Question 1

Which statement best describes the First Law of Thermodynamics as it applies to chemical reactions?

Accepted Answer

Energy is conserved; it cannot be created or destroyed, only transferred or transformed.

Question 2

A student mixes 100 g of water at $$25\ ^\circ \mathrm{C}$$ with 100 g of water at $$75\ ^\circ \mathrm{C} in an $$insulated container. Assuming no heat is lost to the surroundings, what is the final temperature of the mixture? (Specific heat of water $$c = 4.18\ \mathrm{J/g\cdot K})$$

Accepted Answer

$$50\ ^\circ \mathrm{C}$$

Question 3

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction where $$\Delta H \u003c 0$$ and heat is released to the surroundings.

Question 4

A chemical reaction occurs in a coffee-cup calorimeter. The temperature of the solution increases. Which statement is true about the reaction?

Accepted Answer

The reaction is exothermic and releases heat to the solution.

Question 5

Which equation is used to calculate the heat absorbed or released in a calorimetry experiment?

Accepted Answer

$$q = m \cdot c \cdot \Delta T$$

Question 6

If the enthalpy of formation $$\Delta H^\circ_f$$ for $$\mathrm{CO_2$} is$$ $$-393.5\ \mathrm{kJ/mol}$$, what does this value represent?

Accepted Answer

The energy released when 1 mole of $$\mathrm{CO_2$} is $$formed from its elements in their standard states.

Question 7

Which of the following is a correct thermochemical equation for the combustion of methane ($$\mathrm{CH_4$}$$)?

Accepted Answer

$$\mathrm{CH_4$}(g) + 2\ \mathrm{O_2$}(g) \rightarrow \mathrm{CO_2$}(g) + 2\ \mathrm{H_2O}(l)\ \quad \Delta H = -890\ \mathrm{kJ}$$

Question 8

A reaction has $$\Delta H = +150\ \mathrm{kJ}. $$Which of the following statements is true?

Accepted Answer

The reaction is endothermic and absorbs heat.

Question 9

Which principle allows you to determine the enthalpy change for a reaction by combining the enthalpy changes of multiple reactions?

Accepted Answer

Hess's Law

Question 10

A student burns 2.00 g of ethanol ($$\mathrm{C_2H_5OH}) in a $$calorimeter containing 100.0 g of water. The temperature of the water increases from $$20.0\ ^\circ \mathrm{C} to$$ $$28.0\ ^\circ \mathrm{C}. $$What is the heat absorbed by the water? (Specific heat $$c = 4.18\ \mathrm{J/g\cdot K})$$

Accepted Answer

$$3344\ \mathrm{J}$$

Question 11

Which of the following statements about calorimetry is correct?

Accepted Answer

Calorimetry measures the heat transfer during a chemical reaction.

Question 12

If a reaction in a calorimeter causes the temperature of the solution to decrease, what can be concluded about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat from the solution.

Question 13

Which of the following is the correct definition of enthalpy ($$\Delta H$$)?

Accepted Answer

The heat flow at constant pressure.

Question 14

A reaction has the following steps:

Step 1: $$\mathrm{A} \rightarrow \mathrm{B}\ \quad \Delta H_1$ = +50\ \mathrm{kJ}$$
Step 2: $$\mathrm{B} \rightarrow \mathrm{C}\ \quad \Delta H_2$ = -30\ \mathrm{kJ}$$

What is the overall enthalpy change ($$\Delta H) $$for $$\mathrm{A} \rightarrow \mathrm{C}$$?

Accepted Answer

$$+20\ \mathrm{kJ}$$

Question 15

Which of the following is true about the enthalpy of formation ($$\Delta H^\circ_f) $$for an element in its standard state?

Accepted Answer

$$\Delta H^\circ_f is $$zero.

Question 16

A reaction is carried out at constant pressure and releases $$200\ \mathrm{kJ} of $$heat. What is the value of $$\Delta H$$ for the reaction?

Accepted Answer

$$\Delta H = -200\ \mathrm{kJ}$$

Question 17

Which of the following best illustrates the use of Hess’s Law in a real-world scenario?

Accepted Answer

Calculating the enthalpy change for a reaction that cannot be measured directly by combining known enthalpy changes of related reactions.

Question 18

Which of the following is a multi-step process that can be used to calculate the enthalpy change for the formation of $$\mathrm{NH_3$}$$ from $$\mathrm{N_2$}$$ and $$\mathrm{H_2$}$$?

Accepted Answer

Using Hess’s Law to combine the enthalpy changes of several reactions involving $$\mathrm{N_2$}$$, $$\mathrm{H_2$}$$, and $$\mathrm{NH_3$}.$$

Question 19

A student wants to determine the enthalpy change for the reaction:

$$\mathrm{C(graphite)} + \mathrm{O_2$}(g) \rightarrow \mathrm{CO_2$}(g)$$

Which value should they use from a data table?

Accepted Answer

The enthalpy of formation ($$\Delta H^\circ_f) of$$ $$\mathrm{CO_2$}.$$

Thermochemistry: Energy Changes in Chemical Reactions (Chapter 7 Study Guide)

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