Problem 52 - Chemistry: The Central Science

Question

Which of the following are redox reactions? For those that are, indicate which element is oxidized and which is reduced. For those that are not, indicate whether they are precipitation or neutralization reactions. (a) \(\begin{aligned} \mathrm{P}_{4}(s)+10 \mathrm{HClO}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow & \longrightarrow \mathrm{H}_{3} \mathrm{PO}_{4}(a q)+10 \mathrm{HCl}(a q) \end{aligned}\) (b) \(\mathrm{Br}_{2}(l)+2 \mathrm{~K}(s) \longrightarrow 2 \mathrm{KBr}(s)\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g)\) (d) \(\mathrm{ZnCl}_{2}(a q)+2 \mathrm{NaOH}(a q) \longrightarrow \mathrm{Zn}(\mathrm{OH})_{2}(s)+\) \(2 \mathrm{NaCl}(a q)\)

Step-by-Step Solution

Answer

(a) Redox reaction: P is oxidized (0 to +5), Cl is reduced (+1 to -1). (b) Redox reaction: K is oxidized (0 to +1), Br is reduced (0 to -1). (c) Redox reaction: C is oxidized (-2 to +4), O is reduced (0 to -2). (d) Precipitation reaction: No redox process occurs.

Step 1: (a) Identify Reaction Type

For the given reaction: \(\begin{aligned} \mathrm{P}_{4}(s)+10 \mathrm{HClO}(aq)+6 \mathrm{H}_{2}\mathrm{O}(l) \longrightarrow& \mathrm{H}_{3}\mathrm{PO}_{4}(aq)+10 \mathrm{HCl}(aq) \end{aligned}\) Let's determine the initial and final oxidation states of the elements involved: - P: 0 (in \(\mathrm{P}_4\)) ➡ +5 (in \(\mathrm{H}_3\mathrm{PO}_4\)) - Cl: +1 (in \(\mathrm{HClO}\)) ➡ -1 (in \(\mathrm{HCl}\)) Since there is a change in the oxidation states, this reaction is a redox reaction.

Step 2: (a) Oxidized and Reduced Elements

In the redox reaction: - P is oxidized (increase in oxidation state from 0 to +5). - Cl is reduced (decrease in oxidation state from +1 to -1).

Step 3: (b) Identify Reaction Type

For the given reaction: \(\mathrm{Br}_{2}(l)+2 \mathrm{K}(s) \longrightarrow 2 \mathrm{KBr}(s)\) Let's determine the initial and final oxidation states of the elements involved: - Br: 0 (in \(\mathrm{Br}_2\)) ➡ -1 (in \(\mathrm{KBr}\)) - K: 0 (in \(\mathrm{K}\)) ➡ +1 (in \(\mathrm{KBr}\)) Since there is a change in the oxidation states, this reaction is a redox reaction.

Step 4: (b) Oxidized and Reduced Elements

In the redox reaction: - K is oxidized (increase in oxidation state from 0 to +1). - Br is reduced (decrease in oxidation state from 0 to -1).

Step 5: (c) Identify Reaction Type

For the given reaction: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g)\) Let's determine the initial and final oxidation states of the elements involved: - C: -2 (in \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\)) ➡ +4 (in \(\mathrm{CO}_2\)) - O: -2 (in \(\mathrm{O}_2\)) ➡ -2 (in \(\mathrm{H}_2\mathrm{O}\) and \(\mathrm{CO}_2\)) Since there is a change in the oxidation states, this reaction is a redox reaction.

Step 6: (c) Oxidized and Reduced Elements

In the redox reaction: - C is oxidized (increase in oxidation state from -2 to +4). - O is reduced (decrease in oxidation state from 0 to -2).

