Q: A 2-kg object is moving at a speed of 1 m/s. What is its kinetic energy?

$$KE = \frac{1}{2}mv^2$ = \frac{1}{2} \times 2 \text{ kg} \times (1 \text{ m/s})^2$ = 1 \text{ J}$$

Q: A system absorbs 188.1 J of heat and does 141 J of work on its surroundings. What is the change in internal energy ($$\Delta U$$)?

$$\Delta U = 188.1 \text{ J} + (-141 \text{ J}) = 47.1 \text{ J}$$

Q: How much heat (in kJ) is required to heat 255 g of water from 25.2°C to 90.5°C? (Specific heat of water $$s = 4.184 J/g$$·°C)

$$q = 255 \text{ g} \times 4.184 \text{ J/g·°C} \times (90.5 - 25.2) \text{°C} = 69,700 \text{ J} = 69.7 \text{ kJ}$$

Q: A bite-sized chocolate chip cookie weighing 7.25 g is combusted in a bomb calorimeter. The heat capacity of the calorimeter is $$C_{cal} = 39.97 kJ$$/°C and the temperature increases by 3.90°C. What is the energy content per gram of the cookie?

$$q = C_{cal} \Delta T = 39.97 \text{ kJ/°C} \times 3.90 \text{°C} = 155.9 \text{ kJ}$$; $$\text{Energy per gram} = \frac{155.9 \text{ kJ}}{7.25 \text{ g}} = 21.5 \text{ kJ/g}$$

Q: Given the following standard enthalpies of formation: $$\Delta H_f^\circ(Ag$^+$$, aq) = +105.9$$ kJ/mol$$, $$\Delta H_f^\circ(Cl$^-$$, aq) = -167.2$$ kJ/mol$$, $$\Delta H_f^\circ($$AgCl, s) = -127.0$$ kJ/mol$$, calculate the standard enthalpy change for the reaction: Ag$$^+(aq) + Cl$^-(aq$$) $$\rightarrow$$ AgCl(s).

$$\Delta H^\circ = \Delta H_f^\circ(\text{AgCl}) - [\Delta H_f^\circ(\text{Ag}^+) + \Delta H_f^\circ(\text{Cl}^-)] = -127.0 - (105.9 - 167.2) = -127.0 - (-61.3) = -65.7 \text{ kJ/mol}$$

Question 1

Which of the following best describes the law of conservation of energy as it applies to chemical reactions?

Accepted Answer

The total energy of the universe remains constant; energy may change forms but is neither created nor destroyed.

Question 2

A 2-kg object is moving at a speed of 1 m/s. What is its kinetic energy?

Accepted Answer

$$KE = \frac{1}{2}mv^2$ = \frac{1}{2} 	imes 2 	ext{ kg} 	imes (1 	ext{ m/s})^2$ = 1 	ext{ J}$$

Question 3

Which process is endothermic?

Accepted Answer

Melting of ice to liquid water.

Question 4

The electrostatic potential energy between two charges is given by $$E_{el} = \frac{q_1$ q_2$}{d}. If $$the charges are +3 and -1, and the distance is the same as for charges +1 and -1, how does the magnitude of $$E_{el}$$ compare?

Accepted Answer

It is three times greater for +3 and -1 than for +1 and -1.

Question 5

Which of the following is the correct relationship for the change in internal energy ($$\Delta U) of a $$system?

Accepted Answer

$$\Delta U = q + w$$

Question 6

A system absorbs 188.1 J of heat and does 141 J of work on its surroundings. What is the change in internal energy ($$\Delta U$$)?

Accepted Answer

$$\Delta U = 188.1 	ext{ J} + (-141 	ext{ J}) = 47.1 	ext{ J}$$

Question 7

Which statement about state functions is correct?

Accepted Answer

State functions depend only on the initial and final states, not on the path taken.

Question 8

Which equation correctly defines enthalpy ($$H$$)?

