The vaporization of Br₂ from the liquid to the gas state requires 7.4 kcal/mol (31.0 kJ/mol).
a. What is the sign of ∆H for this process? Write a reaction showing heat as a product or reactant.

Verified step by step guidance

1. Understand the concept of enthalpy change (∆H): Enthalpy change is the heat absorbed or released during a chemical or physical process at constant pressure. For endothermic processes, ∆H is positive, and for exothermic processes, ∆H is negative.
2. Identify the type of process: Vaporization is the process of converting a liquid into a gas. This requires energy input to overcome intermolecular forces, making it an endothermic process. Therefore, the sign of ∆H is positive.
3. Write the reaction for the vaporization of bromine (Br₂): The reaction can be written as Br₂(l) → Br₂(g).
4. Include heat in the reaction: Since the process is endothermic, heat is absorbed. Heat is written as a reactant in the reaction: Br₂(l) + heat → Br₂(g).
5. Summarize: The sign of ∆H for the vaporization of Br₂ is positive, and the reaction showing heat as a reactant is Br₂(l) + heat → Br₂(g).

Key Concepts

Enthalpy Change (∆H)

Enthalpy change (∆H) is a measure of the heat content of a system during a process at constant pressure. A positive ∆H indicates that heat is absorbed from the surroundings, while a negative ∆H signifies that heat is released. In the context of vaporization, the process requires energy input, resulting in a positive ∆H.

Endothermic Process

An endothermic process is one that absorbs heat from its surroundings. In the case of vaporization, the transition of a substance from liquid to gas requires energy to overcome intermolecular forces. This absorption of heat is reflected in the positive value of ∆H, indicating that heat is a reactant in the reaction.

Chemical Reaction Representation

Chemical reactions can be represented with a balanced equation that includes reactants, products, and energy changes. For the vaporization of bromine (Br2), the reaction can be written as: Br2(l) + heat → Br2(g). This notation emphasizes that heat is required for the transition from liquid to gas, highlighting the endothermic nature of the process.