Question

Balance each of the following redox reactions and classify it as a combination, decomposition, or displacement reaction: (a) \(\mathrm{Sb}(s)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{SbCl}_{3}(s)\) (b) \(\mathrm{AsH}_{3}(g) \longrightarrow \mathrm{As}(s)+\mathrm{H}_{2}(g)\) (c) \(\mathrm{Zn}(s)+\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2}(a q) \longrightarrow \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\mathrm{Fe}(s)\)

Short answer

a) Combination, balanced: \(2\text{Sb}(s) + 3\text{Cl}_2(g) \rightarrow 2\text{SbCl}_3(s)\). b) Decomposition, balanced: \(2\text{AsH}_3(g) \rightarrow 2\text{As}(s) + 3\text{H}_2(g)\). c) Displacement, balanced: \( \text{Zn}(s) + \text{Fe}(\text{NO}_3)_2(aq) \rightarrow \text{Zn}(\text{NO}_3)_2(aq) + \text{Fe}(s) \)

Step-by-step solution

- Identify the type of reaction for (a)

To determine the type of reaction for \(\text{Sb}(s) + \text{Cl}_{2}(g) \rightarrow \text{SbCl}_3(s)\), observe that solids (\text{Sb}) and gas (\text{Cl}_2) combine to form a compound (\text{SbCl}_3). This is a combination reaction.

- Balance equation (a)

Check both sides of the reaction. One \(\text{Sb}\) atom and three \(\text{Cl}\) atoms are required on both sides to balance. Balance the equation as follows: \[ \text{Sb}(s) + \frac{3}{2}\text{Cl}_2(g) \rightarrow \text{SbCl}_3(s) \] To avoid fractional coefficients, multiply through by 2: \[ \text{2Sb}(s) + 3\text{Cl}_2(g) \rightarrow 2\text{SbCl}_3(s) \]

- Identify the type of reaction for (b)

For the reaction \(\text{AsH}_3(g) \rightarrow \text{As}(s) + \text{H}_2(g)\), the compound \(\text{AsH}_3\) breaks into elemental solids and gases, indicating a decomposition reaction.

- Balance equation (b)

Make sure the number of atoms on each side matches. Balance the equation as: \[ 2\text{AsH}_3(g) \rightarrow 2\text{As}(s) + 3\text{H}_2(g) \]

- Identify the type of reaction for (c)

The reaction \( \text{Zn}(s) + \text{Fe}\text{(NO}_3)_2(aq) \rightarrow \text{Zn}\text{(NO}_3)_2(aq) + \text{Fe}(s) \) is a displacement reaction because zinc displaces iron from its compound.

- Balance equation (c)

Ensure the number of atoms on both sides is equal. The atoms are already balanced: \[ \text{Zn}(s) + \text{Fe}(\text{NO}_3)_2(aq) \rightarrow \text{Zn}(\text{NO}_3)_2(aq) + \text{Fe}(s) \]

Key concepts

Combination Reaction

A combination reaction occurs when two or more substances combine to form a single product. Each reactant adds to the other to create a brand new compound. For example, in the reaction Sb(s) + Cl₂(g) → SbCl₃(s), solid antimony (Sb) reacts with chlorine gas (Cl₂) to form antimony trichloride (SbCl₃). This is a classic example of a combination reaction. combination reactions are usually straightforward to identify, as you will see multiple reactants joining together to create one single product.

Key points to remember about combination reactions:

• They involve two or more reactants.
• Only one product is formed.
• The reaction usually releases energy (exothermic).

To balance such reactions, ensure the number of atoms of each element is equal on both sides. You may need to multiply the coefficients to avoid fractional results.

Decomposition Reaction

In a decomposition reaction, a single compound breaks down into two or more simpler substances. This type of reaction is essentially the reverse of a combination reaction. For instance, in the reaction AsH₃(g) → As(s) + H₂(g), arsenic trihydride (AsH₃) decomposes into arsenic solids (As) and hydrogen gas (H₂).

This is a typical decomposition reaction where one compound breaks down.

Key points about decomposition reactions:

• One reactant breaks into two or more products.
• They often require an input of energy (endothermic) such as heat, light, or electricity.

When balancing these reactions, make sure the total number of each type of atom is the same on both sides of the equation. If necessary, adjust the coefficients to maintain the balance.

Displacement Reaction

A displacement reaction, also known as a replacement reaction, occurs when an element displaces another element in a compound. In the reaction Zn(s) + Fe(NO₃)₂(aq) → Zn(NO₃)₂(aq) + Fe(s), solid zinc (Zn) displaces iron (Fe) from its nitrate compound, resulting in zinc nitrate (Zn(NO₃)₂) and solid iron (Fe). This is a clear example of a displacement reaction.

Key points about displacement reactions:

• One element replaces another in a compound.
• The reactivity of the elements determines the success of the displacement.

Balancing these reactions involves ensuring that the number of atoms for each element matches on both sides of the equation. Simple inspection and adjustment of coefficients are usually sufficient.