Q: A 5.78 g sample of silver is heated from 18 $$^ ext{o}C to 35$$ $$^ ext{o}C. $$The specific heat capacity of silver is 0.235 J/g$$ulletK. $$How much heat is required for this process?

$$q = 5.78 \times 0.235 \times (35 - 18) = 23.1 J$$

Q: A kettle contains 45.5 g of water cooled from 22.5 $$^ ext{o}C to 4.3$$ $$^ ext{o}C. $$The specific heat capacity of water is 75.291 J/mol$$ulletK. $$How much heat does the water lose? (Molar mass of water = 18.02 g/mol)

$$q = n \times C_m \times \Delta T = \frac{45.5}{18.02} \times 75.291 \times (4.3 - 22.5) = -6,170 J$$

Q: How much heat is evolved in the complete combustion of 250 g of ethanol ($$\mathrm{C_2H_5OH}$$)? $$\Delta H_{rxn} = -1315.3 kJ/$$mol. (Molar mass of ethanol = 46.07 g/mol)

$$q = \frac{250}{46.07} \times (-1315.3) = -7,140 kJ$$

Q: Ammonia reacts with oxygen to form nitrogen dioxide and steam: $$4\mathrm{NH_3$} + 7\mathrm{O_2$} \rightarrow 4\mathrm{NO_2$} + 6\mathrm{H_2O}. $$Given the following enthalpies of formation: $$\Delta H_f^\circ [\mathrm{NH_3$}] = -45.90 kJ/$$mol, $$\Delta H_f^\circ [\mathrm{NO_2$}] = 33.18 kJ/$$mol, $$\Delta H_f^\circ [\mathrm{H_2O}] = -241.8 kJ/$$mol, calculate $$\Delta H_{rxn}$$ for the reaction.

$$\Delta H_{rxn} = [4 \times 33.18 + 6 \times (-241.8)] - [4 \times (-45.90) + 7 \times 0] = -1,324 kJ$$

Q: A student mixes 100 g of water at 80 $$^ ext{o}C $$with 100 g of water at 20 $$^ ext{o}C in an $$insulated container. What is the final temperature of the mixture? (Assume no heat loss to the surroundings and $$C = 4.18 J/g$ulletK)$$

$$T_f = \frac{(100 \times 80) + (100 \times 20)}{200} = 50$$ $$^ ext{o}C$$

Q: If 100 g of copper ($$C = 0.385 J/g$ulletK) at 150$$ $$^ ext{o}C is $$placed in 100 g of water at 25 $$^ ext{o}C$$, what is the final temperature? (Assume no heat loss to surroundings, $$C_{water} = 4.18 J/g$ulletK)$$

$$T_f = \frac{(100 \times 0.385 \times 150) + (100 \times 4.18 \times 25)}{(100 \times 0.385) + (100 \times 4.18)} = 38.6$$ $$^ ext{o}C$$

Question 1

A 5.78 g sample of silver is heated from 18 $$^	ext{o}C to 35$$ $$^	ext{o}C. $$The specific heat capacity of silver is 0.235 J/g$$ulletK. $$How much heat is required for this process?

Accepted Answer

$$q = 5.78 	imes 0.235 	imes (35 - 18) = 23.1 J$$

Question 2

A kettle contains 45.5 g of water cooled from 22.5 $$^	ext{o}C to 4.3$$ $$^	ext{o}C. $$The specific heat capacity of water is 75.291 J/mol$$ulletK. $$How much heat does the water lose? (Molar mass of water = 18.02 g/mol)

Accepted Answer

$$q = n 	imes C_m 	imes \Delta T = \frac{45.5}{18.02} 	imes 75.291 	imes (4.3 - 22.5) = -6,170 J$$

Question 3

How much heat is evolved in the complete combustion of 250 g of ethanol ($$\mathrm{C_2H_5OH}$$)? $$\Delta H_{rxn} = -1315.3 kJ/$$mol. (Molar mass of ethanol = 46.07 g/mol)

Accepted Answer

$$q = \frac{250}{46.07} 	imes (-1315.3) = -7,140 kJ$$

Question 4

How many moles of natural gas (methane, $$\mathrm{CH_4$}) $$must be burned to release 267 kJ of heat? $$\Delta H_{rxn} = -802.3 kJ/$$mol.

Accepted Answer

$$n = \frac{267}{-802.3} = -0.333$$ mol

Question 5

Which of the following is a formation reaction?

