Problem 24 - Chemistry Principles and Practice

Question

Balance the following reactions, and specify which species is oxidized and which is reduced. (a) \(\mathrm{Na}+\mathrm{Hg}_{2} \mathrm{Cl}_{2} \rightarrow \mathrm{NaCl}+\mathrm{Hg}\) (b) \(\mathrm{HCl}+\mathrm{Zn} \rightarrow \mathrm{ZnCl}_{2}+\mathrm{H}_{2}\) (c) \(\mathrm{H}_{2}+\mathrm{CO}_{2} \rightarrow \mathrm{CO}+\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(a) Na oxidized, Hg reduced; (b) Zn oxidized, HCl reduced; (c) H2 oxidized, CO2 reduced.

Step 1: Understanding Oxidation and Reduction

Oxidation refers to the loss of electrons by a species, while reduction refers to the gain of electrons. Identifying changes in oxidation states helps us determine which species are oxidized and which are reduced.

Step 2: Balance Reaction (a)

The unbalanced reaction is \( \mathrm{Na} + \mathrm{Hg}_{2} \mathrm{Cl}_{2} \rightarrow \mathrm{NaCl} + \mathrm{Hg} \). Balance the elements on both sides:- Sodium: \( \mathrm{Na} \rightarrow \mathrm{NaCl} \).- Mercury: \( \mathrm{Hg}_{2}\rightarrow 2 \mathrm{Hg} \).- Chlorine: \( \mathrm{Cl}_{2} \rightarrow 2 \mathrm{Cl} \).Balanced equation: \( 2\,\mathrm{Na} + \mathrm{Hg}_{2} \mathrm{Cl}_{2} \rightarrow 2\,\mathrm{NaCl} + 2\,\mathrm{Hg} \).In this reaction, sodium (Na) is oxidized and mercury (Hg) in \(\mathrm{Hg}_2\mathrm{Cl}_2\) is reduced.

Step 3: Balance Reaction (b)

The unbalanced reaction is \( \mathrm{HCl} + \mathrm{Zn} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{H}_{2} \). Balance the elements:- Zinc: \( \mathrm{Zn} \rightarrow \mathrm{ZnCl}_2 \).- Chlorine: \( 2\,\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 \).- Hydrogen: \( 2\,\mathrm{HCl} \rightarrow \mathrm{H}_2 \).Balanced equation: \( 2\,\mathrm{HCl} + \mathrm{Zn} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2 \).Here, \(\mathrm{Zn}\) is oxidized and hydrogen in \(\mathrm{HCl}\) is reduced.

Step 4: Balance Reaction (c)

The unbalanced reaction is \( \mathrm{H}_{2} + \mathrm{CO}_{2} \rightarrow \mathrm{CO} + \mathrm{H}_{2}\mathrm{O} \). Balance the elements:- Carbon: already balanced \( \mathrm{CO}_{2} \rightarrow \mathrm{CO} \).- Hydrogen: \( \mathrm{H}_{2} \rightarrow \mathrm{H}_{2}\mathrm{O} \).Balanced equation (already balanced): \( \mathrm{H}_{2} + \mathrm{CO}_{2} \rightarrow \mathrm{CO} + \mathrm{H}_{2}\mathrm{O} \).Here, hydrogen in \(\mathrm{H}_2\) is oxidized, and carbon in \(\mathrm{CO}_2 \) is reduced.

Step 5: Conclusion

In (a), \(\mathrm{Na}\) is oxidized and \(\mathrm{Hg}_2\mathrm{Cl}_2\)'s \(\mathrm{Hg}\) is reduced. In (b), \(\mathrm{Zn}\) is oxidized and \(\mathrm{HCl}\)'s \(\mathrm{H}\) is reduced. In (c), \(\mathrm{H}_2\) is oxidized and \(\mathrm{CO}_2\)'s \(\mathrm{C}\) is reduced.

Key Concepts

Understanding Oxidation

Oxidation is a fundamental concept in redox reactions that refers to the loss of electrons from a chemical species. When a substance undergoes oxidation, its oxidation state increases because it is losing negatively charged electrons. For example, when sodium (\(\mathrm{Na}\)) reacts in a chemical equation, it often loses an electron to become \(\mathrm{Na}^+\).

• Oxidation is characterized by an increase in oxidation state.
• The substance that loses electrons is known as the reducing agent because it "donates" electrons.

During a reaction, identify the element that has lost electrons by looking for an increase in its oxidation state from reactants to products. In our exercises:

• In reaction (a), sodium (\(\mathrm{Na}\)) is oxidized as it goes from elemental \(\mathrm{Na}\) to \(\mathrm{Na}^+\).
• In reaction (b), zinc (\(\mathrm{Zn}\)) is oxidized as it changes from \(\mathrm{Zn}\) to \(\mathrm{Zn}^{2+}\).
• In reaction (c), the hydrogen in \(\mathrm{H}_2\) is oxidized as it is transformed into water (\(\mathrm{H}_2\mathrm{O}\)).

Recognizing oxidation is crucial to balancing redox equations and understanding how chemical reactions occur on a molecular level.

The Nature of Reduction

Reduction is the opposite of oxidation and involves the gain of electrons by a chemical species. During reduction, a substance's oxidation state decreases because it receives extra electrons. For example, mercury in \(\mathrm{Hg}_2\mathrm{Cl}_2\) gains electrons when it is reduced to \(\mathrm{Hg}\).

• Reduction is marked by a decrease in oxidation state.
• The substance that gains electrons is considered the oxidizing agent because it "accepts" electrons.

Examining redox reactions helps us identify which species is reduced by observing a decrease in the oxidation state from reactants to products. In the given reactions:

• In reaction (a), mercury (\(\mathrm{Hg}\)) in \(\mathrm{Hg}_2\mathrm{Cl}_2\) is reduced as it gains electrons.
• In reaction (b), the hydrogen in hydrochloric acid (\(\mathrm{HCl}\)) is reduced as it forms \(\mathrm{H}_2\) gas.
• In reaction (c), carbon in \(\mathrm{CO}_2\) is reduced as it loses an oxygen atom to form \(\mathrm{CO}\).

Understanding reduction processes is essential for predicting the outcome of redox reactions and balancing equations accurately.

Mastering Balancing Chemical Equations

Balancing chemical equations is a critical skill in chemistry that ensures that the number of each type of atom is conserved across a chemical reaction. In any chemical equation, the same number of each type of atom must appear on both sides of the equation. This balance reflects the law of conservation of mass.
Balancing redox reactions involves ensuring that both the mass and charge are balanced. When approaching a redox equation:

• Balance elements that appear in only one reactant and one product first.
• Balance the remaining atoms, usually starting with oxygen and hydrogen.
• Ensure that charges are balanced if dealing with ionic species.

In our sample reactions:

• For reaction (a), merely adjusting coefficients balances \(\mathrm{Na}\) and \(\mathrm{Hg}\).
• In reaction (b), adding a coefficient of 2 in front of \(\mathrm{HCl}\) balances both \(\mathrm{Cl}\) and \(\mathrm{H}_2\)
• Reaction (c) is already balanced because each side has the same number of each atom present.

Mastering the balancing of chemical equations facilitates a deeper understanding of chemical reactions, ensuring they adhere to fundamental laws and behave predictably in applications.