is that it forces the electron around the circuit and creates half of the
circuit as the other half would be using ions in the electrolyte. It is conducted through the use of ions in
the inert (Anode and Cathode) and electrons in the external circuit.