Q: Which of the following is the correct formula for calculating the enthalpy change of a reaction ($$\Delta H_{rxn}) $$using standard enthalpies of formation?

$$\Delta H_{rxn} = \sum \Delta H_f^\circ \text{(products)} - \sum \Delta H_f^\circ \text{(reactants)}$$

Question 1

Which of the following statements best describes an exothermic process?

Accepted Answer

The system loses heat to the surroundings, resulting in a negative $$q.$$

Question 2

A 50.0 g sample of water is heated from 20.0°C to 80.0°C. Given the specific heat capacity of water is $$c = 4.184 \frac{J}{g \cdot ^\circ C}$$, what is the amount of heat absorbed by the water?

Accepted Answer

$$q = 12,552 	ext{ J}$$

Question 3

Which equation correctly relates the change in internal energy ($$\Delta E) to $$heat ($$q) $$and work ($$w$$)?

Accepted Answer

$$\Delta E = q + w$$

Question 4

A bomb calorimeter is used to measure the heat of combustion of a food sample. If the heat capacity of the calorimeter is $$C = 3.00 \frac{kJ}{^\circ C}$$ and the temperature rises by $$2.50^\circ C$$, what is the total heat released by the sample?

Accepted Answer

$$q = 7.50 	ext{ kJ}$$

Question 5

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

Accepted Answer

Potential energy is due to position, while kinetic energy is due to motion.

Question 6

A sample of NaCl weighing 0.563 g absorbs 22.18 J of heat. Its specific heat ($$c) is$$ $$5.05 \frac{J}{g \cdot ^\circ C}. If $$its initial temperature is $$20.5^\circ C$$, what is its final temperature?

Accepted Answer

$$T_{final} = 28.4^\circ C$$

Question 7

Which of the following is the correct formula for calculating the electrostatic potential energy ($$E_{el}) $$between two charges $$Q_1$$ and $$Q_2$$ separated by a distance $$d$$?

Accepted Answer

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

Question 8

During a chemical reaction at constant pressure, which equation correctly relates enthalpy change ($$\Delta H) to $$heat ($$q_p) $$and work ($$w$$)?

Accepted Answer

$$\Delta H = q_p$$

Question 9

A reaction has $$\Delta H_{rxn} = -250 	ext{ kJ}. $$Which statement is true about this reaction?

Accepted Answer

The reaction is exothermic and releases heat.

Question 10

Which of the following is the correct expression for the specific heat capacity ($$c) of a $$substance?

Accepted Answer

$$c = \frac{q}{m \Delta T}$$

Question 11

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

Accepted Answer

$$w = -P \Delta V$$

Question 12

Which of the following statements about calorimetry is correct?

Accepted Answer

The heat lost by the system is equal in magnitude but opposite in sign to the heat gained by the surroundings.

Question 13

Which of the following is the correct definition of the standard enthalpy of formation ($$\Delta H_f^\circ$$)?

Accepted Answer

The enthalpy change when one mole of a compound is formed from its elements in their standard states.

Question 14

A chemical reaction is carried out in a calorimeter at constant pressure. The temperature of the water in the calorimeter decreases. What can you conclude about the reaction?

Accepted Answer

The reaction is endothermic.

Question 15

Which of the following is the correct formula for calculating the enthalpy change of a reaction ($$\Delta H_{rxn}) $$using standard enthalpies of formation?

Accepted Answer

$$\Delta H_{rxn} = \sum \Delta H_f^\circ 	ext{(products)} - \sum \Delta H_f^\circ 	ext{(reactants)}$$

Question 16

A 100.0 g sample of water at $$25.0^\circ C$$ absorbs $$4184 J of $$heat. What is the final temperature of the water? ($$c = 4.184 \frac{J}{g \cdot ^\circ C})$$

Accepted Answer

$$T_{final} = 35.0^\circ C$$

Question 17

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

Accepted Answer

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

Question 18

Which of the following is true regarding the sign convention for work ($$w) in $$chemistry?

Accepted Answer

Work is negative when the system does work on the surroundings.

Question 19

A reaction is performed in a bomb calorimeter and releases 5.00 kJ of heat. What is the sign of $$q$$ for the system?

Accepted Answer

$$q is $$negative

Question 20

Which of the following is a correct statement about the relationship between energy, work, and heat in a chemical system?

Accepted Answer

Energy can be transferred as both heat and work between a system and its surroundings.

Thermochemistry: Energy, Heat, and Enthalpy in Chemical Systems

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