Problem 64 - Chemistry The Central Science In Si Units

Question

For a certain gas-phase reaction, the fraction of products in an equilibrium mixture is increased by either increasing the temperature or by increasing the volume of the reaction vessel. (a) Is the reaction exothermic or endothermic? (b) Does the balanced chemical equation have more molecules on the reactant side or product side?

Step-by-Step Solution

Answer

(a) The reaction is endothermic. (b) More molecules on the product side.

Step 1: Analyze Temperature Effect

Increasing temperature favors the formation of products, suggesting that the reaction is endothermic. In an endothermic reaction, heat is absorbed, so higher temperatures drive the equilibrium towards the product side to absorb excess heat.

Step 2: Analyze Volume Change Effect

Increasing the volume of the reaction vessel shifts the equilibrium towards the side with more gas molecules. Given that the fraction of products increases when volume is increased, the product side must have more moles of gas compared to the reactant side.

Step 3: Determine Reaction Heat Nature

Based on the temperature effect, the reaction is endothermic, as the equilibrium shifts to products with increased temperature.

Step 4: Determine Molecule Distribution

Based on the volume effect, the balanced chemical equation has more gas molecules on the product side than the reactant side.

Key Concepts

Endothermic Reactions

Endothermic reactions are processes in which the system absorbs heat from its surroundings. During these reactions, the energy required to break the bonds in the reactants is greater than the energy released when new bonds are formed in the products. This results in a net absorption of energy that feels as if the system is consuming heat.
Some key characteristics of endothermic reactions include:

• Heat absorption: They absorb heat, which makes them cool down the environment unless an external heat source is provided.
• Temperature-driven equilibrium: In an endothermic reaction, increasing the temperature will shift the equilibrium toward the products because additional heat drives more reactants to convert into products.

Understanding this behavior is crucial in predicting how reactions will respond to changes in temperature, which directly ties into Le Chatelier’s Principle and the overall dynamics of chemical equilibrium.

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental rule used to predict the effect of a change in conditions on chemical equilibria. Essentially, it states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. This principle helps chemists determine which direction a reaction will progress under different circumstances.
Some important aspects include:

• Temperature changes: If the temperature of a system is increased, the equilibrium will shift towards the direction that absorbs heat. For endothermic reactions, this means shifting towards the products.
• Pressure and volume changes: Increasing the volume of a gas-phase system decreases its overall pressure, causing the equilibrium to shift toward the side with more gas molecules (as pressure adjustment is easier this way).
• Concentration changes: Adding more reactants will shift the equilibrium to favor more products, and vice versa.

Applying Le Chatelier's Principle is crucial in understanding how external changes influence the position of equilibrium, making it a powerful tool for predicting and controlling chemical reactions.

Gas Laws

Gas laws are essential principles that describe the behavior of gases in response to changes in conditions such as pressure, volume, and temperature. They are crucial in understanding reactions involving gases, especially at equilibrium.
Some fundamental gas laws include:

• Boyle's Law: States that the pressure of a gas is inversely proportional to its volume at constant temperature, \( P \propto \frac{1}{V} \).
• Charles's Law: States that the volume of a gas is directly proportional to its temperature at constant pressure, \( V \propto T \).
• Avogadro's Law: Indicates that the volume of a gas is directly proportional to the number of moles at constant temperature and pressure, \( V \propto n \).

These laws help in understanding the effects of changes in conditions on the behavior of gases. They are especially helpful in interpreting Le Chatelier’s Principle as applied to changes in pressure and volume in chemical equilibria.