Question

What is a disproportionation reaction, and which of the following fit the description? (a) \(\mathrm{I}_{2}(s)+\mathrm{KI}(a q) \longrightarrow \mathrm{KI}_{3}(a q)\) (b) \(2 \mathrm{ClO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(I) \longrightarrow \mathrm{HClO}_{3}(a q)+\mathrm{HClO}_{2}(a q)\) (c) \(\mathrm{Cl}_{2}(g)+2 \mathrm{NaOH}(a q) \longrightarrow \mathrm{NaCl}(a q)+\mathrm{NaClO}(a q)+\mathrm{H}_{2} \mathrm{O}(l)\) (d) \(\mathrm{NH}_{4} \mathrm{NO}_{2}(s) \longrightarrow \mathrm{N}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)\) (e) \(3 \mathrm{MnO}_{4}^{2-}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow$$$ 2 \mathrm{MnO}_{4}^{-}(a q)+\mathrm{MnO}_{2}(s)+4 \mathrm{OH}^{-}(a q)$$ (f) \)3 \mathrm{AuCl}(s) \longrightarrow \mathrm{AuCl}_{3}(s)+2 \mathrm{Au}(s)$

Short answer

The reactions fitting the description of a disproportionation reaction are (b), (c), (e), and (f).

Step-by-step solution

Define Disproportionation Reaction

A disproportionation reaction is a type of redox reaction in which a single substance is simultaneously oxidized and reduced, forming two different products.

Analyze Reaction (a)

In the reaction \(\text{I}_{2}(s) + \text{KI}(aq) \rightarrow \text{KI}_{3}(aq)\), there is no change in oxidation states of iodine. Therefore, it is not a disproportionation reaction.

Analyze Reaction (b)

In the reaction \(\text{2 ClO}_2(g) + \text{H}_2\text{O}(l) \rightarrow \text{HClO}_3(aq) + \text{HClO}_2(aq)\), chlorine changes its oxidation state from +4 in ClO_2 to +5 in HClO_3 and to +3 in HClO_2. Thus, it involves both oxidation and reduction of ClO_2, making it a disproportionation reaction.

Analyze Reaction (c)

In the reaction \(\text{Cl}_2(g) + 2 \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{NaClO}(aq) + \text{H}_2\text{O}(l)\), chlorine changes oxidation state from 0 in Cl2 to -1 in NaCl (reduced) and +1 in NaClO (oxidized). Thus, it is a disproportionation reaction.

Analyze Reaction (d)

In the reaction \(\text{NH}_4\text{NO}_2(s) \rightarrow \text{N}_2(g) + 2 \text{H}_2\text{O}(g)\), there is no change in oxidation states of nitrogen. Therefore, it is not a disproportionation reaction.

Analyze Reaction (e)

In the reaction \(\text{3 MnO}_4^{2-}(aq) + 2 \text{H}_2\text{O}(l) \rightarrow 2 \text{MnO}_4^{-}(aq) + \text{MnO}_2(s) + 4 \text{OH}^-(aq)\), manganese changes oxidation state from +6 in MnO_4^{2-} to +7 in MnO_4^{-} (oxidized) and +4 in MnO_2 (reduced). Therefore, it is a disproportionation reaction.

Analyze Reaction (f)

In the reaction \(\text{3 AuCl}(s) \rightarrow \text{AuCl}_3(s) + 2 \text{Au}(s)\), gold changes oxidation state from +1 in AuCl to +3 in AuCl3 and 0 in Au. Therefore, it is a disproportionation reaction.

Key concepts

Redox Reactions

Redox reactions are chemical reactions involving the transfer of electrons between two substances. The term 'redox' stands for reduction-oxidation. In these reactions:
- One substance loses electrons (oxidation).
- Another substance gains electrons (reduction).
This transfer results in changes in the oxidation states of the substances involved. Redox reactions are crucial in a variety of processes like cellular respiration, combustion, and metal corrosion. For example, in the reaction between sodium and chlorine, where sodium is oxidized to form Na+, and chlorine is reduced to form Cl-.

Oxidation States

Oxidation states indicate the degree of oxidation of an atom in a chemical compound. It represents the hypothetical charge an atom would have if all bonds were ionic. Here are some key points:
- Elements in their pure form have an oxidation state of 0.
- For monoatomic ions, the oxidation state is equal to the ion's charge.
- Oxygen usually has an oxidation state of -2, except in peroxides where it's -1.
- The sum of oxidation states in a neutral compound is 0, while in ions, it equals the ion charge.
Understanding oxidation states is essential for identifying redox reactions and balancing chemical equations.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds, producing new substances. Several types of chemical reactions include:
- Synthesis reactions, where two or more simple substances combine to form a complex substance.
- Decomposition reactions, where a complex substance breaks into simpler substances.
- Single-replacement reactions, where an element replaces another in a compound.
- Double-replacement reactions, where parts of two compounds swap places to form two new compounds.
Understanding different reaction types helps in predicting the products and understanding reaction mechanisms.

Oxidation

Oxidation is the process where a substance loses electrons, increasing its oxidation state. This often involves the addition of oxygen or the removal of hydrogen. Examples include:
- The rusting of iron (iron loses electrons to form iron oxide).
- The combustion of hydrocarbons (carbon atoms lose electrons as they combine with oxygen).
In oxidation, the substance undergoing oxidation is called the reducing agent because it donates electrons to another substance, which is then reduced.

Reduction

Reduction is the process where a substance gains electrons, decreasing its oxidation state. This often involves the removal of oxygen or the addition of hydrogen. Examples include:
- The reduction of oxygen to form water in combustion reactions.
- The conversion of metal ores to pure metals (such as the reduction of iron ore to iron).
In reduction, the substance being reduced is called the oxidizing agent because it accepts electrons from another substance, which is then oxidized.
In summary, oxidation and reduction always occur together in redox reactions, making them fundamental concepts in understanding chemical processes.