Chemical Energy (College Board AP® Chemistry): Exam Questions

A student is analyzing the energy changes involved in melting a 100.0 g sample of ice at 0.0°C and heating the resulting water to 50.0°C. The heat of fusion for ice is 334 J/g, and the specific heat capacity of water is 4.18 J/g/°C.

Calculate the energy required to melt the ice.

Calculate the energy needed to heat the liquid water to 50.0°C.

Explain why the temperature remains constant during the phase change.

If the student instead boiled the water at 100.0°C, would more or less energy be required? Justify your answer.

A 300.0 g block of iron at 120.0°C is placed into 500.0 g of water at 25.0°C. The specific heat capacity of iron is 0.449 J/g·°C, and the specific heat capacity of water is 4.18 J/g·°C.

Write the heat transfer equation for thermal equilibrium.

Calculate the final temperature of the system when thermal equilibrium is reached.

Explain why the final temperature is between the initial temperatures of the two substances.

If the block were made of aluminum (c = 0.897 J/g·°C) instead of iron, how would the final temperature change?

A student investigates how energy changes during the dissolution of different salts in water.

The student dissolves ammonium chloride (NH₄Cl) in 100.0 mL of water and observes a temperature decrease. Explain whether this process is endothermic or exothermic and justify your answer in terms of energy flow.

The student then dissolves calcium chloride (CaCl₂) in another beaker of water and observes a temperature increase. Explain whether this dissolution is endothermic or exothermic using energy considerations.

The student is given sodium sulfate (Na₂SO₄) and predicts that its dissolution will lead to an increase in entropy. Justify this prediction in terms of molecular-level interactions and changes in disorder.

A student investigates the energy changes that occur when potassium chloride (KCl) dissolves in water. The student adds 4.00 g of solid KCl to 100.0 g of distilled water in a calorimeter and records the temperature change over time. The calorimeter has negligible heat loss, and the specific heat capacity of water is 4.18 J g^{-1 o}C^-1.

The temperature of the water changes from 24.6 ^oC to 21.3 ^oC after the KCl dissolves completely.

Write the balanced chemical equation, including state symbols, that represents the dissolution of solid KCl in water.

Is the dissolution of KCl endothermic or exothermic? Justify your answer using the data from the experiment.

Using the data from the experiment, calculate the amount of heat energy transferred (q) as the KCl dissolves in water. Assume the solution has the same specific heat capacity and mass as the water.

Use your answer from part (c) to calculate the molar enthalpy change (ΔH, in kJ mol^-1) for the dissolution of KCl.

Explain the thermodynamic reason why the dissolution of KCl occurs, even though the temperature of the solution decreases.

If the same amount of KCl were dissolved in only 50.0 g of water, would the temperature change of the solution be greater, smaller, or the same? Justify your answer.