Accepted Answer

$$\mathrm{H_2$ (g) + O_2$ (g) \rightarrow H_2O (l)}$$

Question 6

Ammonia reacts with oxygen to form nitrogen dioxide and steam: $$4\mathrm{NH_3$} + 7\mathrm{O_2$} \rightarrow 4\mathrm{NO_2$} + 6\mathrm{H_2O}. $$Given the following enthalpies of formation: $$\Delta H_f^\circ [\mathrm{NH_3$}] = -45.90 kJ/$$mol, $$\Delta H_f^\circ [\mathrm{NO_2$}] = 33.18 kJ/$$mol, $$\Delta H_f^\circ [\mathrm{H_2O}] = -241.8 kJ/$$mol, calculate $$\Delta H_{rxn}$$ for the reaction.

Accepted Answer

$$\Delta H_{rxn} = [4 	imes 33.18 + 6 	imes (-241.8)] - [4 	imes (-45.90) + 7 	imes 0] = -1,324 kJ$$

Question 7

From the following reactions: 
$$\mathrm{C_2H_2 (g) + 2H_2 (g) \rightarrow C_2H_6 (g)}$$ $$\Delta H = -311.0 kJ$$ 
$$\mathrm{2C (graphite) + 3H_2 (g) \rightarrow C_2H_6 (g)}$$ $$\Delta H = -84.7 kJ$$ 
$$\mathrm{2C (graphite) + H_2$ (g) \rightarrow C_2H_2 (g)}$$ $$\Delta H = 227.4 kJ 
$$Calculate the enthalpy change for the reaction: $$\mathrm{2C (graphite) + 3H_2 (g) \rightarrow C_2H_6 (g)}$$ using Hess's Law.

Accepted Answer

$$\Delta H = -84.7 kJ$

Question 8

Which of the following statements about specific heat capacity is correct?

Accepted Answer

Specific heat capacity is the amount of heat required to raise the temperature of 1 gram of a substance by 1 $$^	ext{o}C.$$

Question 9

A student mixes 100 g of water at 80 $$^	ext{o}C $$with 100 g of water at 20 $$^	ext{o}C in an $$insulated container. What is the final temperature of the mixture? (Assume no heat loss to the surroundings and $$C = 4.18 J/g$ulletK)$$

Accepted Answer

$$T_f = \frac{(100 	imes 80) + (100 	imes 20)}{200} = 50$$ $$^	ext{o}C$$

Question 10

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction that releases heat to the surroundings.

Question 11

If 100 g of copper ($$C = 0.385 J/g$ulletK) at 150$$ $$^	ext{o}C is $$placed in 100 g of water at 25 $$^	ext{o}C$$, what is the final temperature? (Assume no heat loss to surroundings, $$C_{water} = 4.18 J/g$ulletK)$$

Accepted Answer

$$T_f = \frac{(100 	imes 0.385 	imes 150) + (100 	imes 4.18 	imes 25)}{(100 	imes 0.385) + (100 	imes 4.18)} = 38.6$$ $$^	ext{o}C$$

Question 12

Which of the following is NOT a state function?

Accepted Answer

Work ($$w)$$

Question 13

A chemical reaction has $$\Delta H = +250 kJ. $$Which statement is true?

Accepted Answer

The reaction is endothermic and absorbs 250 kJ of heat.

Question 14

Which of the following reactions represents the standard enthalpy of formation for water?

Accepted Answer

$$\mathrm{H_2$ (g) + \frac{1}{2} O_2$ (g) \rightarrow H_2O (l)}$$

Question 15

Which of the following best explains why water has a high specific heat capacity?

Accepted Answer

Water molecules form strong hydrogen bonds.

Question 16

A sample of 10.0 g of ice at 0 $$^	ext{o}C is $$melted and then heated to 50 $$^	ext{o}C. $$How much heat is required? ($$C_{water} = 4.18 J/g$ulletK$$, $$\Delta H_{fus} = 334 J/g)$$

Accepted Answer

$$q = 10.0 	imes 334 + 10.0 	imes 4.18 	imes 50 = 3340 + 2090 = 5430 J$$

Question 17

Which of the following is true about the enthalpy change ($$\Delta H) $$for a reaction at constant pressure?

Accepted Answer

$$\Delta H$$ equals the heat absorbed or released by the system.

Question 18

A reaction has $$\Delta H = -500 kJ $$and $$\Delta S = -100 J/K. At $$what temperature (in K) does the reaction become non-spontaneous? (Assume $$\Delta G = \Delta H - T\Delta S)$$

Accepted Answer

$$T = \frac{\Delta H}{\Delta S} = \frac{-500,000}{-100} = 5000 K$$

Thermochemistry: Heat, Calorimetry, and Enthalpy Changes

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