Q: A gas absorbs 13 J of heat and then performs 13 J of work. The change in internal energy of the gas is

$$\Delta E = q + w = 13\ \text{J} + (-13\ \text{J}) = 0\ \text{J}$$

Q: Calculate the work gained or lost by the system when a gas contracts from 35 L to 15 L against a constant external pressure of 1.5 atm. ($$w = -P \Delta V$$; 1 L atm = 101.3 J)

$$w = -1.5\ \text{atm} \times (15\ \text{L} - 35\ \text{L}) = -1.5 \times (-20) = +30\ \text{L atm} = +3.0\ \text{kJ}$$

Question 1

Which of the following best describes the direction of the dipole moment in H$$_2C=$$CCl$$_2$$? (Refer to the images provided.)

Accepted Answer

The arrow points from the hydrogen side toward the chlorine side (Image 2).

Question 2

Which of the following images best represents hydrogen bonding between two water molecules?

Accepted Answer

Image 6: Dashed line between O and H of different molecules.

Question 3

Which of the following is the correct order of boiling points for KNO$$_3$$, C$$_2H$_5OH$$, C$$_3H$_8$$, and He?

Accepted Answer

He \u003c C$$_3H$_8$$ \u003c C$$_2H$_5OH$$ \u003c KNO$$_3$$

Question 4

What types of intermolecular forces exist in a solution of HF in CH$$_3OH$$?

Accepted Answer

Dispersion forces, dipole-dipole forces, and hydrogen bonding

Question 5

What bonds, if any, are broken as water boils?

Accepted Answer

Hydrogen bonds between the H$$_2O $$molecules.

Question 6

If atoms or molecules of the following gases have the same average kinetic energy, which gas has the highest average velocity?

Accepted Answer

H$$_2(g)$$

Question 7

A gas absorbs 13 J of heat and then performs 13 J of work. The change in internal energy of the gas is

Accepted Answer

$$\Delta E = q + w = 13\ 	ext{J} + (-13\ 	ext{J}) = 0\ 	ext{J}$$

Question 8

Calculate the work gained or lost by the system when a gas contracts from 35 L to 15 L against a constant external pressure of 1.5 atm. ($$w = -P \Delta V$$; 1 L atm = 101.3 J)

Accepted Answer

$$w = -1.5\ 	ext{atm} 	imes (15\ 	ext{L} - 35\ 	ext{L}) = -1.5 	imes (-20) = +30\ 	ext{L atm} = +3.0\ 	ext{kJ}$$

Question 9

When a Cheeto (a carbohydrate containing food) is burned in air to produce carbon dioxide and water, the amount of energy released is _____________ the amount of energy released when a person eats a Cheeto and metabolizes the carbohydrates into carbon dioxide and water because energy change is a ________.

Accepted Answer

the same as, a state function

Question 10

Imagine a reaction that results in a change in both volume and temperature, as shown in the diagram (Image 9). What is the sign of the work being done, and what is the sign of the enthalpy change involved in this reaction?

Accepted Answer

w is (-) and $$\Delta H is (-).$$

Question 11

Which of the following statements regarding phase changes and their associated enthalpies is true?

Accepted Answer

The enthalpy of sublimation is always equal to the sum of the enthalpies of fusion and vaporization.

Question 12

Two metals of equal mass with different heat capacities are subjected to the same amount of heat. Which undergoes the smallest change in temperature?

Accepted Answer

The metal with the highest heat capacity.

Question 13

The specific heat of copper is 0.385 J/(g·°C). If 34.2 g of copper, initially at 25°C, absorbs 7.880 kJ, what will be the final temperature of the copper?

Accepted Answer

$$T_f = 25 + \frac{7880}{34.2 	imes 0.385} = 623\ ^\circ	ext{C}$$

Question 14

When 12.5 g of ammonium nitrate (NH$$_4NO$_3$$, 80.0 g/mol) is dissolved in 150.0 g of water at 25.0°C in a coffee cup calorimeter, the final temperature of the solution is 19.7°C. Assume specific heat of the solution is the same as water, 4.18 J/g·°C. What is $$\Delta H$$ per mol of NH$$_4NO$_3$$?

Accepted Answer

$$\Delta H = +21.3\ 	ext{kJ}$$

Question 15

Given the heats of reaction for reactions (1) and (2) below, which choice correctly describes how to combine $$\Delta H(1)$$ and $$\Delta H(2) to $$calculate the value of $$\Delta H(3)$$ for reaction (3)?
(1) $$2\ 	ext{H}_2(g) + 	ext{O}_2(g) ightarrow 2\ 	ext{H}_2	ext{O}(l)
(2$$) $$	ext{H}_2(g) + 	ext{O}_2(g) ightarrow 	ext{H}_2	ext{O}_2(l)
(3$$) $$2\ 	ext{H}_2	ext{O}(l) ightarrow 	ext{H}_2	ext{O}_2(l) + 	ext{H}_2(g)$$

Accepted Answer

$$\Delta H(3) = -\Delta H(1) + \Delta H(2)$$

Question 16

As the bond order of a bond increases, its bond energy __________ and its bond length __________.

Accepted Answer

increases, decreases

Question 17

Heat of formation for CO is $$-110.5 kJ/$$mol and the heat of formation for MgO is $$-602 kJ/$$mol. Which of the following statements is true?

Accepted Answer

The bond in MgO is stronger than the bond in CO.

C117 Exam 3 Practice – Step-by-Step Chemistry Guidance

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