AP Chemistry 6.5-6.5 Knowledge Check

A 1.0 g sample of a cashew was burned in a calorimeter containing 1000. g of water, and the temperature of the water changed from 20.0°C to 25.0°C. In another experiment, a 3.0 g sample of a marshmallow was burned in a calorimeter containing 2000. g of water, and the temperature of the water changed from 25.0°C to 30.0°C. Based on the data, which of the following can be concluded about the energy content for 1.0 g of each of the two substances? (The specific heat of water is 4.2 J/(g · °C).)

• The combustion of 1.0 g of cashew releases less energy than the combustion of 1.0 g of marshmallow.

• The combustion of 1.0 g of cashew releases the same amount of energy as the combustion of 1.0 g of marshmallow.

• The combustion of 1.0 g of cashew releases more energy than the combustion of 1.0 g of marshmallow.

• No comparison can be made because the two systems started with different masses of food, different masses of water, and different initial temperatures.

In an insulated cup of negligible heat capacity, 50. g of water at 40.0°C is mixed with 30. g of water at 20.0°C. The final temperature of the mixture is closest to

• 22°C

• 27°C

• 30°C

• 33°C

A molecular solid coexists with its liquid phase at its melting point. The solid-liquid mixture is heated, but the temperature does not change while the solid is melting. The best explanation for this phenomenon is that the heat absorbed by the mixture

• is lost to the surroundings very quickly

• is used in overcoming the intermolecular attractions in the solid

• is used in breaking the bonds within the molecules of the solid

• causes the nonbonding electrons in the molecules to move to lower energy levels

A 2.00 mol sample of C₂H₅OH undergoes the phase transition illustrated in the diagram at left. The molar enthalpy of vaporization, ΔH_vap, of C₂H₅OH is +38.6 kJ/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram?

• A. +19.3kJ

• B. +77.2kJ

• C. −19.3kJ

• D. −77.2kJ

How much energy is required to melt 64 g of methane at 90 K? (The molar mass of methane is 16 g/mol.)

• 0.24 kJ

• 3.8 kJ

• 33 kJ

• 60. 60 kJ

2 H2S(g) + 3 O2(g) ⟶ 2 H2O(l) + 2 SO2(g)          ΔH° = － 1120 kJ/molrxn

Based on the reaction represented by the chemical equation shown above, what is the amount of heat released when 4.00 mol of H2S(g) reacts with 9.00 mol of O2(g)?

• -560 kJ

• -1120kJ

• -2240kJ

• -3360kJ

Mg(s) + 2 HCl(aq) ⟶ MgCl2(aq) + H2(g)

The chemical equation shown above represents the reaction between Mg(s) and HCl(aq). When 12.15 g of Mg(s) is added to 500.0mL of 4.0M HCl(aq), 95 kJ of heat is released. The experiment is repeated with 24.30 g of Mg(s) and 500.0 mL of 4.0M HCl(aq). Which of the following gives the correct value for the amount of heat released by the reaction?

• 380 kJ

• 190 kJ

• 95 kJ

• 48 kJ