Q: A glazed donut contains 250 Calories (Cal). How many joules of energy does this represent? (1 Cal = 1 kcal, 1 kcal = 4184 J)

$$250 \times 4184 = 1,046,000 J$$

Q: If a system does 75 J of work on the surroundings and absorbs 25 J of heat from the surroundings, what is $$\Delta E_{system}$$?

$$\Delta E = q + w = 25\ \text{J} + (-75\ \text{J}) = -50\ \text{J}$$

Q: During the combustion of 2 moles of methanol, the volume increases by 73.4 L against an external pressure of 1 atm. Calculate the work done by the system in kJ. ($$1\ \text{L} \cdot \text{atm} = 101\ \text{J})$$

$$w = -P \Delta V = -(1\ \text{atm})(73.4\ \text{L}) = -73.4\ \text{L} \cdot \text{atm} = -7413.4\ \text{J} = -7.41\ \text{kJ}$$

Q: Which of the following reactions has work done on it by the surroundings?

$$2\ \text{H}_2(g) + \text{O}_2(g) \rightarrow 2\ \text{H}_2\text{O}(g)$$

Q: How much heat is released when 10.00 g of propane ($$\text{C}_3\text{H}_8) $$reacts with excess $$\text{O}_2$$? ($$\Delta H^\circ = -2044\ \text{kJ}$$ for $$1$$ mol $$\text{C}_3\text{H}_8$$; molar mass $$= 44.1\ \text{g/mol})$$

$$\frac{10.00\ \text{g}}{44.1\ \text{g/mol}} \times (-2044\ \text{kJ}) = -464\ \text{kJ}$$

Q: A 55.0 g aluminum block initially at $$27.5\ ^\circ\text{C}$$ absorbs 725 J of heat. What is the final temperature of the aluminum? ($$c = 0.902\ \text{J/g}^\circ\text{C})$$

$$T_f = T_i + \frac{q}{mc} = 27.5 + \frac{725}{55.0 \times 0.902} = 27.5 + 14.6 = 42.1\ ^\circ\text{C}$$

Q: A 0.350 g sample of biodiesel ($$\text{C}_{19}\text{H}_{38}\text{O}_2) is $$burned in a bomb calorimeter, causing the temperature to rise by $$5.29\ ^\circ\text{C}. $$The calorimeter's heat capacity is $$2.50\ \text{kJ}/^\circ\text{C}. $$What is $$\Delta E$$ for the combustion of 1 mol of biodiesel? (Molar mass = 298 g/mol)

$$q = C \Delta T = 2.50 \times 5.29 = 13.2\ \text{kJ}$$; \newline \text{For 0.350 g: } \frac{13.2\ \text{kJ}}{0.350\ \text{g}} \times 298\ \text{g/mol} = 11,200\ \text{kJ/mol}$

Question 1

Which of the following best describes the difference between the system and the surroundings in a chemical reaction, as illustrated in the provided diagram?

Accepted Answer

The system is the mixture of reactants, and the surroundings are everything else, including the flask, solvent, and room.

Question 2

A glazed donut contains 250 Calories (Cal). How many joules of energy does this represent? (1 Cal = 1 kcal, 1 kcal = 4184 J)

Accepted Answer

$$250 	imes 4184 = 1,046,000 J$$

Question 3

If a system does 75 J of work on the surroundings and absorbs 25 J of heat from the surroundings, what is $$\Delta E_{system}$$?

Accepted Answer

$$\Delta E = q + w = 25\ 	ext{J} + (-75\ 	ext{J}) = -50\ 	ext{J}$$

Question 4

You ride an elevator to the top of Willis Tower. From your perspective as the system, how is the energy exchange classified and what is the sign of $$\Delta E$$?

Accepted Answer

Work, $$\Delta E \u003e 0$$

Question 5

An ice cube melts and cools your drink. If the ice cube is the system, how is the energy exchange classified and what is the sign of $$\Delta E$$?

Accepted Answer

Heat, $$\Delta E \u003e 0$$

Question 6

During the combustion of 2 moles of methanol, the volume increases by 73.4 L against an external pressure of 1 atm. Calculate the work done by the system in kJ. ($$1\ 	ext{L} \cdot 	ext{atm} = 101\ 	ext{J})$$

Accepted Answer

$$w = -P \Delta V = -(1\ 	ext{atm})(73.4\ 	ext{L}) = -73.4\ 	ext{L} \cdot 	ext{atm} = -7413.4\ 	ext{J} = -7.41\ 	ext{kJ}$$

Question 7

Which of the following reactions has work done on it by the surroundings?

