Q: A 32.5 g iron rod ($$C_s = 0.449 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1}$$), initially at $$22.7 ^\circ\mathrm{C}$$, is submerged into an unknown mass of water at $$63.2 ^\circ\mathrm{C} in an $$insulated container. The final temperature of the mixture upon reaching thermal equilibrium is $$59.5 ^\circ\mathrm{C}. $$What is the mass of the water in grams? ($$C_s,\mathrm{water} = 4.184 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1})$$

$$m_{\mathrm{water}} = 32.5 \times 0.449 \times (59.5-22.7) / [4.184 \times (59.5-63.2)] = 120.6$$

Q: A $$75.0 \ \mathrm{g}$$ iron block ($$C_s = 0.449 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1}) is $$heated from $$25 ^\circ\mathrm{C} to $$its melting temperature of $$1535 ^\circ\mathrm{C}. $$How much heat, in kilojoules, must be added?

$$q = 75.0 \times 0.449 \times (1535-25) = 50,800 \ \mathrm{J} = 50.8 \ \mathrm{kJ}$$

Q: Which equation correctly expresses the relationship between heat ($$q$$), mass ($$m$$), specific heat capacity ($$C_s$$), and temperature change ($$\Delta T$$)?

$$q = m \times C_s \times \Delta T$$

Q: A $$50.0 \ \mathrm{g}$$ sample of water at $$22 ^\circ\mathrm{C} is $$mixed with $$125 \ \mathrm{g} of $$water at $$36 ^\circ\mathrm{C}. $$What is the final temperature of the water after mixing, assuming no heat is lost to the surroundings? ($$C_s = 4.184 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1})$$

$$T_f = \frac{(50.0 \times 22) + (125 \times 36)}{50.0 + 125} = 32.2$$

Question 1

Which of the following statements best describes the First Law of Thermodynamics?

Accepted Answer

The total energy of the universe is constant; energy can neither be created nor destroyed, but it can be transferred from one object or system to another.

Question 2

A 32.5 g iron rod ($$C_s = 0.449 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1}$$), initially at $$22.7 ^\circ\mathrm{C}$$, is submerged into an unknown mass of water at $$63.2 ^\circ\mathrm{C} in an $$insulated container. The final temperature of the mixture upon reaching thermal equilibrium is $$59.5 ^\circ\mathrm{C}. $$What is the mass of the water in grams? ($$C_s,\mathrm{water} = 4.184 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1})$$

Accepted Answer

$$m_{\mathrm{water}} = 32.5 	imes 0.449 	imes (59.5-22.7) / [4.184 	imes (59.5-63.2)] = 120.6$$

Question 3

Which of the following would show the largest temperature change upon gaining $$200.0 \ \mathrm{J} of $$heat? (Assume the sample masses are all the same.)

Accepted Answer

Sand, specific heat capacity $$= 0.290 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1}$$

Question 4

Which of the following signs on $$q$$ and $$w$$ represent a system that is doing work on the surroundings as well as losing heat to the surroundings?

Accepted Answer

$$q = -, \ w = -$$

Question 5

Calculate the internal energy (in kJ) for combustion of a gas that releases $$210.0 \ \mathrm{kJ} of $$heat to its surroundings and does $$65.5 \ \mathrm{kJ} of $$work on its surroundings.

Accepted Answer

$$-209.5 \ \mathrm{kJ}$$

Question 6

A piece of unknown solid substance weighs $$437.2 \ \mathrm{g}$$ and requires $$8460 \ \mathrm{J} to $$increase its temperature from $$19.3 ^\circ\mathrm{C} to$$ $$68.9 ^\circ\mathrm{C}. $$What is the specific heat of the substance? Assuming it is one of the substances found in Table 6.4, what is its likely identity?

Accepted Answer

Aluminum, $$C_s = 0.900 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1}$$

Question 7

Which of the following is a state function?

Accepted Answer

Change in altitude

Question 8

A $$75.0 \ \mathrm{g}$$ iron block ($$C_s = 0.449 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1}) is $$heated from $$25 ^\circ\mathrm{C} to $$its melting temperature of $$1535 ^\circ\mathrm{C}. $$How much heat, in kilojoules, must be added?

Accepted Answer

$$q = 75.0 	imes 0.449 	imes (1535-25) = 50,800 \ \mathrm{J} = 50.8 \ \mathrm{kJ}$$

Question 9

Which equation correctly expresses the relationship between heat ($$q$$), mass ($$m$$), specific heat capacity ($$C_s$$), and temperature change ($$\Delta T$$)?

Accepted Answer

$$q = m 	imes C_s 	imes \Delta T$$

Question 10

In an exothermic process, the volume of a system expanded from $$186 \ \mathrm{mL} to$$ $$1997 \ \mathrm{mL}$$ against a constant pressure of $$745 \ \mathrm{torr}. $$During the process, $$18.6$$ calories of heat energy were given off. What was the internal energy change for the system in joules? ($$1 \ \mathrm{bar} \cdot \mathrm{L} = 100 \ \mathrm{J}$$, $$1 \ \mathrm{cal} = 4.184 \ \mathrm{J}$$, $$1 \ \mathrm{atm} = 760 \ \mathrm{torr})$$

Accepted Answer

$$\Delta U = -77.8 \ \mathrm{J}$$

Question 11

Which of the following best describes an endothermic reaction?

Accepted Answer

It absorbs energy from the surroundings and feels cold to the touch.

Question 12

A $$50.0 \ \mathrm{g}$$ sample of water at $$22 ^\circ\mathrm{C} is $$mixed with $$125 \ \mathrm{g} of $$water at $$36 ^\circ\mathrm{C}. $$What is the final temperature of the water after mixing, assuming no heat is lost to the surroundings? ($$C_s = 4.184 \ \mathrm{J} \ \mathrm{g}^{-1} \ ^\circ\mathrm{C}^{-1})$$

Accepted Answer

$$T_f = \frac{(50.0 	imes 22) + (125 	imes 36)}{50.0 + 125} = 32.2$$

Question 13

Which of the following is NOT a correct unit conversion for energy?

Accepted Answer

1 joule (J) $$= 1000$$ calories (cal)

Question 14

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

Accepted Answer

$$\Delta U = q + w$$

Question 15

A system expands from $$1.25 \ \mathrm{L} to$$ $$4.86 \ \mathrm{L}$$ against a constant pressure of $$1.00 \ \mathrm{bar}. $$What is the work done by the system?

Accepted Answer

$$w = -P \Delta V = -1.00 	imes (4.86 - 1.25) = -3.61 \ \mathrm{bar} \cdot \mathrm{L} = -361 \ \mathrm{J}$$

Question 16

Which of the following is the correct expression for the work done by a chemical reaction at constant pressure and temperature, when the number of moles of gas changes?

Accepted Answer

$$w = -\Delta nRT$$

Question 17

Which of the following is the correct equation for calculating the enthalpy change ($$\Delta H) of a $$reaction from standard enthalpies of formation?

Accepted Answer

$$\Delta H = \sum 
u_p \Delta H_f$$^{\circ}$$ (\mathrm{products}) - \sum 
u_r \Delta H_f$$^{\circ}$$ (\mathrm{reactants})$$

Question 18

Which of the following chemical equations represents a formation reaction for $$\mathrm{NO}_2 (g)$$?

Accepted Answer

$$\frac{1}{2} \mathrm{N}_2 (g) + \mathrm{O}_2 (g) \rightarrow \mathrm{NO}_2 (g)$$

Chapter 6: Thermochemistry – Study Notes

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