Q: Calculate the specific heat ($$c) of a $$glass sample if 150 J of heat raises the temperature of 50.0 g by 3.57°C.

$$c = \frac{150}{50.0 \times 3.57} = 0.841\ \mathrm{J/g\ ^\circ C}$$

Q: In a coffee-cup calorimeter, a student mixes 0.158 g Mg with HCl, and the temperature of 100.0 g solution rises from 25.6°C to 32.8°C. If $$c_s = 4.18\ \mathrm{J/(g\ ^\circ C)}$$, what is the heat absorbed by the solution?

$$q = 100.0 \times 4.18 \times (32.8 - 25.6) = 3018\ \mathrm{J}$$

Question 1

Which of the following best describes the difference between kinetic energy and potential energy, as illustrated by a compressed spring launching a ball?

Accepted Answer

Potential energy is stored in the compressed spring due to its position, and is converted to kinetic energy as the ball moves upward.

Question 2

According to the first law of thermodynamics, which of the following statements is correct?

Accepted Answer

Energy is neither created nor destroyed, only transformed or transferred.

Question 3

A chemical reaction occurs in a balloon, causing it to shrink as gas is consumed. What is the sign of work ($$w) $$for the system?

Accepted Answer

Work is positive ($$w \u003e 0) $$because the surroundings do work on the system.

Question 4

Which of the following is a state function?

Accepted Answer

The internal energy ($$E) of $$the system

Question 5

A hot pack and a cold pack are used for injuries. Which contains an exothermic reaction?

Accepted Answer

The hot pack, because it releases heat to the surroundings.

Question 6

Calculate the specific heat ($$c) of a $$glass sample if 150 J of heat raises the temperature of 50.0 g by 3.57°C.

Accepted Answer

$$c = \frac{150}{50.0 	imes 3.57} = 0.841\ \mathrm{J/g\ ^\circ C}$$

Question 7

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction in which heat is released by the system ($$q \u003c 0)$$

Question 8

In a coffee-cup calorimeter, a student mixes 0.158 g Mg with HCl, and the temperature of 100.0 g solution rises from 25.6°C to 32.8°C. If $$c_s = 4.18\ \mathrm{J/(g\ ^\circ C)}$$, what is the heat absorbed by the solution?

Accepted Answer

$$q = 100.0 	imes 4.18 	imes (32.8 - 25.6) = 3018\ \mathrm{J}$$

Question 9

Which of the following is a main assumption in coffee-cup calorimetry experiments?

Accepted Answer

All the heat from the reaction is transferred to the solution.

Question 10

Which equation correctly relates the enthalpy change ($$\Delta H_{rxn}) to $$the internal energy change ($$\Delta E_{rxn}) at $$constant pressure?

Accepted Answer

$$\Delta H_{rxn} = \Delta E_{rxn} + P \Delta V_{rxn}$$

Question 11

Given the following reactions and their enthalpy changes, use Hess's Law to calculate $$\Delta H_{rxn}$$ for $$2SO_2(g) + O_2$(g) \rightarrow 2SO_3(g)$$:

$$SO_2(g$$) \rightarrow S(s) + $$O_2(g$$)\ \Delta H_{rxn} = +296.8\ \mathrm{kJ/mol}$$
2S(s) + 3O_2(g$$) \rightarrow 2SO_3(g)\ \Delta H_{rxn} = -791.4\ \mathrm{kJ/mol}$$

Accepted Answer

$$\Delta H_{rxn} = -197.8\ \mathrm{kJ/mol}$$

Question 12

Which of the following substances has a standard enthalpy of formation ($$\Delta H_f^\circ) of $$zero?

Accepted Answer

$$\mathrm{Na(s)}$$

Question 13

Using the following standard enthalpies of formation, calculate $$\Delta H_{rxn}^\circ$$ for the reaction:

$$\mathrm{CH_3CH_2OH(l) + 3 O_2$(g) \rightarrow 2 CO_2$(g) + 3 H_2O(l)}$$

$$\Delta H_f^\circ [\mathrm{CH_3CH_2OH(l)}] = -277.6\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{CO_2$(g)}] = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{H_2O(l)}] = -285.8\ \mathrm{kJ/mol}$$

Accepted Answer

$$\Delta H_{rxn}^\circ = [2 	imes (-393.5) + 3 	imes (-285.8)] - [-277.6 + 3 	imes 0] = -1367.0\ \mathrm{kJ}$$

Question 14

Which equation is used to estimate the enthalpy change of a reaction using average bond energies?

Accepted Answer

$$\Delta H_{rxn} \approx H_{bond}[	ext{reactants}] - H_{bond}[	ext{products}]$$

Question 15

Use the following bond energies to estimate $$\Delta H_{rxn}$$ for the reaction:

$$\mathrm{CH_4$(g) + Cl_2$(g) \rightarrow CH_3Cl(g) + HCl(g)}$$

Bond energies (kJ/mol): C-H = 414; Cl-Cl = 243; C-Cl = 331; H-Cl = 431

Accepted Answer

$$\Delta H_{rxn} = [414 + 243] - [331 + 431] = -105\ \mathrm{kJ}$$

Question 16

Which of the following best explains why the elevation at the top of a mountain is a state function?

Accepted Answer

Because the elevation depends only on the initial and final positions, not the path taken.

Question 17

Which of the following is true about the sign of $$\Delta E$$ for a reaction where the products have lower internal energy than the reactants?

Accepted Answer

$$\Delta E is $$negative.

Chapter 9: Thermochemistry – Energy, Heat, and Enthalpy in Chemical Reactions

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