Q: A student dissolves 5.00 g of NaOH in water in a coffee-cup calorimeter. The temperature of the solution increases by $$7.5\ ^\circ\mathrm{C}. If $$the solution has a mass of 100.0 g and a specific heat capacity of $$4.18\ \mathrm{J}/\mathrm{g}\cdot\mathrm{K}$$, what is the heat absorbed by the solution?

$$q = 100.0 \times 4.18 \times 7.5 = 3,135\ \mathrm{J}$$

Q: A bomb calorimeter is used to measure the heat of combustion of glucose. The temperature of the calorimeter increases by $$2.50\ ^\circ\mathrm{C}. If $$the heat capacity of the calorimeter is $$8.00\ \mathrm{kJ}/\mathrm{K}$$, what is the total heat released?

$$q = 8.00 \times 2.50 = 20.0\ \mathrm{kJ}$$

Q: Given the reaction: $$\mathrm{C}_2\mathrm{H}_4(g) + 3\mathrm{O}_2(g) \rightarrow 2\mathrm{CO}_2(g) + 2\mathrm{H}_2\mathrm{O}(l)$$, and the enthalpy of reaction $$\Delta H = -1,323\ \mathrm{kJ}$$, how much heat is released when 10.0 g of $$\mathrm{C}_2\mathrm{H}_4 is $$combusted? (Molar mass of $$\mathrm{C}_2\mathrm{H}_4 = 28.05\ \mathrm{g/mol})$$

$$\frac{10.0}{28.05} \times (-1,323) = -472\ \mathrm{kJ}$$

Question 1

Which of the following best describes how a coffee-cup calorimeter is used to measure the enthalpy change ($$\Delta H) of a $$chemical reaction?

Accepted Answer

It measures the temperature change of the solution, assuming constant pressure, to calculate the enthalpy change ($$\Delta H) $$using $$q_p = m c \Delta T.$$

Question 2

Which equation correctly relates heat ($$q$$), mass ($$m$$), specific heat capacity ($$c$$), and temperature change ($$\Delta T) in $$calorimetry?

Accepted Answer

$$q = m c \Delta T$$

Question 3

A student dissolves 5.00 g of NaOH in water in a coffee-cup calorimeter. The temperature of the solution increases by $$7.5\ ^\circ\mathrm{C}. If $$the solution has a mass of 100.0 g and a specific heat capacity of $$4.18\ \mathrm{J}/\mathrm{g}\cdot\mathrm{K}$$, what is the heat absorbed by the solution?

Accepted Answer

$$q = 100.0 	imes 4.18 	imes 7.5 = 3,135\ \mathrm{J}$$

Question 4

Which statement correctly distinguishes between enthalpy ($$H) $$and internal energy ($$U$$)?

Accepted Answer

Enthalpy ($$H) $$includes the energy required to create a system and the energy needed to make room for it by displacing its environment, while internal energy ($$U) is $$the total energy contained within the system.

Question 5

During a chemical reaction, the enthalpy change ($$\Delta H) is $$negative. What does this indicate about the reaction?

Accepted Answer

The reaction is exothermic and releases heat to the surroundings.

Question 6

Which formula is used to calculate the enthalpy change ($$\Delta H) $$for a reaction at constant pressure?

Accepted Answer

$$\Delta H = q_p$$

Question 7

A bomb calorimeter is used to measure the heat of combustion of glucose. The temperature of the calorimeter increases by $$2.50\ ^\circ\mathrm{C}. If $$the heat capacity of the calorimeter is $$8.00\ \mathrm{kJ}/\mathrm{K}$$, what is the total heat released?

Accepted Answer

$$q = 8.00 	imes 2.50 = 20.0\ \mathrm{kJ}$$

Question 8

Which of the following best explains why enthalpy ($$\Delta H) is $$used instead of internal energy ($$\Delta U) $$for reactions at constant pressure?

Accepted Answer

Because $$\Delta H$$ accounts for both heat flow and work done by expansion or compression at constant pressure.

Question 9

Given the reaction: $$\mathrm{C}_2\mathrm{H}_4(g) + 3\mathrm{O}_2(g) \rightarrow 2\mathrm{CO}_2(g) + 2\mathrm{H}_2\mathrm{O}(l)$$, and the enthalpy of reaction $$\Delta H = -1,323\ \mathrm{kJ}$$, how much heat is released when 10.0 g of $$\mathrm{C}_2\mathrm{H}_4 is $$combusted? (Molar mass of $$\mathrm{C}_2\mathrm{H}_4 = 28.05\ \mathrm{g/mol})$$

Accepted Answer

$$\frac{10.0}{28.05} 	imes (-1,323) = -472\ \mathrm{kJ}$$

Question 10

Which of the following relationships is correct for a reaction at constant pressure?

Accepted Answer

$$\Delta H = \Delta U + P \Delta V$$

Question 11

A chemical reaction in a coffee-cup calorimeter causes the temperature of the solution to decrease. What can be concluded about the sign of $$q$$ and $$\Delta H$$ for the reaction?

Accepted Answer

$$q$$ and $$\Delta H$$ are both positive; the reaction is endothermic.

Question 12

Which of the following is a correct application of stoichiometry to calculate the heat released in a reaction when the enthalpy of reaction ($$\Delta H_{rxn}) is $$known?

Accepted Answer

Multiply the number of moles of reactant by $$\Delta H_{rxn} to $$find the total heat released.

Question 13

If the enthalpy of reaction ($$\Delta H_{rxn}) $$for the combustion of methane is $$-890\ \mathrm{kJ}$$ per mole, how much heat is released when 2.00 moles of methane are combusted?

Accepted Answer

$$2.00 	imes (-890) = -1,780\ \mathrm{kJ}$$

Question 14

Which of the following is NOT a correct use of calorimetry in chemistry?

Accepted Answer

Directly measuring the enthalpy of formation of a compound without any calculations.

Question 15

Which equation would you use to determine the enthalpy change ($$\Delta H) of a $$reaction using data from a coffee-cup calorimeter?

Accepted Answer

$$\Delta H = q_p = m c \Delta T$$

Question 16

A student mixes two solutions in a coffee-cup calorimeter and observes a temperature increase. Which of the following is the best explanation for this observation?

Accepted Answer

The reaction is exothermic, and heat is released into the solution.

Question 17

Which of the following statements about enthalpy ($$\Delta H) is $$true?

Accepted Answer

A positive $$\Delta H$$ indicates that the system absorbs heat from the surroundings.

Question 18

Which of the following is the correct definition of enthalpy of reaction ($$\Delta H_{rxn}$$)?

Accepted Answer

The heat absorbed or released during a chemical reaction at constant pressure.

Question 19

Which of the following is a quantitative relationship between a chemical equation and the enthalpy of reaction ($$\Delta H_{rxn}$$)?

Accepted Answer

The enthalpy change is proportional to the number of moles of reactants and products as written in the balanced equation.

Thermochemistry: Enthalpy, Calorimetry, and Energy Changes in Chemical Reactions

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