Accepted Answer

$$2\ 	ext{H}_2(g) + 	ext{O}_2(g) ightarrow 2\ 	ext{H}_2	ext{O}(g)$$

Question 8

How much heat is released when 10.00 g of propane ($$	ext{C}_3	ext{H}_8) $$reacts with excess $$	ext{O}_2$$? ($$\Delta H^\circ = -2044\ 	ext{kJ}$$ for $$1$$ mol $$	ext{C}_3	ext{H}_8$$; molar mass $$= 44.1\ 	ext{g/mol})$$

Accepted Answer

$$\frac{10.00\ 	ext{g}}{44.1\ 	ext{g/mol}} 	imes (-2044\ 	ext{kJ}) = -464\ 	ext{kJ}$$

Question 9

Which of the following processes is endothermic?

Accepted Answer

Boiling water into steam

Question 10

A 55.0 g aluminum block initially at $$27.5\ ^\circ	ext{C}$$ absorbs 725 J of heat. What is the final temperature of the aluminum? ($$c = 0.902\ 	ext{J/g}^\circ	ext{C})$$

Accepted Answer

$$T_f = T_i + \frac{q}{mc} = 27.5 + \frac{725}{55.0 	imes 0.902} = 27.5 + 14.6 = 42.1\ ^\circ	ext{C}$$

Question 11

Which of the following is a state function?

Accepted Answer

Temperature of the classroom

Question 12

A 0.350 g sample of biodiesel ($$	ext{C}_{19}	ext{H}_{38}	ext{O}_2) is $$burned in a bomb calorimeter, causing the temperature to rise by $$5.29\ ^\circ	ext{C}. $$The calorimeter's heat capacity is $$2.50\ 	ext{kJ}/^\circ	ext{C}. $$What is $$\Delta E$$ for the combustion of 1 mol of biodiesel? (Molar mass = 298 g/mol)

Accepted Answer

$$q = C \Delta T = 2.50 	imes 5.29 = 13.2\ 	ext{kJ}$$; 
ewline 	ext{For 0.350 g: } \frac{13.2\ 	ext{kJ}}{0.350\ 	ext{g}} 	imes 298\ 	ext{g/mol} = 11,200\ 	ext{kJ/mol}$

Question 13

Which salt would result in the greatest decrease in temperature when 1 mol is dissolved in 100.0 mL of water, based on the given enthalpy changes?

Accepted Answer

LiF ($$\Delta H^\circ = +5.5 kJ)$$

Question 14

Why does a bomb calorimeter measure $$\Delta E$$ and not $$\Delta H$$?

Accepted Answer

Because the volume is constant, so no $$P\Delta V$$ work is done.

Question 15

A sample of glass beads (34.5 g) is heated to $$100.0\ ^\circ	ext{C}$$ and added to $$100.0 g of $$water at $$25.0\ ^\circ	ext{C}. $$The final temperature is $$29.9\ ^\circ	ext{C}. $$What is the specific heat capacity of the glass? ($$c_{water} = 4.184\ 	ext{J/g}^\circ	ext{C})$$

Accepted Answer

$$q_{glass} = -q_{water}$$; $$m_{glass}c$$_{glass}(T_f-T_i) = -$$m_{water}c$$_{water}(T_f-T_i)$$; 
ewline 34.5c$$_{glass}(29.9-100.0) = -100.0 	imes 4.184 	imes (29.9-25.0)$$; 
ewline c$$_{glass} = 0.848\ 	ext{J/g}^\circ	ext{C}$$

Question 16

Which of the following best describes the principle of Hess’s Law?

Accepted Answer

The overall enthalpy change for a reaction is equal to the sum of the enthalpy changes for the individual steps.

Question 17

Given the following reactions: 
ewline $$	ext{C}_{graphite}(s) + 	ext{O}_2(g) ightarrow 	ext{CO}_2(g)\ \Delta H^\circ = -394\ 	ext{kJ}$$ 
ewline $$	ext{C}_{diamond}(s) + 	ext{O}_2(g) ightarrow 	ext{CO}_2(g)\ \Delta H^\circ = -396\ 	ext{kJ}$$ 
ewline What is $$\Delta H^\circ$$ for $$	ext{C}_{graphite}(s) ightarrow 	ext{C}_{diamond}(s)$$?

Accepted Answer

$$\Delta H^\circ = -394 - (-396) = +2\ 	ext{kJ}$$

Thermochemistry: Energy Changes in Chemical Reactions

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