Oxidation and Reduction

Oxidation & Reduction

• Oxidation and reduction take place together at the same time in the same reaction
• These are called redox reactions
• There are three definitions of oxidation. It is a reaction in which:
  • Oxygen is added to an element or a compound
  • An element, ion or compound loses electrons
  • The oxidation state of an element is increased
• There are three definitions of reduction. It is a reaction in which:
  • Oxygen is removed from an element or a compound
  • An element, ion or compound gains electrons
  • The oxidation state of an element is decreased

Oxidation state

• The oxidation state (also called oxidation number) is a number assigned to an atom or ion in a compound which indicates the degree of oxidation (or reduction)
• The oxidation state helps you to keep track of the movement of electrons in a redox process
• It is written as a +/- sign followed by a number.
• Eg O^-2 means that it is an atom of oxygen that has an oxidation state of -2. It is not written as O^2- as this refers to the ion and its charge

Assigning the oxidation number

• Oxidation number refers to a single atom or ion only
• The oxidation number of a compound is 0 and of an element (for example Br in Br₂) is also 0
• The oxidation number of oxygen in a compound is always -2 (except in peroxide R-O-O-R, where it is -1)
• For example in FeO, oxygen is -2 then Fe must have an oxidation number of +2 as the overall oxidation number for the compound must be 0

Ionic Equations

• Ionic equations are used to show only the particles that actually take part in a reaction
• These equations show only the ions that change their status during a chemical process, i.e: their bonding or physical state changes
• The other ions present are not involved and are called spectator ions

Writing ionic equations

• For the neutralisation reaction between hydrochloric acid and sodium hydroxide:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

• If we write out all of the ions present in the equation and include the state symbols, we get:

H+(aq) + Cl- (aq)+ Na+(aq) + OH-(aq) → Na+ (aq)+ Cl-(aq) + H2O(l)

• The spectator ions are thus Na⁺ and Cl^–. Removing these from the previous equation leaves the overall net ionic equation:

H+(aq) + OH-(aq) →H2O(l)

• This ionic equation is the same for all acid-base neutralisation reactions

Example redox equation: oxygen loss/gain

Zinc oxide + carbon → zinc + carbon monoxide

ZnO + C → Zn + CO

• In this reaction, the zinc oxide has been reduced since it has lost oxgyen
• The carbon atom has been oxidised since it has gained oxygen

Extended Only

Redox & Electron Transfer

Example redox equation: electron loss/gain and oxidation state

Zinc + copper sulphate → zinc sulphate + copper

Zn + CuSO4 → ZnSO4 + Cu

• Writing this as an ionic equation:

Zn(s) + Cu2+(aq) + SO42-(aq) →Zn2+(aq) + SO42-(aq) + Cu(s)

• By analysing the ionic equation, it becomes clear that zinc has become oxidised as its oxidation state has increased and it has lost electrons:

Zn(s) →Zn2+(aq)

• Copper has been reduced as its oxidation state has decreased and it has gained electrons:

Cu2+(aq) → Cu(s)

Exam Tip

Use the mnemonic OIL-RIG to remember oxidation and reduction in terms of the movement of electrons: Oxidation Is Loss –  Reduction Is Gain.Extended Only

Oxidising & Reducing Agents

Oxidising agent

• A substance that oxidises another substance, in so doing becoming itself reduced
• Common examples include hydrogen peroxide, fluorine and chlorine

Reducing agent

• A substance that reduces another substance, in so doing becoming itself oxidised
• Common examples include carbon and hydrogen
• The process of reduction is very important in the chemical industry as a means of extracting metals from their ores 

Example

CuO + H2 →Cu + H2O

• In the above reaction, hydrogen is reducing the CuO and is itself oxidised, so the reducing agent is therefore hydrogen
• The CuO is reduced to Cu and has oxidised the hydrogen, so the oxidising agent is therefore copper oxide