Problem 76 - Chemistry Matter and Change

Question

How would these equilibria be affected by decreasing the temperature? $$ \begin{array}{l}{\text { a. } 2 \mathrm{O}_{3}(\mathrm{g}) \rightleftharpoons 3 \mathrm{O}_{2}(\mathrm{g})+\text { heat }} \\ {\text { b. heat }+\mathrm{H}_{2}(\mathrm{g})+\mathrm{F}_{2}(\mathrm{g}) \rightleftharpoons 2 \mathrm{HF}(\mathrm{g})}\end{array} $$

Step-by-Step Solution

Answer

By decreasing the temperature, the equilibrium of reaction a (exothermic) will shift towards the products, increasing the concentration of O₂(g) and decreasing the concentration of O₃(g). In reaction b (endothermic), the equilibrium will shift towards the reactants, increasing the concentrations of H₂(g) and F₂(g), and decreasing the concentration of HF(g).

Step 1: Identify the type of reaction

Before analyzing the effect of temperature change on the equilibrium, we need to identify whether the reaction is exothermic (releases heat) or endothermic (absorbs heat) because this will help determine the direction of the equilibrium shift. For reaction a: 2 O3(g) ⇌ 3 O2(g) + heat Since heat is a product, this is an exothermic reaction (releases heat). For reaction b: Heat + H2(g) + F2(g) ⇌ 2 HF(g) Since heat is a reactant, this is an endothermic reaction (absorbs heat).

Step 2: Apply Le Chatelier's principle to exothermic reaction

For reaction a (exothermic), when the temperature decreases, the equilibrium tries to counteract this change by shifting in the direction that generates heat. That is, the equilibrium will shift towards the products side, favoring the forward reaction to produce more O2(g) and less O3(g). So, decreasing the temperature will lead to an increase in O2(g) and a decrease in O₃(g).

Step 3: Apply Le Chatelier's principle to the endothermic reaction

For reaction b (endothermic), when the temperature decreases, the equilibrium shifts in a direction that counteracts the change by absorbing heat. Hence, the equilibrium will shift towards the reactants, favoring the reverse reaction to produce more H2(g) and F2(g) and less HF(g). So, decreasing the temperature will lead to an increase in H₂(g) and F₂(g) and a decrease in HF(g). To sum up, decreasing the temperature will shift the equilibrium of reaction a towards the products, increasing O₂(g) concentration and decreasing O₃(g) concentration. In reaction b, the equilibrium shifts towards the reactants, increasing H₂(g) and F₂(g) concentrations and decreasing HF(g) concentration.

Key Concepts

Exothermic Reactions

Exothermic reactions are fascinating because they release energy in the form of heat. This happens during the transformation of reactants to products. Thus, in an exothermic reaction, you often find that heat is a product of the reaction. Take for instance the reaction of ozone decomposition:

• The chemical equation can be written as: \[2 \text{O}_3(\text{g}) \rightleftharpoons 3 \text{O}_2(\text{g}) + \text{ heat}\]Here, releasing heat is part of forming oxygen gas \((\text{O}_2)\).


• The presence of heat on the product side indicates that the process releases energy into the environment, helping to stabilize the products compared to reactants.

This type of reaction usually feels warm or hot to touch because energy is being released into the surroundings. Understanding exothermic reactions is key to manipulating chemical equilibria, especially when considering temperature changes.

Endothermic Reactions

Endothermic reactions are quite the opposite, as they absorb energy from their surroundings. They require an input of heat to proceed, and you will typically find heat as a reactant in these reactions' equations. Consider the formation of hydrogen fluoride:

• The chemical equation looks like this: \[\text{heat} + \text{H}_2(\text{g}) + \text{F}_2(\text{g}) \rightleftharpoons 2 \text{HF}(\text{g})\]Here, energy is absorbed to convert hydrogen and fluorine into hydrogen fluoride.


• Heat absorption makes the surroundings feel cooler, as heat is taken in from the environment.

Endothermic processes are crucial for reactions where energy input is necessary to form products. Changes in temperature can significantly alter the direction and yield of these reactions.

Le Chatelier's Principle

Le Chatelier's principle helps predict how changes in conditions affect the equilibrium of a chemical reaction. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system adjusts to counteract the change and restore a new equilibrium.

Effects of Temperature Changes

When the temperature drops, an exothermic reaction like the ozone decomposition shifts towards the product side to produce more heat. The decrease is counteracted by generating more heat:

• For exothermic reactions: Decreasing temperature favors the formation of products as the system shifts to produce more heat.

Conversely, in an endothermic reaction, such as the formation of hydrogen fluoride, reducing the temperature results in a shift toward the reactants. This is because less heat is available to drive the reaction forward:

• For endothermic reactions: Decreasing temperature causes the system to decrease the production of products, resulting in increased reactant concentrations.

Le Chatelier's principle provides a valuable framework for anticipating how changes like temperature will influence the direction of chemical equilibria.