Q: A 5.78 g sample of silver is heated from 18 $$^\\circC to 31$$ $$^\\circC. $$The specific heat capacity of silver is 0.235 J/g·$$^\\circC. $$How much heat is required for this temperature change?

$$q = 5.78 \\times 0.235 \\times (31 - 18) = 17.6 J$$

Q: A kettle contains 45.5 g of water cooled from 22.5 $$^\\circC to 4.3$$ $$^\\circC. $$The specific heat capacity of water is 75.29 J/mol·K. How much heat does the water lose?

$$q = 45.5 \\times 4.18 \\times (4.3 - 22.5) = -3440 J$$

Q: Which of the following is the correct formula for calculating heat absorbed or released during a temperature change?

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

Q: How much heat is evolved in the complete combustion of 25.0 g of ethanol ($$C_2H_5OH$$)? The enthalpy of combustion is $$\\Delta H = -1367 kJ/$$mol.

$$q = \\frac{25.0}{46.07} \\times (-1367) = -743 kJ$$

Q: Which of the following reactions is a formation reaction?

$$C(graphite) + O_2$(g) \\rightarrow CO_2$(g)$$

Q: Ammonia reacts with oxygen to form nitrogen dioxide and steam: $$4NH_3(g) + 7O_2(g) \\rightarrow 4NO_2(g) + 6H_2O(g). $$Given the following standard enthalpies of formation: $$NH_3(g) = -45.90$ kJ/mol, NO_2$(g) = 33.18 kJ/$$mol, $$H_2O(g) = -241.8 kJ/$$mol, calculate $$\\Delta H$$ for the reaction.

$$\\Delta H = [4 \\times 33.18 + 6 \\times (-241.8)] - [4 \\times (-45.90) + 7 \\times 0] = -1126 kJ$$

Q: From the following reactions: $$C_2H_2(g) + 2H_2(g) \\rightarrow C_2H_6(g)$$ $$\\Delta H = -311.0 kJ$$, $$2C(graphite) + 3H_2(g) \\rightarrow C_2H_6(g)$$ $$\\Delta H = -83.8 kJ$$, calculate the enthalpy change for $$2C(graphite) + H_2$(g) \\rightarrow C_2H_2(g).$$

$$\\Delta H = -83.8 - (-311.0) = 227.2 kJ$$

Q: If 267 kJ of heat is released by burning natural gas ($$CH_4$$), how many grams of $$CH_4$$ are combusted? The enthalpy of combustion is $$\\Delta H = -802.3 kJ/$$mol.

$$mass = \\frac{267}{802.3} \\times 16.04 = 5.34 g$$

Q: A student mixes 50.0 g of water at 80.0 $$^\\circC $$with 50.0 g of water at 20.0 $$^\\circC in an $$insulated container. What is the final temperature of the mixture? (Assume no heat loss to the surroundings and $$C = 4.18 J/g$$·$$^\\circC)$$

The final temperature is $$50.0$$ $$^\\circC$$

Question 1

A 5.78 g sample of silver is heated from 18 $$^\circC to 31$$ $$^\circC. $$The specific heat capacity of silver is 0.235 J/g·$$^\circC. $$How much heat is required for this temperature change?

Accepted Answer

$$q = 5.78 \times 0.235 \times (31 - 18) = 17.6 J$$

Question 2

A kettle contains 45.5 g of water cooled from 22.5 $$^\circC to 4.3$$ $$^\circC. $$The specific heat capacity of water is 75.29 J/mol·K. How much heat does the water lose?

Accepted Answer

$$q = 45.5 \times 4.18 \times (4.3 - 22.5) = -3440 J$$

Question 3

Which of the following is the correct formula for calculating heat absorbed or released during a temperature change?

Accepted Answer

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

Question 4

How much heat is evolved in the complete combustion of 25.0 g of ethanol ($$C_2H_5OH$$)? The enthalpy of combustion is $$\Delta H = -1367 kJ/$$mol.

Accepted Answer

$$q = \frac{25.0}{46.07} \times (-1367) = -743 kJ$$

Question 5

Which of the following reactions is a formation reaction?

Accepted Answer

$$C(graphite) + O_2$(g) \rightarrow CO_2$(g)$$

Question 6

Ammonia reacts with oxygen to form nitrogen dioxide and steam: $$4NH_3(g) + 7O_2(g) \rightarrow 4NO_2(g) + 6H_2O(g). $$Given the following standard enthalpies of formation: $$NH_3(g) = -45.90$ kJ/mol, NO_2$(g) = 33.18 kJ/$$mol, $$H_2O(g) = -241.8 kJ/$$mol, calculate $$\Delta H$$ for the reaction.

Accepted Answer

$$\Delta H = [4 \times 33.18 + 6 \times (-241.8)] - [4 \times (-45.90) + 7 \times 0] = -1126 kJ$$

Question 7

From the following reactions: $$C_2H_2(g) + 2H_2(g) \rightarrow C_2H_6(g)$$ $$\Delta H = -311.0 kJ$$, $$2C(graphite) + 3H_2(g) \rightarrow C_2H_6(g)$$ $$\Delta H = -83.8 kJ$$, calculate the enthalpy change for $$2C(graphite) + H_2$(g) \rightarrow C_2H_2(g).$$

Accepted Answer

$$\Delta H = -83.8 - (-311.0) = 227.2 kJ$$

Question 8

If 267 kJ of heat is released by burning natural gas ($$CH_4$$), how many grams of $$CH_4$$ are combusted? The enthalpy of combustion is $$\Delta H = -802.3 kJ/$$mol.

Accepted Answer

$$mass = \frac{267}{802.3} \times 16.04 = 5.34 g$$

Question 9

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 $$^\circC.$$

Question 10

A student mixes 50.0 g of water at 80.0 $$^\circC $$with 50.0 g of water at 20.0 $$^\circC in an $$insulated container. What is the final temperature of the mixture? (Assume no heat loss to the surroundings and $$C = 4.18 J/g$$·$$^\circC)$$

Accepted Answer

The final temperature is $$50.0$$ $$^\circC$$

Question 11

Which of the following best describes a formation reaction?

Accepted Answer

A reaction in which one mole of a compound is formed from its elements in their standard states.

Question 12

Which of the following is the correct sign for $$\Delta H in an $$exothermic reaction?

Accepted Answer

$$\Delta H is $$negative

Question 13

A chemical reaction absorbs 150 kJ of heat from the surroundings. Which term best describes this process?

Accepted Answer

Endothermic

Question 14

Which of the following is NOT a state function?

Accepted Answer

Heat ($$q)$$

Question 15

If the enthalpy of formation of $$CO_2(g$$)$$ is -393.5$ kJ/mol and that of H_2O(l$$)$$ is -285.8$ kJ/mol, what is the enthalpy change for the combustion of 1 mol of ethanol (C_2H_5OH$) given the reaction: C_2H_5OH(l) + 3O_2(g$$) \rightarrow 2CO_2(g) + 3H_2O(l)$$?

Accepted Answer

$$\Delta H = [2 \times (-393.5) + 3 \times (-285.8)] - [(-277.0) + 0] = -1367 kJ$$

Question 16

Which of the following statements about enthalpy is correct?

Accepted Answer

Enthalpy is a state function that depends only on the initial and final states of the system.

Thermochemistry: Study Notes and Practice Problems

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