Q: A gas expands against a constant external pressure of 1.00 atm, increasing its volume by 2.50 L. What is the work done by the system? (1 L·atm = 101.3 J)

$$w = -P\Delta V = -(1.00\,\text{atm})(2.50\,\text{L}) = -2.50\,\text{L·atm} = -253.25\,\text{J}$$

Q: Which of the following processes is exothermic?

Combustion of methane: $$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Q: Calculate the heat required to raise the temperature of 100 g of water from 20°C to 25°C. (Specific heat of water = 4.184 J/g·K)

$$q = m s \Delta T = (100\,\text{g})(4.184\,\text{J/g·K})(5\,\text{K}) = 2092\,\text{J}$$

Q: Given the reaction: $$2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2$$ with $$\Delta H = -89.4 kJ$$, what is the enthalpy change for the formation of 0.855 mol of $$\text{O}_2$$?

$$\Delta H = -89.4\,\text{kJ} \times \frac{0.855}{3} = -25.5\,\text{kJ}$$

Question 1

Which of the following best describes the difference between heat and work in thermochemistry?

Accepted Answer

Heat is the energy transferred due to a temperature difference, while work is the energy transferred when an object is moved by a force.

Question 2

A metal rod is heated in a Bunsen burner flame until it glows red (as shown in the image). Which type of energy transfer is primarily responsible for the rod's temperature increase?

Accepted Answer

Heat transfer from the flame to the rod.

Question 3

Which of the following is a state function?

Accepted Answer

Internal energy ($$E)$$

Question 4

A ball is lifted to the top of a wall and then dropped, as shown in the sequence of images. Which statement best describes the energy changes as the ball falls?

Accepted Answer

Potential energy is converted to kinetic energy, and upon hitting the ground, kinetic energy is transformed into heat and deformation.

Question 5

Which equation correctly expresses the First Law of Thermodynamics for a closed system?

Accepted Answer

$$\Delta E = q + w$$

Question 6

A gas expands against a constant external pressure of 1.00 atm, increasing its volume by 2.50 L. What is the work done by the system? (1 L·atm = 101.3 J)

Accepted Answer

$$w = -P\Delta V = -(1.00\,	ext{atm})(2.50\,	ext{L}) = -2.50\,	ext{L·atm} = -253.25\,	ext{J}$$

Question 7

Which of the following processes is exothermic?

Accepted Answer

Combustion of methane: $$	ext{CH}_4 + 2	ext{O}_2 ightarrow 	ext{CO}_2 + 2	ext{H}_2	ext{O}$$

Question 8

A reaction is carried out in a bomb calorimeter. Which quantity is directly measured?

Accepted Answer

Change in internal energy ($$\Delta E)$$

Question 9

Calculate the heat required to raise the temperature of 100 g of water from 20°C to 25°C. (Specific heat of water = 4.184 J/g·K)

Accepted Answer

$$q = m s \Delta T = (100\,	ext{g})(4.184\,	ext{J/g·K})(5\,	ext{K}) = 2092\,	ext{J}$$

Question 10

Which of the following statements about enthalpy ($$H) is $$correct?

Accepted Answer

At constant pressure, the change in enthalpy ($$\Delta H) $$equals the heat absorbed or released by the system.

Question 11

Given the reaction: $$2	ext{KClO}_3 ightarrow 2	ext{KCl} + 3	ext{O}_2$$ with $$\Delta H = -89.4 kJ$$, what is the enthalpy change for the formation of 0.855 mol of $$	ext{O}_2$$?

Accepted Answer

$$\Delta H = -89.4\,	ext{kJ} 	imes \frac{0.855}{3} = -25.5\,	ext{kJ}$$

Question 12

Which of the following best explains why the internal energy of a system is a state function?

Accepted Answer

It depends only on the initial and final states, not on the path taken.

Question 13

Which of the following is true for an endothermic process at constant pressure?

Accepted Answer

$$\Delta H \u003e 0$$ and heat is absorbed by the system.

Question 14

Which of the following equations correctly uses standard enthalpies of formation to calculate the enthalpy change for a reaction?

Accepted Answer

$$\Delta H = \sum n \Delta H_f(	ext{products}) - \sum m \Delta H_f(	ext{reactants})$$

Question 15

Which phase change absorbs the most energy per gram?

Accepted Answer

Liquid to gas

Question 16

Which of the following best describes the energy changes during the combustion of gasoline in a car engine?

Accepted Answer

Chemical potential energy is converted to heat and work, with most energy lost as heat.

Question 17

Which of the following reactions is most exothermic based on bond enthalpies?

Accepted Answer

$$	ext{CH}_4(g) + 	ext{F}_2(g) ightarrow 	ext{CH}_3	ext{F}(g) + 	ext{HF}(g)$$

Question 18

Which of the following best explains why breaking chemical bonds requires energy?

Accepted Answer

Bond breaking is an endothermic process because energy must be supplied to overcome the attractive forces between atoms.

Thermochemistry: Energy, Heat, and Enthalpy in Chemical Reactions

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