Question

Identify each of the following reactions as a precipitation, neutralization, or redox reaction: (a) \(\mathrm{Mg}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{MgCl}_{2}(a q)+\mathrm{H}_{2}(g)\) (b) \(\mathrm{KOH}(a q)+\mathrm{HNO}_{3}(a q) \longrightarrow \mathrm{KNO}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)\) (c) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{HBr}(a q) \longrightarrow$$\mathrm{PbBr}_{2}(s)+2 \mathrm{HNO}_{3}(a q)\) (d) \(\mathrm{Ca}(\mathrm{OH})_{2}(a q)+2 \mathrm{HCl}(a q) \longrightarrow$$2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CaCl}_{2}(a q)\)

Short answer

(a) Redox, (b) Neutralization, (c) Precipitation, (d) Neutralization.

Step-by-step solution

Analyzing Reaction (a)

The reaction is \( \mathrm{Mg}(s)+2 \mathrm{HCl}(aq) \longrightarrow \mathrm{MgCl}_{2}(aq)+\mathrm{H}_{2}(g) \). This involves magnesium \( \mathrm{Mg} \) reacting with hydrochloric acid \( \mathrm{HCl} \). Here, magnesium is oxidized to \( \mathrm{Mg}^{2+} \) and \( \mathrm{H}^+ \) from \( \mathrm{HCl} \) is reduced to \( \mathrm{H}_2 \) gas. This change in oxidation states identifies the reaction as a redox reaction.

Analyzing Reaction (b)

The reaction is \( \mathrm{KOH}(aq)+\mathrm{HNO}_{3}(aq) \longrightarrow \mathrm{KNO}_{3}(aq)+\mathrm{H}_{2} \mathrm{O}(l) \). This reaction involves an acid \( \mathrm{HNO}_{3} \) and a base \( \mathrm{KOH} \) producing water and salt. This is characteristic of a neutralization reaction.

Analyzing Reaction (c)

The reaction is \( \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(aq)+2 \mathrm{HBr}(aq) \longrightarrow \mathrm{PbBr}_{2}(s)+2 \mathrm{HNO}_{3}(aq) \). Here, lead(II) bromide \( \mathrm{PbBr}_{2} \) is formed as a solid precipitate from soluble reactants, which makes it a precipitation reaction.

Analyzing Reaction (d)

The reaction is \( \mathrm{Ca} (\mathrm{OH})_{2}(aq)+2 \mathrm{HCl}(aq) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{CaCl}_{2}(aq) \). An acid \( \mathrm{HCl} \) and a base \( \mathrm{Ca}(\mathrm{OH})_{2} \) are reacting to form water and a soluble salt, indicating a neutralization reaction.

Key concepts

Precipitation Reactions

Precipitation reactions occur when two soluble ionic compounds in aqueous solution react to form an insoluble compound, which then settles out of the solution as a solid, known as a precipitate. These reactions are a key part of many laboratory and real-world chemical processes, including water treatment and the isolation of product compounds.

• **Key Indicators:** Precipitation reactions are identified by the formation of a solid product from soluble reactants. In our exercise, reaction (c) forms lead(II) bromide (PbBr₂) as a solid precipitate, getting separated from the aqueous solution.
• **Chemical Equation:** A classic precipitation reaction has the general form: \[ \text{AB (aq) + CD (aq) } \rightarrow \text{ AD (s) + CB (aq)} \]. In this format, 'AD' is the precipitate.
• **Predicting Precipitates:** The solubility rules are essential for predicting when a precipitate will form. Compounds like lead bromide are often less soluble, leading to precipitation.

Recognizing precipitation reactions helps in understanding the process of separating substances based on their solubility characteristics.

Neutralization Reactions

Neutralization reactions involve the reaction of an acid with a base, resulting in the formation of water and a salt. These reactions typically occur in aqueous solution and are foundational to understanding pH balance, buffer systems, and titration procedures.

• **Key Characteristics:** Neutralization reactions are marked by the production of water and a salt. For example, in reaction (b), when potassium hydroxide (KOH) reacts with nitric acid (HNO₃), potassium nitrate (KNO₃) is formed along with water.
• **General Reaction Form:** The basic equation for a neutralization reaction is: \[ \text{HA + BOH} \rightarrow \text{ BA + H₂O } \]. Here, 'HA' is the acid and 'BOH' is the base.
• **Applications:** These reactions are used in a variety of applications such as antacid solutions neutralizing stomach acid, and in laboratories during titration to determine concentration.

Understanding neutralization reactions is essential for mastering concepts in both practical chemistry and theoretical acid-base behavior.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between chemical species. These reactions are fundamental to processes such as combustion, photosynthesis, and cellular respiration.

• **Identifying Redox Reactions:** A redox reaction is characterized by changes in oxidation states of the reactants. For example, in reaction (a), magnesium (\text{Mg}) donates electrons and becomes \text{Mg}^{2+}, while hydrogen ions (\text{H}^{+}) gain electrons to form hydrogen gas (\text{H}_{2}).
• **Oxidation and Reduction:** Oxidation refers to the loss of electrons, while reduction refers to gaining electrons. The mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) can help remember this.
• **Redox Equation:** Generally written as: \[ \text{A + B} \rightarrow \text{A}^{n+} \text{ + B}^{m-}\]. In this equation, A is oxidized, and B is reduced.
• **Practical Examples:** Redox reactions are involved in battery operations, rusting of metals, and providing energy in living organisms through oxidative phosphorylation.

Grasping the mechanisms of redox reactions is vital for understanding both chemical reactivity and energy transfer.