Sodium metal reacts with water to produce hydrogen gas and sodium hydroxide according to the chemical equation shown below. When 0.025 mol of Na is added to 100.00 g of water, the temperature of the resulting solution rises from 25.00°C to 35.75°C. If the reaction is exothermic, what is the sign of the enthalpy change (ΔH) for this reaction?

• A. ΔH is zero
• B. ΔH is positive
• C. ΔH is negative
• D. ΔH cannot be determined

Answer: (C)

Verified step by step guidance

1. Understand the concept of enthalpy change (ΔH): In an exothermic reaction, energy is released to the surroundings, usually in the form of heat, causing the temperature of the surroundings to increase.
2. Identify the type of reaction: The problem states that the reaction is exothermic, which means that the system releases heat to the surroundings.
3. Relate temperature change to enthalpy: Since the temperature of the solution increases from 25.00°C to 35.75°C, this indicates that heat is being released, consistent with an exothermic process.
4. Determine the sign of ΔH: In exothermic reactions, the enthalpy change (ΔH) is negative because the system loses energy to the surroundings.
5. Conclude based on the information: Given that the reaction is exothermic and the temperature of the solution rises, the sign of ΔH for this reaction is negative.