Problem 2 - General Chemistry: Principles and Modern Applications

Question

Use information from the chapter to write chemical equations to represent each of the following: (a) reaction of rubidium metal with water (b) thermal decomposition of aqueous \(\mathrm{KHCO}_{3}\) (c) combustion of lithium metal in oxygen gas (d) action of concentrated aqueous \(\mathrm{H}_{2} \mathrm{SO}_{4}\) on \(\mathrm{KCl}(\mathrm{s})\) (e) reaction of lithium hydride with water

Step-by-Step Solution

Answer

(a) \(2 \mathrm{Rb(s)} + 2 \mathrm{H}_{2}\mathrm{O(l)} \rightarrow 2 \mathrm{RbOH(aq)} + \mathrm{H}_{2}\mathrm{(g)}\) \n (b) \(\mathrm{2 KHCO}_{3}(\mathrm{aq}) \rightarrow \mathrm{K}_{2}\mathrm{CO}_{3}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{O(l)} + \mathrm{CO}_{2}\mathrm{(g)}\) \n (c) \( \mathrm{4 Li(s)} + \mathrm{O}_{2}\mathrm{(g)} \rightarrow 2 \mathrm{Li}_{2}\mathrm{O(s)}\) \n (d) \( \mathrm{KCl(s)} + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{KHSO}_{4}(\mathrm{aq}) + \mathrm{HCl(g)}\) \n (e) \( \mathrm{LiH (s)} + \mathrm{H}_{2}\mathrm{O(l)} \rightarrow \mathrm{LiOH (aq)} + \mathrm{H}_{2}\mathrm{(g)}\)

Step 1: Reaction of Rubidium with Water

Rubidium is a metal, hence, when it reacts with water it will be oxidized, producing the rubidium hydroxide and hydrogen (gas). The equation is: \(2 \mathrm{Rb(s)} + 2 \mathrm{H}_{2}\mathrm{O(l)} \rightarrow 2 \mathrm{RbOH(aq)} + \mathrm{H}_{2}\mathrm{(g)}\)

Step 2: Decomposition of Aqueous \( \mathrm{KHCO}_{3}\)

Upon heating, \( \mathrm{KHCO}_{3}\) breaks down into \( \mathrm{K}_{2}\mathrm{CO}_{3}\), water and carbon dioxide. The equation is: \(\mathrm{2 KHCO}_{3}(\mathrm{aq}) \rightarrow \mathrm{K}_{2}\mathrm{CO}_{3}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{O(l)} + \mathrm{CO}_{2}\mathrm{(g)}\)

Step 3: Combustion of Lithium in Oxygen

When lithium burns in oxygen gas, lithium oxide is produced. The equation is: \( \mathrm{4 Li(s)} + \mathrm{O}_{2}\mathrm{(g)} \rightarrow 2 \mathrm{Li}_{2}\mathrm{O(s)}\)

Step 4: Action of Concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4}\) on \( \mathrm{KCl}\)

It means that \( \mathrm{KCl}\) reacts with sulfuric acid to form \( \mathrm{KHSO}_{4}\) and \( \mathrm{HCl}\). The equation is: \( \mathrm{KCl(s)} + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{KHSO}_{4}(\mathrm{aq}) + \mathrm{HCl(g)}\)

Step 5: Reaction of Lithium Hydride with Water

Lithium hydride reacts with water to produce lithium hydroxide and hydrogen. The equation is: \( \mathrm{LiH (s)} + \mathrm{H}_{2}\mathrm{O(l)} \rightarrow \mathrm{LiOH (aq)} + \mathrm{H}_{2}\mathrm{(g)}\)

Key Concepts

Reaction of Metals with Water

Understanding the interaction between metals and water is vital in chemistry, as it's a common reaction illustrating the reactivity of metals. When metals like rubidium (Rb) react with water, they tend to lose electrons (oxidation) and form hydroxides. For example, rubidium reacts with water to form rubidium hydroxide and hydrogen gas as shown in the chemical equation:
\(2 \text{Rb(s)} + 2 \text{H}_2\text{O(l)} \rightarrow 2 \text{RbOH(aq)} + \text{H}_2\text{(g)}\).
This type of reaction is exothermic, meaning it releases heat. Additionally, the vigor of this reaction increases as we move down the group in the periodic table, highlighting the increasing reactivity of alkali metals with water.

Thermal Decomposition

Thermal decomposition refers to the breakdown of a compound when heated. It's a type of chemical reaction where a single substance splits into two or more different substances. The decomposition of potassium hydrogen carbonate (KHCO3), an example from the exercise, occurs upon heating and produces potassium carbonate, water, and carbon dioxide:
\(2 \text{KHCO}_3(\text{aq}) \rightarrow \text{K}_2\text{CO}_3(\text{s}) + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\).
These reactions are essential in industries for processes like calcination of limestone to make lime, in baking, and even in fire safety systems.

Combustion Reactions

Combustion reactions are exothermic, meaning they release energy, usually in the form of heat and light. They involve the reaction of a substance with oxygen. An example is the combustion of lithium (Li) in oxygen, yielding lithium oxide:
\(4 \text{Li(s)} + \text{O}_2\text{(g)} \rightarrow 2 \text{Li}_2\text{O(s)}\).
Such reactions are crucial for releasing energy from fuels. In a broader sense, understanding combustion is essential for environmental science, engineering, and safety protocols.

Acid-Base Reactions

Acid-base reactions are characterized by the transfer of protons (H+) between reactants. In our example, when potassium chloride (KCl) reacts with sulfuric acid (H2SO4), it forms potassium bisulfate (KHSO4) and hydrogen chloride gas (HCl), as shown in the equation:
\(\text{KCl(s)} + \text{H}_2\text{SO}_4(\text{aq}) \rightarrow \text{KHSO}_4(\text{aq}) + \text{HCl(g)}\).
These reactions are ubiquitous in chemistry, with uses ranging from titrations in analytical chemistry to the manufacture of a wide array of products in the chemical industry.

Stoichiometry

Stoichiometry is the calculation of reagents and products in chemical reactions. It's a fundamental concept in chemistry that involves using balanced equations to determine the mole ratios of the substances involved. For instance, in the reaction of lithium hydride (LiH) with water, stoichiometry helps us quantify the exact amount of lithium hydroxide and hydrogen produced:
\(\text{LiH (s)} + \text{H}_2\text{O(l)} \rightarrow \text{LiOH (aq)} + \text{H}_2\text{(g)}\).
It is engrained in chemical equations and is critical for determining the theoretical yield in a reaction, which has practical applications in lab settings and commercial production lines.