Q: Which of the following equations correctly expresses the relationship between enthalpy change and the amount of substance undergoing a reaction?

$$\Delta H = n \cdot \Delta H_{\text{rxn}}$$

Question 1

Which of the following best describes the system in the context of thermodynamics?

Accepted Answer

The part of the universe we are interested in studying, separated by a boundary from the surroundings.

Question 2

Which equation correctly represents the enthalpy change ($$\Delta H) $$for the combustion of magnesium as shown in the notes?

Accepted Answer

$$2\mathrm{Mg}(s) + \mathrm{O}_2(g) \rightarrow 2\mathrm{MgO}(s)$$

Question 3

Which of the following statements about exothermic and endothermic processes is correct?

Accepted Answer

Exothermic processes release heat to the surroundings.

Question 4

If a chemical reaction occurs in an open beaker at constant pressure, which thermodynamic quantity is most directly measured?

Accepted Answer

Enthalpy change ($$\Delta H)$$

Question 5

A student mixes two solutions in a calorimeter and observes the temperature of the mixture increases. Which of the following is true?

Accepted Answer

The reaction is exothermic and heat is released to the surroundings.

Question 6

Which of the following best describes the difference between the system and the surroundings in thermodynamics?

Accepted Answer

The system is the chemical reaction or process being studied, while the surroundings are everything else.

Question 7

Which law of thermodynamics states that energy cannot be created or destroyed, only transferred or transformed?

Accepted Answer

First Law of Thermodynamics

Question 8

Given the reaction $$2\mathrm{Mg}(s) + \mathrm{O}_2(g) \rightarrow 2\mathrm{MgO}(s)$$ with $$\Delta H = -1204\ \mathrm{kJ}$$, what is the enthalpy change if only 1 mole of Mg reacts?

Accepted Answer

$$-602\ \mathrm{kJ}$$

Question 9

Which of the following is an example of an open system?

Accepted Answer

A cup of hot coffee cooling in a room.

Question 10

Which of the following best describes an endothermic process?

Accepted Answer

A process in which heat is absorbed by the system from the surroundings.

Question 11

If the enthalpy change for a reaction is $$\Delta H = +50\ \mathrm{kJ}$$, which of the following statements is true?

Accepted Answer

The reaction is endothermic and absorbs 50 kJ of heat.

Question 12

Which of the following is NOT a type of heat transfer discussed in the notes?

Accepted Answer

Sublimation

Question 13

A chemical reaction occurs in a beaker and the temperature of the beaker decreases. What can you conclude about the reaction?

Accepted Answer

The reaction is endothermic.

Question 14

Which of the following equations correctly expresses the relationship between enthalpy change and the amount of substance undergoing a reaction?

Accepted Answer

$$\Delta H = n \cdot \Delta H_{	ext{rxn}}$$

Question 15

Which of the following is a correct statement about thermochemical equations?

Accepted Answer

The enthalpy change is directly proportional to the number of moles of reactants.

Question 16

Which of the following best describes the First Law of Thermodynamics?

Accepted Answer

The total energy of the universe is constant.

Question 17

A reaction has $$\Delta H = -200\ \mathrm{kJ}$$ and is carried out at constant pressure. What does the negative sign indicate?

Accepted Answer

The system releases 200 kJ of heat.

Question 18

Which of the following is an example of a closed system?

Accepted Answer

A sealed container in which a reaction occurs, but heat can be exchanged with the surroundings.

Question 19

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

Accepted Answer

$$\Delta H is $$positive for endothermic reactions and negative for exothermic reactions.

Question 20

If 0.5 mol of Mg reacts with excess $$\mathrm{O}_2$$ according to $$2\mathrm{Mg}(s) + \mathrm{O}_2(g) \rightarrow 2\mathrm{MgO}(s)$$ and $$\Delta H = -1204\ \mathrm{kJ}$$, what is the enthalpy change?

Accepted Answer

$$-301\ \mathrm{kJ}$$

Ch. 5 Thermochemistry and Energy Changes in Chemical Reactions

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