Problem 39 - Chemistry: The Central Science

Question

Write balanced equations for each of the following reactions. (a) When mercury(II) oxide is heated, it decomposes to form \(\mathrm{O}_{2}\) and mercury metal. (b) When copper(II) nitrate is heated strongly, it decomposes to form copper(II) oxide, nitrogen dioxide, and oxygen. (c) Lead(II) sulfide, \(\mathrm{PbS}(s)\), reacts with ozone to form \(\mathrm{PbSO}_{4}(s)\) and \(\mathrm{O}_{2}(g)\). (d) When heated in air, \(\mathrm{ZnS}(s)\) is converted to \(\mathrm{ZnO}\). (e) Potassium peroxide reacts with \(\mathrm{CO}_{2}(g)\) to give potassium carbonate and \(\mathrm{O}_{2}\) (f) Oxygen is converted to ozone in the upper atmosphere.

Step-by-Step Solution

Answer

(a) \( 2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2 \) (b) \( 2\text{Cu(NO}_3)_2 \rightarrow 2\text{CuO} + 4\text{NO}_2 + \text{O}_2 \) (c) \( \text{PbS} + 2\text{O}_3 \rightarrow \text{PbSO}_4 + \text{O}_2 \) (d) \( \text{ZnS} + \frac{3}{2}\text{O}_2 \rightarrow \text{ZnO} + \text{SO}_2 \) (e) \( 2\text{K}_2\text{O}_2 + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{O}_2 \) (f) \( 3\text{O}_2 \rightarrow 2\text{O}_3 \)

Step 1: 1. Decomposition of Mercury(II) Oxide

Write down the unbalanced equation, considering that Mercury(II) oxide decomposes into O2 and mercury metal: \[ \text{HgO} \rightarrow \text{Hg} + \text{O}_2 \] Now, balance the equation by adjusting the coefficients: \[ 2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2 \]

Step 2: 2. Decomposition of Copper(II) Nitrate

Write down the unbalanced equation, considering that Copper(II) nitrate decomposes into Copper(II) oxide, Nitrogen dioxide, and Oxygen gas: \[ \text{Cu(NO}_3)_2 \rightarrow \text{CuO} + \text{NO}_2 + \text{O}_2 \] Now, balance the equation by adjusting the coefficients: \[ 2\text{Cu(NO}_3)_2 \rightarrow 2\text{CuO} + 4\text{NO}_2 + \text{O}_2 \]

Step 3: 3. Reaction of Lead(II) Sulfide with Ozone

Write down the unbalanced equation, considering that Lead(II) sulfide reacts with ozone to form PbSO4 and O2 gas: \[ \text{PbS} + \text{O}_3 \rightarrow \text{PbSO}_4 + \text{O}_2 \] Now, balance the equation by adjusting the coefficients: \[ \text{PbS} + 2\text{O}_3 \rightarrow \text{PbSO}_4 + \text{O}_2 \]

Step 4: 4. Conversion of Zinc Sulfide to Zinc Oxide

Write down the unbalanced equation, considering that Zinc sulfide is heated in air (O2) and converted to Zinc oxide: \[ \text{ZnS} + \text{O}_2 \rightarrow \text{ZnO} \] Now, balance the equation by adjusting the coefficients: \[ \text{ZnS} + \frac{3}{2}\text{O}_2 \rightarrow \text{ZnO} + \text{SO}_2 \]

Step 5: 5. Reaction of Potassium Peroxide with Carbon Dioxide

Write down the unbalanced equation, considering that Potassium peroxide reacts with Carbon dioxide to form Potassium carbonate and O2 gas: \[ \text{K}_2\text{O}_2 + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{O}_2 \] Now, balance the equation by adjusting the coefficients: \[ 2\text{K}_2\text{O}_2 + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{O}_2 \]

Step 6: 6. Conversion of Oxygen to Ozone

Write down the unbalanced equation, considering that Oxygen gas is converted to Ozone gas in the upper atmosphere: \[ \text{O}_2 \rightarrow \text{O}_3 \] Now, balance the equation by adjusting the coefficients: \[ 3\text{O}_2 \rightarrow 2\text{O}_3 \]

Key Concepts

Decomposition Reaction

In chemistry, a decomposition reaction is a type of chemical change where one compound breaks down into two or more simpler substances. This is usually facilitated by an external condition, such as heat, light, or a catalyst. In the exercise, mercury(II) oxide and copper(II) nitrate undergo decomposition reactions when heated, breaking down into their constituent elements or simpler compounds.

For example, heating mercury(II) oxide results in its decomposition into mercury and oxygen. The balanced chemical equation for this reaction is:
\[ 2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2 \]
This illustrates the conservation of mass and the necessity to balance chemical equations. Decomposition reactions are essential in various industrial processes, including the production of metals and other chemicals.

Reaction Stoichiometry

Reaction stoichiometry involves the quantitative relationship between reactants and products in a chemical reaction. This core concept focuses on the mole ratios that are crucial for comprehending how much of each substance is needed or produced. In the context of the exercise questions, stoichiometry dictates the coefficients assigned to each molecular species in the balanced equations to ensure mass is conserved.

For instance, when balancing the reaction for the decomposition of copper(II) nitrate, the coefficients are adjusted to ensure that the number of atoms for each element in the reactants side is equal to the number of atoms in the products side. The balanced equation is expressed as:
\[ 2\text{Cu(NO}_3)_2 \rightarrow 2\text{CuO} + 4\text{NO}_2 + \text{O}_2 \]
Understanding and applying stoichiometry is fundamental in predicting the outcomes of chemical reactions and in designing experiments that produce a desired amount of product.

Redox Reactions

Redox reactions are a family of reactions that are concerned with the transfer of electrons between species. This type of reaction is composed of two complementary processes: oxidation, where a species loses electrons, and reduction, where a species gains electrons. The term 'redox' is actually a portmanteau of reduction and oxidation.

In the provided exercise, the transformation of zinc sulfide (ZnS) to zinc oxide (ZnO) when heated in air can be considered a redox reaction. Zinc sulfide is oxidized to zinc oxide, with sulfur being converted to sulfur dioxide as a byproduct:
\[ \text{ZnS} + \frac{3}{2}\text{O}_2 \rightarrow \text{ZnO} + \text{SO}_2 \]
Identifying oxidizing and reducing agents in a reaction helps chemists understand how reactions proceed and how to control them. It's also crucial in processes such as energy production and in the metabolism of living organisms.

Chemical Equation

A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants, products, and their proportions. A well-balanced chemical equation respects the Law of Conservation of Mass, meaning the number of atoms of each element on the reactant side must equal the number on the product side. Balanced equations also provide information about the stoichiometry of the reaction, which is used for predicting the amounts of reactants needed and products formed.

For example, the synthesis of ozone from oxygen in the upper atmosphere is depicted with the balanced equation:
\[ 3\text{O}_2 \rightarrow 2\text{O}_3 \]
Writing and balancing chemical equations is a fundamental skill in chemistry because it provides a clear and concise way to describe chemical processes. This is essential for both communicating results and interpreting the findings of others within the scientific community.