Using a coffee-cup calorimeter, the ΔH_rxn for the dissolution of KCl(s) into KCl(aq) is determined. The initial temperature of the water in the calorimeter is 26.5°C. After the reaction, the temperature is measured again and found to be 23.0°C. What is the sign of ΔH_rxn for this reaction?

• A. Positive
• B. Zero
• C. Cannot be determined
• D. Negative

Answer: (A)

Verified step by step guidance

1. Understand that the dissolution of KCl(s) into KCl(aq) is an endothermic or exothermic process, which will affect the temperature of the solution.
2. Recognize that a coffee-cup calorimeter is used to measure the heat change at constant pressure, which corresponds to the enthalpy change (ΔH) of the reaction.
3. Note the initial temperature of the water is 26.5°C and the final temperature after the dissolution is 23.0°C.
4. Since the temperature of the solution decreases, it indicates that the system absorbed heat from the surroundings, which is characteristic of an endothermic process.
5. Conclude that for an endothermic reaction, the sign of ΔH_rxn is positive, as the system gains heat.