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Redox equations for OCR A-level Chemistry
This page covers the following topics:
1. Redox half-equations
2. Reactivity of halogens
3. Redox equations
4. Disproportionation
Reduction is the gain of electrons or the decrease of an oxidation number. Oxidation is the loss of electrons or the increase of the oxidation number. A particle gains electrons when it is reduced and a particle loses electrons when it is oxidised. Redox half-equations provide information about the change in particles and the loss or gain of electrons. Many half-equations can be balanced by adding electrons, water molecules and hydrogen ions.
Fe³⁺ + e⁻ → Fe²⁺ (reduction)
Mg → Mg²⁺ + 2e⁻ (oxidation)
The reactivity of halogens decreases going down the group 7 of the periodic table. A more reactive halogen displaces a less reactive halogen in a halide salt solution.
Redox half-equations can be combined to construct full ionic equations by matching the number of electrons and removing duplicate particles from both sides of an equation. If the electrons do not cancel out after adding half-equations, they can be multiplied by suitable integers beforehand. Both sides of a half-equation can also be switched around to ensure that electrons appear on both sides of an ionic equation. In a redox equation a reducing agent is an electron donor, while an oxidising agent is an electron acceptor.
Disproportionation is a process during which the same particles are both reduced and oxidised. Disproportionation can be recognised by tracking the oxidation number of elements before and after the reaction. If compounds produced that include some element have two oxidation numbers that are different from each other and from the initial oxidation number, the reaction includes disproportionation.
1
Explain why iodine does not displace chlorine in sodium chloride that is dissolved in water.
A more reactive halogen displaces a less reactive halogen in halide salt solutions. The reactivity of halogens decreases going down the group 7 of the periodic table. Since iodine is lower down the group 7 than chlorine, it it is less reactive and does not displace chlorine in its salt.
Iodine is less reactive than chlorine.
2
Nitrogen compound with a nitrogen oxidation number of −1 participates in a disproportionation reaction. During the reaction two compounds containing nitrogen are produced with oxidation numbers of +5 and X. Pick suitable values of X from the provided list.
−3, −1, 0, +1, +3, +5
In disproportionation reactions compounds produced that include some element have two oxidation numbers that are different from each other and from the initial oxidation number.
−3, 0, +1, +3
3
Identify all particles that are reduced in the half-equations provided.
2Cl⁻ → Cl₂ + 2e⁻
Mn⁵⁺ + 3e⁻ → Mn²⁺
A particle gains electrons when it is reduced. In these half-equations Mn⁵⁺ gains electrons.
Mn⁵⁺
4
Which of the following reactions are disproportionation?
A: BrF → BrF₃ + Br₂
B: NH₄NO₃ → N₂O + 2H₂O
C: 2H₂O₂ → 2H₂O + H₂
D: Fe + 2HCl → FeCl₂ + H₂
Disproportionation can be recognised by tracking the oxidation number of elements before and after the reaction. If compounds produced that include some element have two oxidation numbers that are different from each other and from the initial oxidation number, the reaction includes disproportionation.
A, C
5
Explain why the reaction presented in the image is an example of a disproportionation reaction.
Disproportionation is a process during which the same particles are both reduced and oxidised. At the beginning chlorine has an oxidation number of 0, while after the reaction it has oxidation numbers of +1 in KClO and −1 in Cl⁻. Since chlorine has two oxidation numbers that are different from each other and from the initial oxidation number, the reaction includes disproportionation.
At the beginning chlorine has an oxidation number of 0, while after the reaction it has oxidation numbers of +1 in KClO and −1 in Cl⁻.
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