Accepted Answer

$$H = U + PV$$

Question 9

For a reaction carried out at constant pressure, which equation relates the heat absorbed ($$q_p) to $$the change in enthalpy ($$\Delta H$$)?

Accepted Answer

$$q_p = \Delta H$$

Question 10

If the thermochemical equation for the combustion of methane is $$CH_4(g) + 2O_2(g$$) \rightarrow $$CO_2(g) + 2H_2O(l$$)$$, $$\Delta H = -890.4$$ kJ, what is the enthalpy change for the reverse reaction?

Accepted Answer

$$\Delta H = +890.4 kJ$$

Question 11

How much heat (in kJ) is required to heat 255 g of water from 25.2°C to 90.5°C? (Specific heat of water $$s = 4.184 J/g$$·°C)

Accepted Answer

$$q = 255 	ext{ g} 	imes 4.184 	ext{ J/g·°C} 	imes (90.5 - 25.2) 	ext{°C} = 69,700 	ext{ J} = 69.7 	ext{ kJ}$$

Question 12

A bite-sized chocolate chip cookie weighing 7.25 g is combusted in a bomb calorimeter. The heat capacity of the calorimeter is $$C_{cal} = 39.97 kJ$$/°C and the temperature increases by 3.90°C. What is the energy content per gram of the cookie?

Accepted Answer

$$q = C_{cal} \Delta T = 39.97 	ext{ kJ/°C} 	imes 3.90 	ext{°C} = 155.9 	ext{ kJ}$$; $$	ext{Energy per gram} = \frac{155.9 	ext{ kJ}}{7.25 	ext{ g}} = 21.5 	ext{ kJ/g}$$

Question 13

Which of the following statements about Hess's Law is correct?

Accepted Answer

The enthalpy change for a reaction is the same whether it occurs in one step or several steps.

Question 14

Given the following standard enthalpies of formation: $$\Delta H_f^\circ(Ag$^+$$, aq) = +105.9$$ kJ/mol$$, $$\Delta H_f^\circ(Cl$^-$$, aq) = -167.2$$ kJ/mol$$, $$\Delta H_f^\circ($$AgCl, s) = -127.0$$ kJ/mol$$, calculate the standard enthalpy change for the reaction: Ag$$^+(aq) + Cl$^-(aq$$) $$\rightarrow$$ AgCl(s).

Accepted Answer

$$\Delta H^\circ = \Delta H_f^\circ(	ext{AgCl}) - [\Delta H_f^\circ(	ext{Ag}^+) + \Delta H_f^\circ(	ext{Cl}^-)] = -127.0 - (105.9 - 167.2) = -127.0 - (-61.3) = -65.7 	ext{ kJ/mol}$$

Question 15

Which of the following is the correct formula for calculating the standard enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

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

Question 16

A chemical reaction is carried out in a closed system at constant pressure. Which of the following statements is true?

Accepted Answer

The heat absorbed or released by the system equals the change in enthalpy ($$q_p = \Delta H).$$

Question 17

Which of the following best describes the difference between specific heat and heat capacity?

Accepted Answer

Specific heat is the amount of heat required to raise the temperature of 1 g of a substance by 1°C, while heat capacity is the amount of heat required to raise the temperature of an object by 1°C.

Question 18

A sample of iron (Fe) with a mass of 100.0 g, originally at 88.4°C, is dropped into 125 g of water at 25.1°C. The final temperature is 31.3°C. What is the heat capacity $$C (in J$$/°C) of the iron sample? (Specific heat of water $$s = 4.184 J/g$$·°C)

Accepted Answer

$$q_{water} = 125 	ext{ g} 	imes 4.184 	ext{ J/g·°C} 	imes (31.3 - 25.1) = 3242.6 	ext{ J}$$; $$C_{Fe} = \frac{3242.6 	ext{ J}}{(-57.1) 	ext{°C}} = -56.8 	ext{ J/°C}$$

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