Problem 52 - Chemistry: The Central Science in SI Units, Global Edition

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) \(\mathrm{P}_{4}(s)+10 \mathrm{HClO}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) (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) \(\mathrm{ZnCl}_{2}(a q)+2 \mathrm{NaOH}(a q) \longrightarrow \mathrm{Zn}(\mathrm{OH})_{2}(s)+\)

Step-by-Step Solution

Answer

(a) Neutralization reaction (b) Redox reaction: \(K\) is oxidized (0 to +1), and \(Br\) is reduced (0 to -1). (c) Redox reaction: \(C\) is oxidized (-3 to +4), and \(O\) is reduced (0 to -2). (d) Precipitation reaction

Step 1: Assign oxidation states

Assign oxidation states for all elements in the reactants and products of the reactions. (a) \(\mathrm{P}_{4}(s)+10 \mathrm{HClO}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) The oxidation states are as follows: \\ For P, the oxidation state is 0. \\ For H in HClO, the oxidation state is +1. \\ For Cl in HClO, the oxidation state is +1. \\ For H in H2O, the oxidation state is +1. \\ For O in H2O, the oxidation state is -2. (b) \(\mathrm{Br}_{2}(l)+2 \mathrm{K}(s) \longrightarrow 2 \mathrm{KBr}(s)\) \\ The oxidation states are as follows: \\ For Br in Br2, the oxidation state is 0. \\ For K, the oxidation state is 0. \\ For K in KBr, the oxidation state is +1. \\ For Br in KBr, the oxidation state is -1. (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g)\) \\ The oxidation states are as follows: \\ For C in CH3CH2OH, the oxidation state is -3. \\ For H in CH3CH2OH, the oxidation state is +1. \\ For O in CH3CH2OH, the oxidation state is -2. \\ For O in O2, the oxidation state is 0. \\ For H in H2O, the oxidation state is +1. \\ For O in H2O, the oxidation state is -2. \\ For C in CO2, the oxidation state is +4. \\ For O in CO2, the oxidation state is -2. (d) \(\mathrm{ZnCl}_{2}(a q)+2 \mathrm{NaOH}(a q) \longrightarrow \mathrm{Zn}(\mathrm{OH})_{2}(s)+\) \\ The oxidation states are as follows: \\ For Zn in ZnCl2, the oxidation state is +2. \\ For Cl in ZnCl2, the oxidation state is -1. \\ For Na in NaOH, the oxidation state is +1. \\ For O in NaOH, the oxidation state is -2. \\ For H in NaOH, the oxidation state is +1. \\ For Zn in Zn(OH)2, the oxidation state is +2.

Step 2: Identify redox reactions

Compare the oxidation states of the elements in the reactants and products to determine if there is a change in oxidation state. (a) The oxidation state of all elements involved remains the same. This reaction is not a redox reaction. (b) The oxidation state of Br changes from 0 to -1, and the oxidation state of K changes from 0 to +1. This reaction is a redox reaction. (c) The oxidation state of C changes from -3 to +4, and the oxidation state of O changes from 0 to -2. This reaction is a redox reaction. (d) The oxidation state of all elements involved remains the same. This reaction is not a redox reaction.

Step 3: Identify oxidized and reduced elements

For redox reactions, identify which elements are oxidized (increase in oxidation state) and which are reduced (decrease in oxidation state). (b) In this redox reaction: \\ K is oxidized (going from 0 to +1). \\ Br is reduced (going from 0 to -1). (c) In this redox reaction: \\ C is oxidized (going from -3 to +4). \\ O is reduced (going from 0 to -2).

Step 4: Identify precipitation or neutralization reactions

For reactions that are not redox reactions, determine if they are precipitation or neutralization reactions based on the products formed during the reaction. (a) This reaction involves an acid (HClO) and a base (H2O) and does not form a precipitate. It is a neutralization reaction. (d) This reaction involves an exchange of ions between the reactants to form a solid precipitate (Zn(OH)2). It is a precipitation reaction. In summary: (a) Neutralization reaction (b) Redox reaction: K is oxidized, and Br is reduced. (c) Redox reaction: C is oxidized, and O is reduced. (d) Precipitation reaction

Key Concepts

Oxidation States

Oxidation states, also known as oxidation numbers, are important in understanding chemical reactions. They help determine whether a chemical reaction is a redox reaction or not. Essentially, oxidation states represent the charge an element would have if all bonds were ionic. Different elements naturally have different typical oxidation states.
For instance:

• Oxygen usually has an oxidation state of -2, except when in peroxides or with fluorine, where it can be different.
• Hydrogen typically is +1, except in metal hydrides where it takes on -1.
• Alkali metals like sodium and potassium typically have oxidation states of +1.

By comparing oxidation states in reactants and products, you can identify which elements are oxidized or reduced: - An increase in the oxidation state shows oxidation (loss of electrons). - A decrease in the oxidation state indicates reduction (gain of electrons).
In our exercise, recognizing changes in the oxidation states helped identify redox reactions among the reactions given. It pointed out which elements had undergone oxidation or reduction.

Neutralization Reactions

Neutralization reactions occur when an acid and a base react together to produce a salt and water. These reactions are characterized by the transfer of protons (H⁺ ions) from the acid to the base. They play a critical role in fields ranging from chemistry to biology, happening in processes such as digestion and industrial manufacturing.
Key features of neutralization reactions include:

• The reaction of H⁺ ions from the acid with OH⁻ ions from the base to form water.
• Often result in the formation of a salt.
• Usually exothermic, releasing heat.

In the provided exercise, reaction (a) serves as an example of a neutralization reaction. Here, the acid HClO reacts with the base water, forming products without undergoing a significant change in the physical state of matter. This demonstrates how neutralization reactions do not necessarily produce a solid precipitate but involve the neutralization of acidity by the basicity of water.

Precipitation Reactions

Precipitation reactions happen when two soluble salts react in a solution to form one or more insoluble solids called precipitates. These reactions are crucial in chemistry for understanding how certain compounds can be separated out of solutions. Precipitates tend to form when the product of a reaction is insoluble in the solution used.
Precipitation reactions are characterized by:

• The formation of a solid product from a liquid solution.
• Often involve ionic compounds swapping partners to form a non-dissolving product.
• Can be predicted using solubility rules for common ion interactions.

In our exercise, reaction (d) is a precipitation reaction. It consists of mixing of aqueous solutions of zinc chloride and sodium hydroxide. The formation of the insoluble compound Zn(OH)₂ solidifies the occurrence of a precipitation reaction, illustrating how such reactions are identified by the visible change of the precipitate falling out of solution.