Q: In a coffee-cup calorimeter experiment, 1.00 g of magnesium reacts with excess HCl. The temperature of the solution increases by $$7.2\ ^\circ\mathrm{C}. If $$the solution has a mass of $$100.0\ \mathrm{g}$$ and a specific heat of $$4.18\ \mathrm{J/g\ ^\circ C}$$, what is the heat evolved ($$q_{soln}$$)?

$$q_{soln} = 100.0 \times 4.18 \times 7.2 = 3010\ \mathrm{J}$$

Q: A student mixes 50.0 g of solution at $$25.0\ ^\circ\mathrm{C}$$ with another 50.0 g of solution at $$35.0\ ^\circ\mathrm{C} in a $$coffee-cup calorimeter. If the final temperature is $$30.0\ ^\circ\mathrm{C}$$ and the specific heat is $$4.18\ \mathrm{J/g\ ^\circ C}$$, what is the heat gained by the cooler solution?

$$q = 50.0 \times 4.18 \times (30.0 - 25.0) = 1045\ \mathrm{J}$$

Question 1

Which of the following statements best describes the enthalpy change ($$\Delta H) $$for a chemical reaction occurring at constant pressure?

Accepted Answer

$$\Delta H is $$the amount of heat exchanged with the surroundings at constant pressure.

Question 2

A student burns propane ($$\mathrm{C_3H_8}) in a $$laboratory experiment at constant pressure. Which equation correctly relates the enthalpy change to the heat evolved?

Accepted Answer

$$q_p = \Delta H$$

Question 3

Which of the following processes is endothermic and would have a positive $$\Delta H$$?

Accepted Answer

Sweat evaporating from skin

Question 4

Given the reaction $$\mathrm{C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)}$$ with $$\Delta H = -2044\ \mathrm{kJ}$$, what does the negative sign of $$\Delta H$$ indicate?

Accepted Answer

The reaction is exothermic and releases heat.

Question 5

If 2.00 mol of $$\mathrm{C_3H_8}$$ are combusted completely, how much heat is evolved? ($$\Delta H = -2044\ \mathrm{kJ}$$ per mol $$\mathrm{C_3H_8})$$

Accepted Answer

$$-4090\ \mathrm{kJ}$$

Question 6

Which of the following best describes the difference between $$\Delta H$$ and $$\Delta E$$ for a chemical reaction at constant pressure?

Accepted Answer

$$\Delta H$$ and $$\Delta E$$ are identical when no $$P\Delta V$$ work is done.

Question 7

A coffee-cup calorimeter is used to measure the enthalpy change of a reaction. Which assumption is made about the heat exchange?

Accepted Answer

The heat gained by the solution equals the heat evolved by the reaction.

Question 8

In a coffee-cup calorimeter experiment, 1.00 g of magnesium reacts with excess HCl. The temperature of the solution increases by $$7.2\ ^\circ\mathrm{C}. If $$the solution has a mass of $$100.0\ \mathrm{g}$$ and a specific heat of $$4.18\ \mathrm{J/g\ ^\circ C}$$, what is the heat evolved ($$q_{soln}$$)?

Accepted Answer

$$q_{soln} = 100.0 	imes 4.18 	imes 7.2 = 3010\ \mathrm{J}$$

Question 9

Which of the following reactions is exothermic?

Accepted Answer

Combustion of gasoline in an automobile engine

Question 10

According to Hess's Law, if a reaction can be expressed as the sum of a series of steps, how is the overall enthalpy change ($$\Delta H_{rxn}) $$determined?

Accepted Answer

$$\Delta H_{rxn} is $$the sum of the enthalpy changes of each step.

Question 11

A chemical reaction at constant pressure absorbs heat from the surroundings. Which of the following is true?

Accepted Answer

The reaction is endothermic and $$\Delta H is $$positive.

Question 12

Which of the following best explains why the coffee-cup calorimeter is suitable for measuring $$\Delta H_{rxn}$$ for many reactions?

Accepted Answer

It allows reactions to occur at constant pressure, matching most laboratory conditions.

Question 13

If the enthalpy change for the reaction $$\mathrm{A + 2B \rightarrow C} is$$ $$\Delta H_1$, and for $$\mathrm{C \rightarrow 2D}$$ is $$\Delta $$H_2$$, what is the enthalpy change for $$\mathrm{A + 2B \rightarrow 2D}$$?

Accepted Answer

$$\Delta H_1$ + \Delta H_2$

Question 14

A student mixes 50.0 g of solution at $$25.0\ ^\circ\mathrm{C}$$ with another 50.0 g of solution at $$35.0\ ^\circ\mathrm{C} in a $$coffee-cup calorimeter. If the final temperature is $$30.0\ ^\circ\mathrm{C}$$ and the specific heat is $$4.18\ \mathrm{J/g\ ^\circ C}$$, what is the heat gained by the cooler solution?

Accepted Answer

$$q = 50.0 	imes 4.18 	imes (30.0 - 25.0) = 1045\ \mathrm{J}$$

Question 15

Which of the following is a direct consequence of Hess's Law in practical chemistry?

Accepted Answer

It allows calculation of $$\Delta H_{rxn}$$ for reactions that are difficult to measure directly.

Question 16

Which of the following would NOT affect the accuracy of a coffee-cup calorimeter measurement?

Accepted Answer

Accurate measurement of mass and temperature change

Question 17

A reaction is performed in a coffee-cup calorimeter and the temperature of the solution decreases. What can you conclude about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat.

Question 18

Which of the following best describes the relationship between $$q_p$$, $$\Delta H$$, and $$\Delta E$$ for a reaction at constant pressure?

Accepted Answer

$$q_p = \Delta H$$

Question 19

In a stoichiometric calculation involving $$\Delta H$$, why is it important to use the balanced chemical equation?

Accepted Answer

Because $$\Delta H$$ depends on the amount of reactants and products actually reacting.

Enthalpy, Calorimetry, and Stoichiometry in Chemical Reactions

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