Topic 4.1: Measuring Enthalpy Change Questions

State the assumption made when the specific heat capacity of water is used for a dilute aqueous solution.

When a reaction is carried out in 100.0 g of water, the temperature rises by 8.4 °C. Calculate the heat released, in kJ. (c = 4.18 J g⁻¹ K⁻¹)

The 3.51 kJ of heat above was released by 0.0250 mol of reactant. Calculate the enthalpy change of reaction, in kJ mol⁻¹, and state its sign with a reason.

State two assumptions made when calculating an enthalpy change from a simple calorimetry experiment, and explain how each affects the result.

Describe two practical changes that would reduce the heat lost in a calorimetry experiment, and explain why each helps.

Distinguish between an exothermic and an endothermic reaction in terms of enthalpy change and the temperature of the surroundings.

A student dissolves ammonium nitrate in water and the temperature of the solution falls from 21.0 °C to 15.5 °C. State whether the process is exothermic or endothermic, give the sign of ΔH, and explain your reasoning.

Explain, in terms of bond breaking and bond making, why a reaction is exothermic.

Explain why the mass used in a calorimetry calculation is that of the solution, not of the solid added.

A 2.50 g sample of a fuel is burned completely and the heat released raises the temperature of 250.0 g of water from 18.0 °C to 47.4 °C.

Calculate the heat energy, in kJ, transferred to the water.

(c(water) = 4.18 J g⁻¹ K⁻¹.)

When ammonium nitrate is dissolved in water in a beaker, the temperature of the water drops sharply.

State whether the process is exothermic or endothermic, the sign of ΔH, and the direction of energy transfer between the system and the surroundings. [3]

Burning 0.500 g of methanol, CH₃OH (M = 32.05 g mol⁻¹), transfers 11.3 kJ of heat to the water in a calorimeter, and the water temperature rises.

Calculate the enthalpy of combustion of methanol, Δ$H_c$, in kJ mol⁻¹, and explain the sign of your answer.

Sketch and fully label the energy profile for an exothermic reaction.

Your labels should include the reactants, the products, the activation energy Eₐ and the enthalpy change ΔH. [3]

In an exothermic reaction, which statement is correct?

100 g of water rises by 5.0 °C. What energy has it absorbed? (c = 4.18 J g⁻¹ °C⁻¹)

Sketch and describe the enthalpy level diagram for an exothermic reaction, labelling the enthalpy change and the activation energy.

Outline two reasons why a calorimetry experiment underestimates the enthalpy change of combustion.

Draw how the graph is extrapolated to find the true temperature rise.