Problem 64 - Chemistry: The Central Science in SI Units, Global Edition

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, as increasing the temperature increases the fraction of products in an equilibrium mixture. (b) The balanced chemical equation has more molecules on the product side, as increasing the volume of the reaction vessel increases the fraction of products.

Step 1: When the temperature of a reaction at equilibrium is increased, the reaction will shift in the direction of the reaction that can consume the added energy (heat). If the reaction is exothermic, heat is released, and if it is endothermic, heat is absorbed. As increasing the temperature increases the fraction of products, the reaction must be shifting towards the products, indicating that it is an endothermic reaction. So, the answer to question (a) is endothermic. #Step 2: Determine if there are more molecules on the reactant or product side of the balanced chemical equation#

When the volume of a reaction mixture at equilibrium is increased, the pressure decreases, and the system will shift to the side with more moles of gas. Increasing the volume of the reaction vessel results in an increase in the fraction of products. Therefore, the reaction must be shifting towards the products side, which means the product side has more molecules than the reactant side. Thus, the answer to question (b) is that the balanced chemical equation has more molecules on the product side.

Key Concepts

Reaction Dynamics

Reaction dynamics involve the study of how chemical reactions take place. It's all about observing and understanding how reactants transform into products. This includes looking at

• the speed (rate) of the reaction,
• how the reaction is influenced by different conditions,
• and how the particles interact with each other.

In a chemical reaction at equilibrium, such as the one described in the exercise, both the forward and reverse reactions occur at the same rate. When a reaction reaches equilibrium, it doesn't mean that the reactants and products are in equal concentrations. Instead, it means their concentrations remain unchanged over time due to the balanced rates of the forward and reverse reactions.
To understand reaction dynamics deeply, envision a dynamic dance where reactants constantly become products and vice versa but always maintaining a balance.

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental rule in chemistry that predicts how a system at equilibrium responds to disturbances. Imagine a balanced seesaw; if you push one side down, the other side rises to respond. Similarly, when a change (or "stress") is applied to a system at equilibrium:

• the system will adjust to counteract the change and restore equilibrium.
• For example, if you increase the concentration of reactants, the system will shift towards the production of more products.
• Conversely, if you remove some products, the system will try to form more products to compensate.

In the exercise provided, increasing temperature caused the reaction to favor product formation because the system absorbed heat, indicating the reaction was endothermic. Le Chatelier's Principle also predicts that increasing the volume of the container, thus lowering pressure, makes the system shift towards the side with more gas molecules to balance the pressure change.

Endothermic Reactions

Endothermic reactions are unique because they absorb energy, often in the form of heat, from their surroundings. This means:

• These reactions feel cold to the touch because they draw heat away from their environment.
• Energy is considered a reactant in these reactions.

In contrast to exothermic reactions, which release heat into their surroundings, endothermic reactions require continuous energy input to proceed.
In the context of the exercise, when temperature is increased, the reaction shifts towards forming more products. This shift indicates the reaction uses the added heat, signifying it is endothermic. Understanding how endothermic reactions work can help predict how they might behave under different stressors, such as changes in temperature or pressure, similar to the principles described by Le Chatelier's Principle.