Q: Which equation correctly represents the calculation of $\Delta H_{\mathrm{rxn}}$ using bond enthalpies?

$\Delta H_{\mathrm{rxn}} = \sum \text{(Bonds broken)} - \sum \text{(Bonds formed)}$

Question 1

Which statement best describes a state function in thermodynamics?

Accepted Answer

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

Question 2

According to Hess's Law, what is true about the enthalpy change ($\Delta H$) for a chemical reaction?

Accepted Answer

It is the same whether the reaction occurs in one step or multiple steps.

Question 3

Given the reaction $\mathrm{NO}(g) + \mathrm{O}(g) \rightarrow \mathrm{NO}_2(g)$, which method would you use to estimate $\Delta H^\circ_{\mathrm{rxn}}$?

Accepted Answer

Use average bond dissociation energies (BDE) for all bonds broken and formed.

Question 4

Which of the following is NOT a state function?

Accepted Answer

Work ($w$)

Question 5

For the reaction $\mathrm{CH}_4(g) + 3\mathrm{Cl}_2(g) \rightarrow \mathrm{CHCl}_3(g) + 2\mathrm{HCl}(g)$, which bonds are broken and which are formed?

Accepted Answer

Broken: 1 C-H, 3 Cl-Cl; Formed: 3 C-Cl, 2 H-Cl

Question 6

If the bond dissociation energy for $\mathrm{Cl}_2$ is $242\ \mathrm{kJ/mol}$, what is the total energy required to break 3 moles of $\mathrm{Cl}_2$ molecules?

Accepted Answer

$726\ \mathrm{kJ}$

Question 7

Which equation correctly represents the calculation of $\Delta H_{\mathrm{rxn}}$ using bond enthalpies?

Accepted Answer

$\Delta H_{\mathrm{rxn}} = \sum 	ext{(Bonds broken)} - \sum 	ext{(Bonds formed)}$

Question 8

A student calculates $\Delta H_{\mathrm{rxn}}$ for a reaction and gets a positive value. What does this indicate about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

Question 9

Which real-world scenario best illustrates the concept of Hess's Law?

Accepted Answer

The enthalpy change for burning glucose is the same whether it occurs in a single step in a laboratory or through multiple metabolic steps in the human body.

Question 10

Which of the following statements about bond dissociation energy (BDE) is correct?

Accepted Answer

BDE is the energy required to break one mole of a specific bond in the gas phase.

Question 11

Which process is NOT considered a reversible process in ideal gases?

Accepted Answer

Combustion of gasoline in a car engine.

Question 12

If the average bond energy for a C-H bond is $413\ \mathrm{kJ/mol}$, how much energy is required to break all four C-H bonds in methane ($\mathrm{CH}_4$)?

Accepted Answer

$1652\ \mathrm{kJ}$

Question 13

Which of the following is a correct application of bond enthalpy to estimate the enthalpy change for a reaction?

Accepted Answer

Calculate the total energy required to break all bonds in reactants and subtract the total energy released when forming all bonds in products.

Question 14

A reaction has $\Delta H_{\mathrm{rxn}} = -285\ \mathrm{kJ}$. What does this value indicate?

Accepted Answer

The reaction is exothermic and releases $285\ \mathrm{kJ}$ of heat.

Question 15

Which of the following is a consequence of the fact that enthalpy is a state function?

Accepted Answer

The enthalpy change for a reaction is independent of the reaction pathway.

Question 16

In a laboratory, a student mixes methane and chlorine gas to produce chloroform and hydrogen chloride. Which concept allows the student to calculate the enthalpy change using bond energies?

Accepted Answer

Bond dissociation energy (BDE)

Question 17

Which of the following best explains why the enthalpy change for a reaction can be calculated using Hess's Law?

Accepted Answer

Because enthalpy is a state function and depends only on the initial and final states.

Question 18

A chemist wants to synthesize $\mathrm{NO}_2$ from $\mathrm{NO}$ and $\mathrm{O}$ in the gas phase. Which factor will most affect the enthalpy change of this reaction?

Accepted Answer

The bond energies of the reactants and products.

Question 19

Which of the following is a correct statement about the use of average bond energies?

Accepted Answer

They provide an estimate of $\Delta H_{\mathrm{rxn}}$ but may not be exact for all molecules.

Lecture 19

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