Redox: Reduction and oxidation happening during a reaction.
OIL RIG
Oxidation Is Loss of electrons
Reduction Is Gain of electrons.
Key ideas
- Half equations are written to represent oxidation and reduction separately.
- Half equations tells us what happens to the electrons in chemical reactions during which atoms or ions are gaining or losing electrons.
- We refer to these equations as half equations because they represent only half of what is taking place during a reaction: transfer of electrons – gain (reduction) and loss (oxidation) of electrons
- Ionic equations shows what happens to ions during chemical reactions.
4. Ions are balanced using electrons.
5. When we combine half equations, we form the ionic equation.
6. Ions which are present, but do not take part in chemical reactions are called spectator ions.
Example:
Displacement reaction between aluminium and iron (III) oxide
2Al + Fe2O3 → Al2O3 + 2Fe
In this reaction, aluminium is oxidised to form aluminium oxide.
Iron (III) oxide is reduced to form iron.
Oxidation and reduction equations separately (half equations).
Oxidation
Al → Al3+ [Oxidation – Loss of electrons]
Reduction
Fe3+ → Fe [Reduction – Gain of electrons]
Balancing the ions in half equations
Balance the positive ions using electrons.
The number of electrons used to balance the equation equals the magnitude of the positive ion.
Al → Al3+ + 3e– [Oxidation – Loss of electrons]
Fe3+ + 3e– → Fe [Reduction – Gain of electrons]
Writing the ionic equation.
Al + Fe3++ 3e–→Al3+ + Fe + 3e–
Overall ionic equation is:
Al + Fe3+ → Al3+ + Fe
Retrieval Practice
Displacement of copper from copper sulfate by zinc is shown below:
Zinc + copper sulfate → zinc sulfate + copper
Zn + CuSO4 → ZnSO4 + Cu
Write down:
(a) the two half equations
(b) overall ionic equation
for this displacement reaction.
Mark scheme
(a) Half Equations
Cu2+ + 2e– → Cu
Zn → Zn2+ + 2e–
b) Overall ionic equation
Zn + Cu2+→ Zn2+ + Cu
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