Question

Balance the following equations, and indicate whether they are combination, decomposition, or combustion reactions: (a) \(\mathrm{Al}(\mathrm{s})+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{AlCl}_{3}(\mathrm{~s})\) (b) \(\mathrm{C}_{2} \mathrm{H}_{4}(g)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) (c) \(\mathrm{Li}(s)+\mathrm{N}_{2}(g) \longrightarrow \mathrm{Li}_{3} \mathrm{~N}(s)\) (d) \(\mathrm{PbCO}_{3}(s) \longrightarrow \mathrm{PbO}(s)+\mathrm{CO}_{2}(g)\) (e) \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}_{2}(l)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\)

Short answer

(a) Combination reaction: \(2\mathrm{Al(s)} + 3\mathrm{Cl}_{2}(g) \longrightarrow 2\mathrm{AlCl}_{3}(s)\) (b) Combustion reaction: \(\mathrm{C}_{2} \mathrm{H}_{4}(g) + 3\mathrm{O}_{2}(g) \longrightarrow 2\mathrm{CO}_{2}(g) + 2\mathrm{H}_{2} \mathrm{O}(g)\) (c) Combination reaction: \(6\mathrm{Li(s)} + \mathrm{N}_{2}(g) \longrightarrow 2\mathrm{Li}_{3} \mathrm{N}(s)\) (d) Decomposition reaction: \(\mathrm{PbCO}_{3}(s) \longrightarrow \mathrm{PbO}(s) + \mathrm{CO}_{2}(g)\) (e) Combustion reaction: \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}_{2}(l) + 9\mathrm{O}_{2}(g) \longrightarrow 7\mathrm{CO}_{2}(g) + 4\mathrm{H}_{2} \mathrm{O}(g)\)

Step-by-step solution

Identify the reaction type

The given reaction is between a metal (Al) and a non-metal (Cl₂), which forms a compound (AlCl₃). This is a combination reaction.

Balance the equation

To balance the equation, make sure the number of atoms of each element is the same on both sides of the equation: \(\mathrm{2Al(s)} + \mathrm{3Cl}_{2}(g) \longrightarrow \mathrm{2AlCl}_{3}(s)\) (b)

Identify the reaction type

The given reaction has a hydrocarbon (C₂H₄) reacting with oxygen (O₂) and producing carbon dioxide (CO₂) and water (H₂O). This is a combustion reaction.

Balance the equation

To balance the equation, make sure the number of atoms of each element is the same on both sides of the equation: \(\mathrm{C}_{2} \mathrm{H}_{4}(g) + \mathrm{3O}_{2}(g) \longrightarrow \mathrm{2CO}_{2}(g) + \mathrm{2H}_{2} \mathrm{O}(g)\) (c)

Identify the reaction type

The given reaction is between a metal (Li) and a non-metal (N₂), which forms a compound (Li₃N). This is a combination reaction.

Balance the equation

To balance the equation, make sure the number of atoms of each element is the same on both sides of the equation: \(\mathrm{6Li(s)} + \mathrm{N}_{2}(g) \longrightarrow \mathrm{2Li}_{3} \mathrm{N}(s)\) (d)

Identify the reaction type

The given reaction has a single compound (PbCO₃) breaking down into two simpler products (PbO and CO₂). This is a decomposition reaction.

Balance the equation

The equation is already balanced, as the number of atoms of each element is the same on both sides of the equation: \(\mathrm{PbCO}_{3}(s) \longrightarrow \mathrm{PbO}(s) + \mathrm{CO}_{2}(g)\) (e)

Identify the reaction type

The given reaction has a hydrocarbon (C₇H₈O₂) reacting with oxygen (O₂) and producing carbon dioxide (CO₂) and water (H₂O). This is a combustion reaction.

Balance the equation

To balance the equation, make sure the number of atoms of each element is the same on both sides of the equation: \(\mathrm{C}_{7} \mathrm{H}_{8} \mathrm{O}_{2}(l) + \mathrm{9O}_{2}(g) \longrightarrow \mathrm{7CO}_{2}(g) + \mathrm{4H}_{2} \mathrm{O}(g)\)

Key concepts

Combination Reaction

Combination reactions are a fundamental type of chemical reaction where two or more simple substances unite to form a more complex compound. These are often straightforward to identify because they typically involve element-to-compound transformations. For example, when aluminum (Al) and chlorine gas (Cl₂) chemically react to form aluminum chloride (AlCl₃), it's a classic combination reaction.

In this type of reaction, atoms come together to create a new, single product, simplifying our work of balancing the chemical equation. You just have to ensure that the total number of each type of atom remains the same on both sides of the equation. In the case of Al and Cl₂, the balanced equation would be:

\(2 \text{Al(s)} + 3 \text{Cl}_2(g) \rightarrow 2 \text{AlCl}_3(s)\).

Key points to remember about combination reactions:

• They result in a single product.
• These reactions help form more complex substances from simpler ones.
• Balancing often starts by ensuring that each type of atom, present in any reactant, appears in the product.

Decomposition Reaction

A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. This is essentially the opposite of a combination reaction. An example is when lead carbonate (PbCO₃) decomposes into lead oxide (PbO) and carbon dioxide (CO₂).

These reactions are often initiated by heating the compound or using a catalyst. The equation demonstrates how one reactant splits to yield multiple products:

\( \text{PbCO}_3(s) \rightarrow \text{PbO(s)} + \text{CO}_2(g) \).

Balancing a decomposition reaction can be straightforward because the reaction typically features a single reactant converting into multiple products already in equilibrium.

Features of decomposition reactions include:

• They break down compounds into simpler products.
• Usually require energy input like heat.
• Important in processes such as digestion and combustion.

Combustion Reaction

Combustion reactions are fascinating and essential in everyday life. These involve a substance, often a hydrocarbon, reacting with oxygen to produce oxides, releasing energy in the form of heat and light.

A classic hydrocarbon combustion involves ethylene (C₂H₄) and oxygen (O₂), producing carbon dioxide (CO₂) and water (H₂O). You recognize combustion reactions by their products and the presence of oxygen as a reactant:

\(\text{C}_2 \text{H}_4(g) + 3 \text{O}_2(g) \rightarrow 2 \text{CO}_2(g) + 2 \text{H}_2 \text{O}(g)\).

Balancing a combustion equation starts by balancing each type of atom from the hydrocarbon, then adjusting for the oxygen.

Main characteristics of combustion reactions include:

• Reactions between a hydrocarbon and oxygen.
• Production of carbon dioxide and water.
• Exothermic nature, releasing heat and sometimes light.

These reactions are critical in energy production and warming our environment, showcasing their significant role in both natural phenomena and industrial processes.