Question

Which of the following processes has the most exothermic heat of reaction \(\Delta\) A. Combustion of ethane B. Combustion of propane C. Combustion of \(n\)-butane D. Combustion of isobutane

Short answer

C. Combustion of n-butane

Step-by-step solution

Understanding Exothermic Reactions

Exothermic reactions release heat, and the amount of heat released can be compared using the heats of combustion for the substances involved.

Compare Heats of Combustion

The heat of combustion is the energy released when one mole of a compound burns completely in oxygen. Generally, the larger the molecule, the more bonds that can be broken and formed, resulting in more energy release.

Analyzing Combustion Reactions

We need to compare the combustion reactions of ethane (C2H6), propane (C3H8), n-butane (C4H10), and isobutane (C4H10). Among these, n-butane and isobutane have the most carbon atoms, leading to more bonds being broken and thus more energy release.

Identifying the Most Exothermic Reaction

Between n-butane and isobutane, their heats of combustion are very close, but typically, the structural difference causes a slight variation. However, both have more significant heats of combustion compared to ethane and propane.

Key concepts

Combustion Reactions

Combustion reactions are chemical processes where a substance combines with oxygen to release heat and light. These reactions typically produce carbon dioxide and water as byproducts. A simple example is the burning of methane (CH4):

The chemical equation for the combustion of methane is:
\[\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{energy} \]

In this reaction, methane reacts with oxygen to produce carbon dioxide, water, and energy. This energy is released in the form of heat, making it an exothermic reaction. Combustion reactions are crucial for various applications, such as powering engines, heating homes, and generating electricity.

Key features of combustion reactions:

• Involve the reaction of a fuel with oxygen.
• Release energy primarily in the form of heat.
• Produce carbon dioxide and water as typical products.

Heat of Combustion

The heat of combustion is an important concept in understanding how much energy is released during a combustion reaction. It refers to the amount of energy released when one mole of a substance burns completely in oxygen.

For instance, when measuring the heat of combustion for ethane (C2H6), we look at the energy released when one mole of ethane undergoes combustion:
\[\text{C}_2\text{H}_6 + 3.5\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} + \text{energy} \]
This reaction will give the heat of combustion for ethane.

Key points to understand about the heat of combustion:

• It is measured in kilojoules per mole (kJ/mol).
• The larger the molecule, the higher the heat of combustion, as larger molecules typically contain more bonds that release energy when broken.
• Comparing the heat of combustion across different substances helps us determine which substance releases the most energy.

Energy Release

Energy release is a central part of exothermic reactions like combustion. When fuels undergo combustion, chemical bonds in the reactants break, and new bonds form in the products, releasing energy in the process.

The amount of energy released during combustion can be substantial. For example, during the combustion of butane (C4H10), a significant amount of energy is released due to its larger molecule size and the number of bonds:
\[2\text{C}_4\text{H}_{10} + 13\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O} + \text{energy} \]
In this reaction, butane reacts with oxygen to produce carbon dioxide and water while releasing a substantial amount of heat energy.

Important aspects of energy release:

• Occurs when chemical bonds are broken and new bonds form, resulting in the release of energy.
• Larger molecules with more bonds generally release more energy upon combustion.
• The efficiency of energy release from combustion reactions makes them vital for energy production and utilization.