Hydrogen chloride can be made from the reaction of chlorine and hydrogen:
Cl₂(g) + H₂(g) → 2 HCl(g)
For this reaction, K = 26 × 10³³ and ∆H = -44 kcal/mol(-184 kJ/mol) at 25 °C.
a. Is the reaction endothermic or exothermic?

Verified step by step guidance

1. Identify whether the reaction is endothermic or exothermic by analyzing the given enthalpy change (∆H). A negative ∆H value indicates that the reaction releases heat, making it exothermic, while a positive ∆H value would indicate an endothermic reaction.
2. In this problem, the enthalpy change (∆H) is given as -44 kcal/mol (-184 kJ/mol). Since the value is negative, this means the reaction releases heat.
3. Recall the definition of an exothermic reaction: it is a reaction that releases energy in the form of heat to the surroundings, resulting in a decrease in the system's enthalpy.
4. Conclude that the reaction is exothermic because the negative ∆H value confirms that heat is being released during the reaction.
5. Additionally, note that the large equilibrium constant (K = 26 x 10^33) suggests the reaction strongly favors the formation of products, which aligns with the exothermic nature of the reaction.

Key Concepts

Exothermic and Endothermic Reactions

Exothermic reactions release energy, usually in the form of heat, to the surroundings, resulting in a temperature increase. In contrast, endothermic reactions absorb energy from the surroundings, leading to a temperature decrease. The sign of the enthalpy change (∆H) indicates the nature of the reaction: negative values signify exothermic reactions, while positive values indicate endothermic reactions.

Enthalpy Change (∆H)

Enthalpy change (∆H) is a measure of the total heat content of a system during a chemical reaction. It reflects the energy absorbed or released when reactants convert to products. A negative ∆H value, such as -44 kcal/mol in this reaction, indicates that the reaction releases heat, confirming it as exothermic.

Equilibrium Constant (K)

The equilibrium constant (K) quantifies the ratio of the concentrations of products to reactants at equilibrium for a given reaction at a specific temperature. A very large K value, like 26 x 10^33, suggests that the products are favored at equilibrium, indicating that the reaction proceeds almost to completion. This can also imply a strong driving force for the reaction, often associated with exothermic processes.