Step 7: (d) Identify Reaction Type

For the given reaction: \(\mathrm{ZnCl}_{2}(a q)+2 \mathrm{NaOH}(a q) \longrightarrow \mathrm{Zn}(\mathrm{OH})_{2}(s)+2 \mathrm{NaCl}(a q)\) Let's determine the initial and final oxidation states of the elements involved: - Zn: +2 (in \(\mathrm{ZnCl}_2\)) ➡ +2 (in \(\mathrm{Zn(OH)_2}\)) - Cl: -1 (in \(\mathrm{ZnCl}_2\)) ➡ -1 (in \(\mathrm{NaCl}\)) - Na: +1 (in \(\mathrm{NaOH}\)) ➡ +1 (in \(\mathrm{NaCl}\)) - O: -2 (in \(\mathrm{NaOH}\)) ➡ -2 (in \(\mathrm{Zn(OH)_2}\)) - H: +1 (in \(\mathrm{NaOH}\)) ➡ +1 (in \(\mathrm{Zn(OH)_2}\)) No change in the oxidation states occurred, indicating that this is not a redox reaction.

Step 8: (d) Precipitation or Neutralization Reaction

The reaction results in the formation of a solid product, \(\mathrm{Zn(OH)_2}\), from aqueous reactants. This is a precipitation reaction.

Key Concepts

Oxidation States

Understanding oxidation states is crucial for identifying redox reactions. Oxidation states, or oxidation numbers, indicate the degree of oxidation of an atom within a compound. They are positive or negative numbers assigned to an atom according to certain rules. These numbers help in tracking electrons during chemical reactions. For instance, in the reaction \(\mathrm{P}_{4}(s)+10 \mathrm{HClO}(aq)+6 \mathrm{H}_{2}\mathrm{O}(l) \longrightarrow \mathrm{H}_{3}\mathrm{PO}_{4}(aq)+10 \mathrm{HCl}(aq)\), phosphorus (P) changes its oxidation state from 0 to +5, showing electron loss, and chlorine (Cl) changes from +1 to -1, showing electron gain. Recognizing these changes allows us to confirm the presence of a redox process.

Reduction

Reduction occurs when an atom gains electrons during a chemical reaction, resulting in a decrease in its oxidation state. In a redox reaction, the reduction is one half of the equation, with the other half being oxidation. For example, in the reaction \(\mathrm{Br}_{2}(l)+2 \mathrm{K}(s) \longrightarrow 2 \mathrm{KBr}(s)\), bromine (Br) is reduced as it goes from an oxidation state of 0 to -1. This signifies that bromine atoms gain electrons. The species that is reduced is called the oxidant, as it drives the oxidation of another species by 'accepting' electrons.

Oxidation

Oxidation is defined as the loss of electrons by an atom, manifested as an increase in oxidation state. For instance, in the combustion of ethanol \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g)\), carbon (C) is oxidized from -2 in ethanol to +4 in carbon dioxide (\(\mathrm{CO}_{2}\)). The substance that gets oxidized is known as the reductant because it 'donates' electrons to the other substance undergoing reduction.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two soluble salts react to form one or more insoluble products, known as precipitates. These reactions typically occur in aqueous solutions and can be easily spotted when a solid suddenly forms from a clear solution. For example, the reaction \(\mathrm{ZnCl}_{2}(aq)+2 \mathrm{NaOH}(aq) \longrightarrow \mathrm{Zn}(\mathrm{OH})_{2}(s)+2 \mathrm{NaCl}(aq)\) results in the formation of zinc hydroxide, an insoluble solid that precipitates out of the solution, confirming the reaction's nature as a precipitation reaction.

Neutralization Reactions

Neutralization reactions are reactions between an acid and a base that produce a salt and water. These reactions are crucial for understanding various chemical processes, including those in biological systems and industrial applications. For instance, when hydrochloric acid reacts with sodium hydroxide, a neutralization reaction occurs, forming sodium chloride and water. While the given exercise does not present a typical neutralization reaction, students should bear in mind that they are often exothermic and play significant roles in chemistry.