Problem 15 - Introductory Chemistry Atoms First

Question

(a) Rewrite this reaction with the word heat in it: $$ 2 \mathrm{CO}(g)+\mathrm{O}_{2}(g) \rightleftarrows 2 \mathrm{CO}_{2}(g) \quad \Delta E=-563.5 \mathrm{~kJ} $$ (b) Which way does the reaction shift when the temperature is raised? Explain your answer.

Step-by-Step Solution

Answer

(a) The rewritten reaction including "heat" is: \[ 2 CO(g) + O_{2}(g) \rightleftarrows 2 CO_{2}(g) + \text{heat} \] (b) When the temperature is raised, the reaction will shift to the left (towards CO and O2) to absorb the added heat, according to Le Chatelier's principle.

Step 1: I. Identify the heat term in the reaction

The reaction has a ΔE value of -563.5 kJ, which represents the energy released by the reaction as heat.

Step 2: II. Rewrite the reaction including the term "heat"

Since ΔE is negative, the reaction releases energy as heat. We can include this in the reaction as follows: \[ 2 CO(g) + O_{2}(g) \rightleftarrows 2 CO_{2}(g) + \text{heat} \] #b. Identify the direction in which the reaction will shift when the temperature is raised#

Step 3: I. Understand the role of energy in an equilibrium reaction

When energy is added to an equilibrium reaction, the system adjusts itself to minimize the change, in accordance with Le Chatelier's principle. In this case, the added heat will cause the reaction to shift in the direction that absorbs the extra heat.

Step 4: II. Determine which direction the reaction shifts

Since the reaction releases heat when it proceeds to form CO2, the opposite direction (formation of CO and O2) will absorb heat. Therefore, when the temperature is raised, the reaction will shift to the left (towards the reactants CO and O2) to absorb the added heat.

Step 5: III. Explain the answer

In summary, when the temperature is raised, the equilibrium reaction: \[ 2 CO(g) + O_{2}(g) \rightleftarrows 2 CO_{2}(g) + \text{heat} \] will shift to the left, favoring the formation of CO and O2, in order to absorb the added heat and minimize the change in temperature, according to Le Chatelier's principle.

Key Concepts

Exothermic Reactions

Exothermic reactions are chemical reactions that release energy in the form of heat. This occurs when the total energy needed to break the bonds in the reactants is less than the energy released when new bonds form in the products. Consequently, the energy difference is emitted as heat to the surroundings.
In a chemical equation for an exothermic reaction, you will often see the word "heat" or the term "energy" on the product side of the equation. For example, the reaction \(2 \text{CO}(g) + \text{O}_2(g) \rightleftarrows 2 \text{CO}_2(g) + \text{heat}\) is exothermic, as indicated by the negative change in energy \(\Delta E = -563.5 \text{ kJ}\). This value implies that when carbon monoxide \((\text{CO})\) and oxygen \((\text{O}_2)\) react to form carbon dioxide \((\text{CO}_2)\), a substantial amount of heat is released.
Understanding exothermic reactions is crucial because it helps us predict how a reaction will behave under different conditions, such as changes in temperature or pressure.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, meaning that the concentrations of reactants and products remain constant over time. At this point, the system is said to be in a state of balance, although reactions continue to take place.
In the equilibrium of \(2 \text{CO}(g) + \text{O}_2(g) \rightleftarrows 2 \text{CO}_2(g)\), both the conversion of carbon monoxide and oxygen to carbon dioxide and the reverse process occur at equal rates, keeping the system stable.
Equilibrium does not mean that reactants and products are present in equal amounts. Instead, their proportions remain constant as long as the equilibrium is undisturbed. Factors such as temperature, pressure, and concentration can shift an equilibrium, leading to changes in the amounts of reactants and products when the equilibrium adjusts to re-establish balance.

Effect of Temperature on Equilibrium

Temperature changes can significantly affect chemical equilibrium. According to Le Chatelier's Principle, if a system at equilibrium is subjected to a change in temperature, the system will adjust itself to counteract this change. For exothermic reactions, an increase in temperature adds heat to the system.
This added heat acts almost like a "reactant" in the sense that the equilibrium will shift in the direction that removes or uses up the excess heat. For the reaction \(2 \text{CO}(g) + \text{O}_2(g) \rightleftarrows 2 \text{CO}_2(g) + \text{heat}\), increasing the temperature shifts the equilibrium to the left, favoring the formation of \(\text{CO}\) and \(\text{O}_2\).
In summary, when temperature increases, exothermic reactions typically shift towards the reactants to absorb the added heat, while a decrease in temperature would cause the reaction to shift towards the products, releasing heat to the surroundings. Understanding this behavior helps in controlling reactions, especially in industrial settings, where temperature management is crucial for optimizing yield.