Problem 99 - Chemistry and Chemical Reactivity

Question

Without doing calculations, decide whether each of the following is product- or reactant-favored. (a) the combustion of natural gas (b) the decomposition of glucose, \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6},\) to carbon and water

Step-by-Step Solution

Answer

(a) Product-favored; (b) Reactant-favored.

Step 1: Understanding Product-Favored Reactions

A product-favored reaction is one in which the products are more stable than the reactants, and the equilibrium lies to the right. These reactions typically release energy, often in the form of heat (exothermic), and result in the formation of more stable compounds.

Step 2: Analysis of Combustion Reaction

Combustion reactions, such as the combustion of natural gas (methane combustion: \( ext{CH}_4 + 2 ext{O}_2 ightarrow ext{CO}_2 + 2 ext{H}_2 ext{O}\)), are typically exothermic and lead to the formation of carbon dioxide and water, which are more stable products. Therefore, combustion reactions are generally product-favored.

Step 3: Understanding Reactant-Favored Reactions

A reactant-favored reaction is one in which the reactants are more stable than the products, and the equilibrium lies to the left. These reactions might require an input of energy to proceed.

Step 4: Analysis of Glucose Decomposition

Decomposition of glucose to carbon and water is an unfavorable process under standard conditions. This is because glucose is a relatively stable molecule, and breaking it down into less ordered forms (carbon is less energetically favorable) requires energy. Thus, it is reactant-favored.

Key Concepts

Combustion Reactions

Combustion reactions are essential processes that occur when a fuel reacts with an oxidant, such as oxygen, to release energy. These reactions are typically exothermic, meaning they emit heat and sometimes light.

Some key characteristics include:

• They usually involve hydrocarbons like methane or propane, common components in natural gas.
• The products of a complete combustion are typically carbon dioxide (\( ext{CO}_2\)) and water (\( ext{H}_2 ext{O}\)).
• Incomplete combustion can result in the formation of carbon monoxide (\( ext{CO}\)) or soot.

In the combustion of natural gas, the reactant, methane (\( ext{CH}_4\)), reacts with oxygen (\( ext{O}_2\)) to produce carbon dioxide and water. This process is highly exothermic and product-favored, due to the stability of the output substances.

Equilibrium in Chemistry

In chemistry, equilibrium refers to a state where the rate of the forward reaction equals that of the reverse reaction. At this point, the concentrations of reactants and products remain constant, though not necessarily equal. The equilibrium position indicates whether the products or reactants are favored in the reaction.

Important features of equilibrium include:

• Dynamic nature: Reactants continue to form products and vice versa at the same rate.
• Equilibrium constant (\(K_c\)): A higher value indicates a product-favored reaction, while a lower one suggests reactant-favored.
• Influence of changes in conditions: Adjusting temperature, pressure, or concentrations can shift the equilibrium.

For combustion reactions, due to the high stability of products and the release of energy, the equilibrium generally lies far to the right, favoring products.

Exothermic Reactions

Exothermic reactions release energy into their surroundings, elevating the temperature of the environment. These types of reactions are common and include processes such as combustion and certain oxidation reactions.

Exothermic characteristics include:

• Energy is released, typically in the form of heat.
• Products are more stable than reactants, contributing to the favorability of products.
• Such reactions are self-sustaining once initiated, as they continue to release energy.

An example of an exothermic reaction is the combustion of methane. This reaction releases a significant amount of heat, converting less stable reactants into stable products such as carbon dioxide and water. This inherent release of energy characterizes the reaction as both exothermic and product-favored.