Question

Classify each of the following reactions as a precipitation, acidbase neutralization, or oxidation-reduction: (a) \(\mathrm{S}_{8}(s)+8 \mathrm{O}_{2}(g) \rightarrow 8 \mathrm{SO}_{2}(g)\) (b) \(\mathrm{NiCl}_{2}(a q)+\mathrm{Na}_{2} \mathrm{~S}(a q) \rightarrow \mathrm{NiS}(s)+2 \mathrm{NaCl}(a q)\) (c) \(2 \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}(a q)+\mathrm{Ba}(\mathrm{OH})_{2}(a q) \rightarrow\) \(\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \mathrm{Ba}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\)

Short answer

(a) Oxidation-reduction, (b) Precipitation, (c) Acid-base neutralization.

Step-by-step solution

Identify Reaction Type for (a)

Examine the reaction \( \mathrm{S}_{8}(s)+8 \mathrm{O}_{2}(g) \rightarrow 8 \mathrm{SO}_{2}(g) \). In this reaction, sulfur \((\mathrm{S}_8)\) combines with oxygen \((\mathrm{O}_2)\) to form sulfur dioxide \((\mathrm{SO}_2)\), involving a transfer of electrons (oxidation of sulfur). Since there are changes in oxidation states, this is an oxidation-reduction (redox) reaction.

Identify Reaction Type for (b)

Examine the reaction \( \mathrm{NiCl}_{2}(aq)+\mathrm{Na}_{2} \mathrm{S}(aq) \rightarrow \mathrm{NiS}(s)+2\mathrm{NaCl}(aq) \). Nickel sulfide \((\mathrm{NiS})\) forms as a solid from the reaction of two aqueous solutions \((\mathrm{NiCl}_2\) and \(\mathrm{Na}_2 \mathrm{S})\). This is a precipitate forming from mixing two solutions, so it is classified as a precipitation reaction.

Identify Reaction Type for (c)

Examine the reaction \( 2 \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}(aq)+\mathrm{Ba(OH)}_{2}(aq) \rightarrow \left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \mathrm{Ba}(aq)+2 \mathrm{H}_{2} \mathrm{O}(l) \). Here, acetic acid \((\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H})\) reacts with barium hydroxide \((\mathrm{Ba(OH)}_{2})\), producing water and a salt \((\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} \mathrm{Ba})\). This is typical of an acid-base neutralization reaction.

Key concepts

Oxidation-Reduction Reactions

In oxidation-reduction reactions, also known as redox reactions, there is a transfer of electrons between different substances. These reactions involve two key processes: oxidation and reduction. Oxidation refers to the loss of electrons from a molecule, atom, or ion, while reduction refers to the gaining of electrons.

For example, in the reaction \[ \mathrm{S}_{8}(s) + 8 \mathrm{O}_{2}(g) \rightarrow 8 \mathrm{SO}_{2}(g) \] sulfur undergoes oxidation as it loses electrons and its oxidation state increases. Concurrently, oxygen goes through reduction as it gains electrons, resulting in a decrease in its oxidation state.

Key characteristics of redox reactions include:

• Changes in oxidation states of substances involved.
• Involvement of electron transfer between reactants.
• Often, redox reactions release energy, making them important in both biological systems (like respiration) and industrial applications (such as in batteries).

Understanding the basic principles of redox reactions helps in identifying and balancing these reactions in chemistry.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions react to form an insoluble solid, known as a precipitate. These reactions typically involve ionic compounds where the ions exchange partners to form new compounds.

Consider the reaction \[ \mathrm{NiCl}_{2}(aq) + \mathrm{Na}_{2} \mathrm{S}(aq) \rightarrow \mathrm{NiS}(s) + 2 \mathrm{NaCl}(aq) \]Here, nickel sulfide (\(\mathrm{NiS}\)) emerges as a solid, indicating the formation of a precipitate.

Notable aspects of precipitation reactions include:

• The formation of a solid from two aqueous solutions.
• They are often used in laboratories to isolate particular substances from solutions.
• They can be predicted using solubility rules which dictate the formation of possible insoluble products.

These reactions are vital in processes like water treatment, where harmful substances are removed as precipitates.

Acid-Base Neutralization Reactions

Acid-base neutralization reactions occur when an acid reacts with a base to form a salt and water. This is an important class of chemical reactions because of their occurrence in everyday life and industrial processes.

An example is seen in the reaction \[ 2 \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}(aq) + \mathrm{Ba(OH)}_{2}(aq) \rightarrow (\mathrm{CH}_{3} \mathrm{CO}_{2})_{2} \mathrm{Ba}(aq) + 2 \mathrm{H}_{2} \mathrm{O}(l) \]where acetic acid reacts with barium hydroxide.

Key points concerning neutralization reactions include:

• The reaction of hydrogen ions from the acid with hydroxide ions from the base, forming water.
• These reactions typically lead to a change in pH, often moving towards a neutral pH of 7.
• Neutralization reactions are employed in various applications, such as antacid tablets to relieve heartburn.

Acid-base reactions are fundamental in chemistry, serving as the basis for acid-base titrations and being crucial in maintaining pH balance in biological systems.