Problem 105 - General Chemistry

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.

Step-by-Step Solution

Answer

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

Step 1: Identify reaction type for reaction a

In reaction (a), ammonium dichromate is decomposing into nitrogen, water vapor, and chromium(III) oxide when heated. This breakdown of a single compound into multiple products is typical of a decomposition reaction.

Step 2: Identify reaction type for reaction b

In reaction (b), ammonium nitrite decomposes to nitrogen and water vapor when heated. Similar to reaction (a), this is also a decomposition reaction because one compound breaks apart into two or more products.

Step 3: Identify reaction type for reaction c

In reaction (c), ammonia reacts with hydrogen chloride to form ammonium chloride. This reaction involves two reactants combining to form a single product, which characterizes a combination reaction.

Step 4: Identify reaction type for reaction d

In reaction (d), aluminum reacts with sulfuric acid, resulting in aluminum sulfate and the release of hydrogen gas. This is a single displacement reaction, where aluminum displaces hydrogen from sulfuric acid to form aluminum sulfate and hydrogen gas is released.

Key Concepts

Combination Reaction

A combination reaction, also known as a synthesis reaction, is a type of chemical reaction where two or more reactants unite to form a single product. This reaction is typically represented by the equation:

• \[ A + B \rightarrow AB \]

Combination reactions are common in many industrial processes and real-world applications. For example, the reaction between ammonia \( \text{NH}_3 \) and hydrogen chloride \( \text{HCl} \) to produce ammonium chloride \( \text{NH}_4 \text{Cl} \) showcases a classic combination reaction. Here, both reactants combine to form a stable ionic compound.
These reactions are vital as they often form new substances with different properties, utilized in various fields like pharmaceuticals, agriculture, and material science.

Decomposition Reaction

A decomposition reaction is essentially the reverse of a combination reaction. It involves the breaking down of a single compound into two or more simpler substances. This can be expressed in general terms as:

• \[ AB \rightarrow A + B \]

Decomposition reactions require energy input, usually in the form of heat, light, or electricity, to occur. For instance, the decomposition of ammonium dichromate \( \left(\text{NH}_4\right)_2 \text{Cr}_2 \text{O}_7 \) into nitrogen, water vapor, and chromium(III) oxide when heated is an example of this type of reaction.
Decomposition reactions are crucial in processes like the recycling of materials and treatment of waste, where breaking down compounds is essential for material recovery and pollution control.

Displacement Reaction

In a displacement reaction, also known as a single replacement reaction, one element in a compound is displaced by another, and this can be represented as:

• \[ A + BC \rightarrow AC + B \]

These reactions are important in extracting metals from their ores and in various purification processes. In the reaction where aluminum reacts with sulfuric acid \( \text{H}_2\text{SO}_4 \), aluminum displaces hydrogen, resulting in the formation of aluminum sulfate \( \text{Al}_2\left(\text{SO}_4\right)_3 \) and hydrogen gas is released.
The amount of energy released or absorbed in these reactions can vary significantly, making them useful in different applications, from the industrial production to everyday chemical processes.

Combustion Reaction

Combustion reactions are a type of exothermic reaction, occurring when a substance reacts rapidly with oxygen, releasing energy in the form of light and heat. These reactions are typically represented by:

• \[ ext{Fuel} + ext{O}_2 \rightarrow ext{CO}_2 + ext{H}_2 ext{O} + ext{Energy} \]

Combustion is fundamental to many daily processes, including cooking, heating, and powering vehicles. The reaction involves hydrocarbons (e.g., gasoline or natural gas) reacting with oxygen to release energy.
Understanding combustion reactions is essential not only from an energy production standpoint but also for managing environmental impacts, as it relates to greenhouse gas emissions and air quality.