Problem 76 - Chemistry Matter and Change

Question

How would these equilibria be affected by decreasing the temperature? \begin{equation} \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} \end{equation}

Step-by-Step Solution

Answer

Decreasing temperature shifts reaction a right (more \(\text{O}_2\)) and reaction b left (producing more \(\text{H}_{2}\) and \(\text{F}_{2}\)).

Step 1: Understand Exothermic vs Endothermic

In an equilibrium reaction, decreasing the temperature shifts the equilibrium towards the side that produces heat (exothermic direction). Conversely, increasing the temperature shifts it towards the endothermic direction, which absorbs heat.

Step 2: Analyze Reaction a

For reaction a, \(2 \text{O}_{3}(\text{g}) \rightleftharpoons 3 \text{O}_{2}(\text{g}) + \text{heat}\), heat is a product. This indicates that the reaction is exothermic. Hence, decreasing the temperature shifts the equilibrium towards the right, producing more \(\text{O}_{2}\).

Step 3: Analyze Reaction b

For reaction b, \(\text{heat} + \text{H}_{2}(\text{g}) + \text{F}_{2}(\text{g}) \rightleftharpoons 2\text{HF}(\text{g})\), heat is a reactant. This signals that the reaction is endothermic. Therefore, decreasing the temperature shifts the equilibrium to the left, favoring the formation of \(\text{H}_{2}\) and \(\text{F}_{2}\).

Key Concepts

Exothermic Reaction

Exothermic reactions are processes where energy is released into the surroundings, typically in the form of heat. In chemical terms, these reactions have a negative enthalpy change (ΔH < 0). During such reactions, the products are at a lower energy state than the reactants, meaning the system loses energy. Common examples include combustion reactions, where fuels burn in oxygen, releasing heat and light, and the formation of concrete, where the hardening process gives off heat.
In the context of chemical equilibrium, it's crucial to understand how exothermic reactions respond to changes in temperature. According to Le Chatelier's principle, if the temperature is decreased, the equilibrium will shift to produce more heat. This means for exothermic reactions at equilibrium, a temperature drop will favor the formation of additional products.

Endothermic Reaction

Endothermic reactions are the opposite of exothermic reactions. They absorb energy from the surroundings, resulting in a positive enthalpy change (ΔH > 0). In these reactions, energy is taken in to help break bonds in the reactants, and thus the products have a higher energy state than the reactants. Examples of endothermic processes include the melting of ice into water, where heat is absorbed from the environment, and photosynthesis, wherein plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.
When it comes to equilibrium scenarios, endothermic reactions behave differently. According to Le Chatelier's principle, reducing the temperature will cause the system to shift towards the reactants to absorb heat. Therefore, for an endothermic reaction at equilibrium, a temperature decrease will shift the balance towards the reactants, minimizing the energy deficit.

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemical equilibrium that predicts how a change in conditions can affect the position of equilibrium. The principle states that if an external change, such as concentration, pressure, or temperature, is applied to a system at equilibrium, the system re-adjusts to counteract the change and restore a new equilibrium.
In terms of temperature effects, if a system in equilibrium is subjected to a temperature change, it will shift in a direction that either absorbs or releases heat. For exothermic reactions, decreasing the temperature shifts the equilibrium towards the product side, producing more heat. Meanwhile, for endothermic reactions, lowering the temperature causes the equilibrium to move towards the reactant side to absorb heat. Le Chatelier's principle offers invaluable insight into controlling chemical processes and optimizing reactions in industrial and laboratory settings.