Q: In a coffee cup calorimeter, a 29.78 g sample of magnesium ribbon (Mg, $$C = 1.04 J/g$$°C) is ignited in water at 25°C. After equilibrium, the temperature is 30.0°C. What is the heat absorbed by the water?

$$q = mC\Delta T = 29.78 \times 1.04 \times (30.0 - 25.0) = 154.9 J$$

Question 1

Which of the following best describes a state function in thermodynamics?

Accepted Answer

A property that depends only on the initial and final states of a system, not on the path taken.

Question 2

Which equation correctly relates the change in internal energy ($$\Delta E) to $$heat ($$q) $$and work ($$w$$)?

Accepted Answer

$$\Delta E = q + w$$

Question 3

A bomb calorimeter is used to measure the energy change of a reaction at constant volume. Which of the following is true about the measurement?

Accepted Answer

It measures the change in internal energy ($$\Delta E) of $$the reaction.

Question 4

Which of the following statements about specific heat capacity ($$C) is $$correct?

Accepted Answer

It is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

Question 5

In a coffee cup calorimeter, a 29.78 g sample of magnesium ribbon (Mg, $$C = 1.04 J/g$$°C) is ignited in water at 25°C. After equilibrium, the temperature is 30.0°C. What is the heat absorbed by the water?

Accepted Answer

$$q = mC\Delta T = 29.78 	imes 1.04 	imes (30.0 - 25.0) = 154.9 J$$

Question 6

Which of the following best describes the difference between kinetic and potential energy?

Accepted Answer

Kinetic energy is energy due to motion, while potential energy is stored energy due to position or composition.

Question 7

Which of the following reactions would produce significant work due to a change in volume ($$\Delta V) $$under constant pressure?

Accepted Answer

$$\mathrm{Zn (s) + 2 H^+ (aq) \rightarrow Zn$$^{2+}$$ (aq) + H_2$ (g)}$$

Question 8

Which formula is used to calculate the amount of heat exchanged ($$q) by a $$material?

Accepted Answer

$$q = mC\Delta T$$

Question 9

Which of the following is true about enthalpy ($$\Delta H) $$measured at constant pressure?

Accepted Answer

It includes the energy from both heat and pressure-volume work.

Question 10

Which of the following is the correct expression for pressure-volume work ($$w) $$done by a system at constant pressure?

Accepted Answer

$$w = -P_{ext} \Delta V$$

Question 11

A student dissolves 9.25 g of NH$$_4NO$_3 in 100.0 mL of $$water at 25°C in a coffee cup calorimeter. The temperature decreases by 6.1°C. What does this indicate about the enthalpy of dissolution?

Accepted Answer

The process is endothermic; $$\Delta H is $$positive.

Question 12

Which of the following is the correct definition of the standard enthalpy of formation ($$\Delta H_f^\circ) $$for a compound?

Accepted Answer

The enthalpy change when one mole of a compound is formed from its elements in their standard states.

Question 13

Which method uses the following equation to calculate the enthalpy change of a reaction: $$\Delta H_{rxn} = \sum \Delta H_f^\circ (	ext{products}) - \sum \Delta H_f^\circ (	ext{reactants})$$?

Accepted Answer

Direct method using standard enthalpies of formation.

Question 14

Which of the following is a correct application of Hess's Law?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for individual steps, regardless of the path taken.

Question 15

Which of the following best explains why the theoretical NaCl$$_2$$ unit cannot exist in real life?

Accepted Answer

The energy required to remove two electrons from Na is much greater than the lattice energy gained, making the process energetically unfavorable.

Question 16

Which equation is used to calculate the enthalpy change of a reaction using bond enthalpies?

Accepted Answer

$$\Delta H_{rxn} = \sum 	ext{Bond Energies (bonds broken)} - \sum 	ext{Bond Energies (bonds formed)}$$

Question 17

Why do the methods using $$\Delta H_f^\circ$$ and bond enthalpies not always give the same result for $$\Delta H_{rxn}$$?

Accepted Answer

Bond enthalpy values are averages and do not account for the specific molecular environment.

Question 18

Which of the following statements about calorimetry is correct?

Accepted Answer

Calorimetry measures the heat exchanged in a chemical or physical process, allowing calculation of energy changes.

Thermochemistry and Thermodynamics: Energy, Enthalpy, and Calorimetry

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