Q: A metal sample with mass 23.4 g is heated to 100.0°C and then placed in 103.2 g of water at 25.7°C. If the metal loses 669.4 J of heat to the water, what is the specific heat capacity $$c of $$the metal? Use $$q = m \times c \times \Delta T.$$

$$c = \dfrac{669.4}{23.4 \times (100.0 - 25.7)} = 0.39 \dfrac{J}{g \cdot ^\circ C}$$

Q: A sample of NaCl weighing 0.563 g absorbs 22.18 J of heat. Its initial temperature is 20.5°C, and its specific heat ($$c) is$$ $$5.05 \dfrac{J}{g \cdot ^\circ C}. $$What is the final temperature ($$T_f) of $$the solid? Use $$q = m \times c \times \Delta T.$$

$$T_f = 20.5 + \dfrac{22.18}{0.563 \times 5.05} = 28.5^\circ C$$

Q: A bomb calorimeter is used to measure the heat of combustion of a food sample. If the heat capacity of the calorimeter is $$C = 3.50 \dfrac{kJ}{^\circ C}$$ and the temperature rises by $$2.00^\circ C$$, what is the total heat released ($$q_{sample}$$)?

$$q_{sample} = 3.50 \times 2.00 = 7.00 \ kJ$$

Question 1

Which of the following statements best describes an exothermic process in thermochemistry?

Accepted Answer

The system loses heat energy to the surroundings.

Question 2

A metal sample with mass 23.4 g is heated to 100.0°C and then placed in 103.2 g of water at 25.7°C. If the metal loses 669.4 J of heat to the water, what is the specific heat capacity $$c of $$the metal? Use $$q = m 	imes c 	imes \Delta T.$$

Accepted Answer

$$c = \dfrac{669.4}{23.4 	imes (100.0 - 25.7)} = 0.39 \dfrac{J}{g \cdot ^\circ C}$$

Question 3

Which equation correctly expresses the relationship between internal energy ($$\Delta E$$), heat ($$q$$), and work ($$w) in a $$chemical system?

Accepted Answer

$$\Delta E = q + w$$

Question 4

A sample of NaCl weighing 0.563 g absorbs 22.18 J of heat. Its initial temperature is 20.5°C, and its specific heat ($$c) is$$ $$5.05 \dfrac{J}{g \cdot ^\circ C}. $$What is the final temperature ($$T_f) of $$the solid? Use $$q = m 	imes c 	imes \Delta T.$$

Accepted Answer

$$T_f = 20.5 + \dfrac{22.18}{0.563 	imes 5.05} = 28.5^\circ C$$

Question 5

Which of the following best describes the role of a calorimeter in thermochemistry experiments?

Accepted Answer

It indirectly measures the heat absorbed or lost by the system via the surroundings.

Question 6

Which formula is used to calculate the electrostatic potential energy ($$E_{el}) $$between two charges in chemistry?

Accepted Answer

$$E_{el} = \dfrac{k Q_1$ Q_2$}{d}$$

Question 7

A bomb calorimeter is used to measure the heat of combustion of a food sample. If the heat capacity of the calorimeter is $$C = 3.50 \dfrac{kJ}{^\circ C}$$ and the temperature rises by $$2.00^\circ C$$, what is the total heat released ($$q_{sample}$$)?

Accepted Answer

$$q_{sample} = 3.50 	imes 2.00 = 7.00 \ kJ$$

Question 8

Which of the following is true about the sign convention for heat ($$q) $$and work ($$w) in $$thermochemistry?

Accepted Answer

Heat absorbed by the system is positive, and work done on the system is positive.

Question 9

If a gas expands against a constant external pressure, which equation correctly calculates the work done by the system?

Accepted Answer

$$w = -P_{ext} \Delta V$$

Question 10

A chemical reaction occurs in a coffee-cup calorimeter at constant pressure. Which quantity does the heat measured ($$q_{p}) $$correspond to?

Accepted Answer

The change in enthalpy ($$\Delta H) of $$the reaction.

Question 11

Which of the following statements about specific heat capacity ($$c) is $$correct?

Accepted Answer

It is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

Question 12

Which process is endothermic?

Accepted Answer

Melting of ice at room temperature.

Question 13

A reaction has $$\Delta H = -250 \ kJ. $$What does this indicate about the reaction?

Accepted Answer

The reaction is exothermic and releases 250 kJ of heat.

Question 14

Which of the following is the correct unit for enthalpy ($$\Delta H$$)?

Accepted Answer

$$kJ$$

Question 15

A 100.0 g sample of water is heated from 25.0°C to 75.0°C. If the specific heat of water is $$4.184 \dfrac{J}{g \cdot ^\circ C}$$, how much heat ($$q) is $$absorbed?

Accepted Answer

$$q = 100.0 	imes 4.184 	imes (75.0 - 25.0) = 20,920 \ J$$

Question 16

Which of the following is NOT a form of energy transfer in thermochemistry?

Accepted Answer

Radiation

Question 17

Which statement about the law of conservation of energy is correct?

Accepted Answer

Energy cannot be created or destroyed, only transformed from one form to another.

Question 18

A chemical reaction in a bomb calorimeter causes the temperature to increase by $$3.00^\circ C. If $$the calorimeter's heat capacity is $$2.50 \dfrac{kJ}{^\circ C}$$, what is the enthalpy change ($$\Delta H) $$for the reaction?

Accepted Answer

$$\Delta H = -2.50 	imes 3.00 = -7.50 \ kJ$$

Question 19

Which of the following best describes the difference between potential and kinetic energy in chemistry?

Accepted Answer

Potential energy is stored due to position; kinetic energy is due to motion.

Thermochemistry: Energy, Heat, and Work in Chemical Systems

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