Q: Which of the following is the correct formula for calculating the standard enthalpy change of a reaction using standard enthalpies of formation?

$$\Delta H_{rxn}^\circ = \sum \Delta H_f^\circ (\text{products}) - \sum \Delta H_f^\circ (\text{reactants})$$

Question 1

Which of the following best describes the main focus of thermochemistry?

Accepted Answer

The study of the interaction of matter with heat.

Question 2

Which of the following is the correct SI unit for energy, and how is it defined in terms of mechanical work?

Accepted Answer

Joule (J); the energy transferred when a force of 1 newton moves an object 1 meter.

Question 3

Which of the following forces is most important in determining the enthalpy change (ΔH) of ordinary chemical reactions?

Accepted Answer

Electromagnetic force

Question 4

Given the equation for electrostatic potential energy $$E_{el} = k \cdot \frac{Q_1$ Q_2$}{d}$$, which compound would have a more negative (lower) lattice energy: NaCl or CaO? Why?

Accepted Answer

CaO, because the product of the charges is greater in magnitude.

Question 5

Which of the following statements about the First Law of Thermodynamics is correct?

Accepted Answer

Energy cannot be created or destroyed, only transformed or transferred.

Question 6

A reaction occurs in a closed flask. The system loses 800 J of heat and the surroundings do 200 J of work on the system. What is the change in internal energy ($$\Delta E$$)?

Accepted Answer

$$\Delta E = -600 J$$

Question 7

Which of the following is a state function?

Accepted Answer

Internal energy (E)

Question 8

Which of the following best describes an open system in thermodynamics?

Accepted Answer

A system that can exchange both mass and energy with its surroundings.

Question 9

A gas expands against a constant external pressure of 100 kPa from 2.0 L to 5.0 L. Calculate the work done by the system (in J). ($$1$$ kPa·L $$= 1 J)$$

Accepted Answer

$$w = -300 J$$

Question 10

Which of the following processes is exothermic?

Accepted Answer

Combustion of butane in oxygen

Question 11

Why must the physical states of reactants and products be specified in thermochemical equations?

Accepted Answer

Because the enthalpy change depends on the physical states involved.

Question 12

Which of the following is true about the enthalpy change ($$\Delta H) $$when a reaction is reversed?

Accepted Answer

$$\Delta H$$ changes sign but keeps the same magnitude.

Question 13

A student mixes 50 mL of 1.0 M HCl with 50 mL of 1.0 M NaOH in a coffee-cup calorimeter. The temperature rises from 21.0 °C to 27.5 °C. Assume the density is 1.0 g/mL and specific heat is 4.18 J/g·K. What is the heat released by the reaction?

Accepted Answer

2,722 J

Question 14

Which of the following best describes Hess’s Law?

Accepted Answer

The enthalpy change for a reaction is the same whether it occurs in one step or several steps.

Question 15

Which of the following is the correct formula for calculating the standard enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H_{rxn}^\circ = \sum \Delta H_f^\circ (	ext{products}) - \sum \Delta H_f^\circ (	ext{reactants})$$

Question 16

Which of the following statements about bond enthalpy is correct?

Accepted Answer

Bond enthalpy is always positive because breaking a bond requires energy.

Question 17

Estimate the enthalpy change for the reaction $$\mathrm{CH_4$(g) + Cl_2$(g) \rightarrow CH_3Cl(g) + HCl(g)}$$ using the following average bond enthalpies: $$D_{C-H} = 413 kJ/$$mol, $$D_{Cl-Cl} = 242 kJ/$$mol, $$D_{C-Cl} = 328 kJ/$$mol, $$D_{H-Cl} = 431 kJ/$$mol.

Accepted Answer

$$\Delta H = -14 kJ$$

Question 18

Why are combustion energy values (fuel values) reported as positive numbers in nutrition, but negative in chemistry?

Accepted Answer

Because in chemistry, energy released is negative (exothermic), but in nutrition, it is counted as usable energy (positive).

Question 19

Which of the following environmental impacts is directly associated with the combustion of fossil fuels?

Accepted Answer

Formation of acid rain due to SO$$_2$$ and NO$$_x$$ emissions.

Chapter 5: Thermochemistry – Energy Changes in Chemical Reactions

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