Q: A 100.0 g sample of water is heated from 25.0°C to 75.0°C. How much energy (in joules) is required? (Specific heat of water $$c = 4.18 \ \mathrm{J/g\cdot°C})$$

$$q = m \times c \times \Delta T = 100.0 \times 4.18 \times (75.0 - 25.0) = 20,900 \ \mathrm{J}$$

Q: A 2.31 kg chunk of iron at 120.1°C is placed in 1040 g of water at 17.9°C. What is the final temperature of the system? ($$c_{\mathrm{Fe}} = 0.449 \ \mathrm{J/g\cdot°C}$$, $$c_{\mathrm{H_2O}} = 4.184 \ \mathrm{J/g\cdot°C})$$

$$T_f = \frac{(2310 \times 0.449 \times 120.1) + (1040 \times 4.184 \times 17.9)}{(2310 \times 0.449) + (1040 \times 4.184)} = 37.6^\circ \mathrm{C}$$

Q: Calculate the energy released when 5.00 g of $$\mathrm{CO_2$} is $$produced from the combustion of propane at constant pressure. ($$\Delta H = -2221 \ \mathrm{kJ/mol}$$ for $$\mathrm{C_3H_8}$$, $$M_{\mathrm{CO_2$}} = 44.01 \ \mathrm{g/mol}$$, 3 mol $$\mathrm{CO_2$}$$ per 1 mol $$\mathrm{C_3H_8})$$

$$5.00 \ \mathrm{g} \times \frac{1 \ \mathrm{mol}}{44.01 \ \mathrm{g}} \times \frac{-2221 \ \mathrm{kJ}}{3 \ \mathrm{mol}} = -84.1 \ \mathrm{kJ}$$

Q: A 2.75 g sample of aluminum foil at 190.0°C releases 385 J of energy as it cools. What is the final temperature of the foil? ($$c_{\mathrm{Al}} = 0.903 \ \mathrm{J/g\cdot°C})$$

$$T_f = \frac{-385}{2.75 \times 0.903} + 190.0 = 35.0^\circ \mathrm{C}$$

Question 1

Which of the following best describes the difference between kinetic energy and potential energy in a chemical system?

Accepted Answer

Kinetic energy is associated with the motion of particles, while potential energy is associated with the position or composition of particles.

Question 2

A 100.0 g sample of water is heated from 25.0°C to 75.0°C. How much energy (in joules) is required? (Specific heat of water $$c = 4.18 \ \mathrm{J/g\cdot°C})$$

Accepted Answer

$$q = m 	imes c 	imes \Delta T = 100.0 	imes 4.18 	imes (75.0 - 25.0) = 20,900 \ \mathrm{J}$$

Question 3

Which of the following is a state function?

Accepted Answer

Internal energy ($$E)$$

Question 4

A 2.31 kg chunk of iron at 120.1°C is placed in 1040 g of water at 17.9°C. What is the final temperature of the system? ($$c_{\mathrm{Fe}} = 0.449 \ \mathrm{J/g\cdot°C}$$, $$c_{\mathrm{H_2O}} = 4.184 \ \mathrm{J/g\cdot°C})$$

Accepted Answer

$$T_f = \frac{(2310 	imes 0.449 	imes 120.1) + (1040 	imes 4.184 	imes 17.9)}{(2310 	imes 0.449) + (1040 	imes 4.184)} = 37.6^\circ \mathrm{C}$$

Question 5

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

Accepted Answer

$$\Delta E = q + w$$

Question 6

A reaction in a coffee cup calorimeter absorbs 50.0 kJ of heat from the surroundings at constant pressure. Which statement is true?

Accepted Answer

$$\Delta H \u003e 0$$, the reaction is endothermic.

Question 7

Which of the following best describes the energy transfer in an exothermic chemical reaction?

Accepted Answer

Energy is released from the system to the surroundings, resulting in $$\Delta H \u003c 0.$$

Question 8

A 1.03 g sample of biphenyl ($$\mathrm{C$$_{12}$$H_{10}}) is $$combusted in a bomb calorimeter, causing the temperature to rise from 24.2°C to 31.4°C. The calorimeter has a heat capacity of 5.86 kJ/°C. What is $$\Delta E_{\mathrm{rxn}} in kJ/$$mol biphenyl?

Accepted Answer

$$\Delta E_{\mathrm{rxn}} = \frac{-42.2 \ \mathrm{kJ}}{0.0066788 \ \mathrm{mol}} = -6320 \ \mathrm{kJ/mol}$$

Question 9

Which of the following statements about constant volume calorimetry (bomb calorimetry) is correct?

Accepted Answer

At constant volume, $$\Delta E = q_v$$ because $$\Delta V = 0$$ and no work is done.

Question 10

Calculate the energy released when 5.00 g of $$\mathrm{CO_2$} is $$produced from the combustion of propane at constant pressure. ($$\Delta H = -2221 \ \mathrm{kJ/mol}$$ for $$\mathrm{C_3H_8}$$, $$M_{\mathrm{CO_2$}} = 44.01 \ \mathrm{g/mol}$$, 3 mol $$\mathrm{CO_2$}$$ per 1 mol $$\mathrm{C_3H_8})$$

Accepted Answer

$$5.00 \ \mathrm{g} 	imes \frac{1 \ \mathrm{mol}}{44.01 \ \mathrm{g}} 	imes \frac{-2221 \ \mathrm{kJ}}{3 \ \mathrm{mol}} = -84.1 \ \mathrm{kJ}$$

Question 11

Which of the following best explains why the calorie (cal) is commonly used in food energy labeling?

Accepted Answer

Because 1 Calorie (Cal) is equivalent to 1 kilocalorie (kcal), which is a convenient unit for the large amounts of energy in food.

Question 12

A 2.75 g sample of aluminum foil at 190.0°C releases 385 J of energy as it cools. What is the final temperature of the foil? ($$c_{\mathrm{Al}} = 0.903 \ \mathrm{J/g\cdot°C})$$

Accepted Answer

$$T_f = \frac{-385}{2.75 	imes 0.903} + 190.0 = 35.0^\circ \mathrm{C}$$

Question 13

Which of the following equations correctly expresses the work done by a system at constant external pressure?

Accepted Answer

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

Question 14

Which of the following reactions is endothermic?

Accepted Answer

$$\mathrm{H_2O (s) \rightarrow H_2O (l)}$$, $$\Delta H = +6.01 \ \mathrm{kJ/mol}$$

Question 15

In a calorimetry experiment, why is it important that the calorimeter is insulated from the outside environment?

Accepted Answer

To ensure that all heat exchange occurs only between the system and the surroundings defined in the experiment, not with the external environment.

Question 16

Which of the following best describes the relationship between $$q_{\mathrm{rxn}}$$ and $$q_{\mathrm{soln}} in a $$coffee cup calorimeter?

Accepted Answer

$$q_{\mathrm{rxn}} = -q_{\mathrm{soln}}$$

Question 17

A chemical reaction occurs in a bomb calorimeter and releases 5000 J of heat. What is the value of $$\Delta E$$ for the reaction?

Accepted Answer

$$\Delta E = q_v = -5000 \ \mathrm{J}$$

lecture 8 (ns) Enthalpy and Calorimetry: Energy Changes in Chemical Reactions

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