Question

Classify each of the following reactions as a combination reaction, decomposition reaction, displacement reaction, or combustion reaction. (a) When they are heated, ammonium dichromate crystals, \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Cr}_{2} \mathrm{O}_{7},\) decompose to give nitrogen, water vapor, and solid chromium(III) oxide, \(\mathrm{Cr}_{2} \mathrm{O}_{3}\). (b) When aqueous ammonium nitrite, \(\mathrm{NH}_{4} \mathrm{NO}_{2}\), is heated, it gives nitrogen and water vapor. (c) When gaseous ammonia, \(\mathrm{NH}_{3}\), reacts with hydrogen chloride gas, HCl, fine crystals of ammonium chloride, \(\mathrm{NH}_{4} \mathrm{Cl}\), are formed (d) Aluminum added to an aqueous solution of sulfuric acid, \(\mathrm{H}_{2} \mathrm{SO}_{4},\) forms a solution of aluminum sulfate, \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} .\) Hydrogen gas is released.

Short answer

(a) Decomposition, (b) Decomposition, (c) Combination, (d) Displacement.

Step-by-step solution

Understanding Combination Reaction

A combination reaction is where two or more substances combine to form a single product. An example of this type of reaction is where gaseous ammonia \((\mathrm{NH}_3)\) reacts with hydrogen chloride gas \((\mathrm{HCl})\) to form ammonium chloride \((\mathrm{NH}_4\mathrm{Cl})\). This is represented by the equation: \( \mathrm{NH}_3 (g) + \mathrm{HCl} (g) \rightarrow \mathrm{NH}_4\mathrm{Cl} (s) \). Since a single product is formed from two reactants, this reaction is classified as a combination reaction.

Identifying Decomposition Reactions

Decomposition reactions occur when a single compound breaks down into two or more products. (a) Ammonium dichromate crystals \((\mathrm{NH}_4)_2 \mathrm{Cr}_2 \mathrm{O}_7)\) decompose into nitrogen, water vapor, and chromium(III) oxide \((\mathrm{Cr}_2\mathrm{O}_3)\) upon heating. This process is a decomposition reaction. Similarly, (b) Aqueous ammonium nitrite \((\mathrm{NH}_4 \mathrm{NO}_2)\) breaks down into nitrogen and water vapor when heated, which is also a decomposition reaction.

Recognizing a Displacement Reaction

A displacement reaction involves an element replacing another in a compound. For reaction (d), aluminum reacts with sulfuric acid \((\mathrm{H}_2 \mathrm{SO}_4)\) to form aluminum sulfate \((\mathrm{Al}_2(\mathrm{SO}_4)_3)\) and hydrogen gas. The displacement here is of hydrogen ions being replaced by aluminum, classifying this as a single displacement reaction.

Key concepts

Combination Reaction

A combination reaction is a fundamental type of chemical reaction where two or more reactants combine to form a single product. This type of reaction is also known as a synthesis reaction. It's like building something new from distinct parts.
For example, when gaseous ammonia (LaTeX: \(\mathrm{NH}_3\)) meets hydrogen chloride gas (LaTeX: \(\mathrm{HCl}\)), they come together to form fine crystals of ammonium chloride (LaTeX: \(\mathrm{NH}_4\mathrm{Cl}\)).
This reaction is represented with the equation: LaTeX: \( \mathrm{NH}_3 (g) + \mathrm{HCl} (g) \rightarrow \mathrm{NH}_4\mathrm{Cl} (s) \).
Not only are such reactions vital in creating new compounds, they exemplify the law of conservation of mass because the mass of the products equals the mass of the reactants.

• Key Point: Two or more simple substances form one complex product.
• Practical Examples: Formation of water from hydrogen and oxygen, rusting of iron when it combines with oxygen.
• Real-World Use: Used in industries for producing various compounds like ammonium nitrate in fertilizers.

Decomposition Reaction

In a decomposition reaction, a single compound breaks down into two or more products. It is essentially the opposite of a combination reaction, where one whole is broken into its parts. These reactions typically require an input of energy in the form of heat, light, or electricity to proceed.
For instance, when ammonium dichromate crystals LaTeX: \((\mathrm{NH}_4)_2 \mathrm{Cr}_2 \mathrm{O}_7\) are heated, they decompose into nitrogen, water vapor, and chromium(III) oxide LaTeX: \((\mathrm{Cr}_2\mathrm{O}_3)\).
This is represented by the equation: LaTeX: \((\mathrm{NH}_4)_2 \mathrm{Cr}_2 \mathrm{O}_7 \rightarrow \mathrm{N}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{Cr}_2\mathrm{O}_3\).

• Key Point: One compound breaks into smaller compounds or elements.
• Practical Examples: Electrolysis of water into hydrogen and oxygen, thermal decomposition of calcium carbonate.
• Real-World Use: Important in processes such as the treatment of wastewater, where hazardous compounds are broken down into safer substances.

Displacement Reaction

A displacement reaction involves an element in a compound being replaced by another element. These reactions are divided into single and double displacement reactions. In single displacement reactions, one element swaps places with another in a compound, typically involving metals or halogens.
Let's consider the reaction where aluminum is added to an aqueous solution of sulfuric acid (LaTeX: \(\mathrm{H}_2 \mathrm{SO}_4\)).
The aluminum displaces hydrogen, forming aluminum sulfate (LaTeX: \(\mathrm{Al}_2(\mathrm{SO}_4)_3\)) and releasing hydrogen gas: LaTeX: \(\mathrm{2Al} + 3\mathrm{H}_2\mathrm{SO}_4 \rightarrow \mathrm{Al}_2(\mathrm{SO}_4)_3 + 3\mathrm{H}_2\).

• Key Point: One element replaces another in a compound.
• Practical Examples: Zinc displacing copper in copper sulfate, a more reactive halogen displacing a less reactive one.
• Real-World Use: Widely used in metallurgy and electroplating industries to extract and purify metals.

Combustion Reaction

Combustion reactions, although not directly listed in our original exercise, are a crucial category of chemical reactions. These involve a substance reacting with oxygen, often releasing energy in the form of light and heat, a process critical for engines and energy generation.
Typically, hydrocarbons are the substances that undergo combustion, producing carbon dioxide and water as products.

• Formula Representation: LaTeX: \(\text{Fuel} + \mathrm{O}_2 \rightarrow \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}\).
• Practical Examples: Burning of fossil fuels in car engines, combustion of wood in fireplaces.
• Real-World Use: Essential for heating, transportation, and energy production but also linked to environmental pollution.

The underlying mechanism of a combustion reaction involves the exothermic process releasing huge amounts of energy, which is why it's commonly used for power in engines.