Q: Given the reaction: $$\mathrm{CH_4$ + 2O_2 \rightarrow CO_2$ + 2H_2O}$$, estimate $$\Delta H_{rxn}$$ using the bond enthalpies from Table 8.4. (Bonds broken: 4 C–H, 2 O=O; Bonds formed: 2 C=O, 4 O–H)

$$\Delta H_{rxn} = [4 \times 413 + 2 \times 495] - [2 \times 799 + 4 \times 463] = -802\ \mathrm{kJ/mol}$$

Q: Which equation correctly estimates the enthalpy change of a reaction using bond enthalpies?

$$\Delta H_{rxn} = \Sigma (\text{bond enthalpies of all bonds broken}) - \Sigma (\text{bond enthalpies of all bonds formed})$$

Q: A reaction involves breaking one mole of $$\mathrm{Cl_2$}$$ molecules and forming two moles of H–Cl bonds. Using Table 8.4, what is the estimated $$\Delta H_{rxn}$$? (Bond enthalpies: Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol)

$$\Delta H_{rxn} = [243] - [2 \times 431] = -619\ \mathrm{kJ/mol}$$

Q: Calculate the enthalpy change for the reaction $$\mathrm{H_2$ + Cl_2$ \rightarrow 2HCl}$$ using bond enthalpies from Table 8.4. (H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol)

$$\Delta H_{rxn} = [436 + 243] - [2 \times 431] = -183\ \mathrm{kJ/mol}$$

Question 1

Which of the following statements best describes the relationship between bond breaking and bond formation in terms of energy transfer?

Accepted Answer

Breaking bonds requires energy (endothermic), while forming bonds releases energy (exothermic).

Question 2

Given the reaction: $$\mathrm{CH_4$ + 2O_2 \rightarrow CO_2$ + 2H_2O}$$, estimate $$\Delta H_{rxn}$$ using the bond enthalpies from Table 8.4. (Bonds broken: 4 C–H, 2 O=O; Bonds formed: 2 C=O, 4 O–H)

Accepted Answer

$$\Delta H_{rxn} = [4 	imes 413 + 2 	imes 495] - [2 	imes 799 + 4 	imes 463] = -802\ \mathrm{kJ/mol}$$

Question 3

Which equation correctly estimates the enthalpy change of a reaction using bond enthalpies?

Accepted Answer

$$\Delta H_{rxn} = \Sigma (	ext{bond enthalpies of all bonds broken}) - \Sigma (	ext{bond enthalpies of all bonds formed})$$

Question 4

Why is it important to count bonds carefully when using bond enthalpies to estimate reaction enthalpy?

Accepted Answer

Because each bond type has a unique enthalpy value and miscounting leads to significant errors in $$\Delta H_{rxn}.$$

Question 5

Which of the following bonds has the highest average bond enthalpy according to Table 8.4?

Accepted Answer

C≡N

Question 6

A reaction involves breaking one mole of $$\mathrm{Cl_2$}$$ molecules and forming two moles of H–Cl bonds. Using Table 8.4, what is the estimated $$\Delta H_{rxn}$$? (Bond enthalpies: Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol)

Accepted Answer

$$\Delta H_{rxn} = [243] - [2 	imes 431] = -619\ \mathrm{kJ/mol}$$

Question 7

Which real-world process is best explained by the concept that breaking bonds is endothermic and forming bonds is exothermic?

Accepted Answer

Combustion of gasoline in a car engine

Question 8

For the reaction $$\mathrm{N_2$ + 3H_2 \rightarrow 2NH_3}$$, which bonds are broken and which are formed?

Accepted Answer

Broken: 1 N≡N, 3 H–H; Formed: 6 N–H

Question 9

If a reaction has $$\Delta H_{rxn} \u003e 0$$ when calculated using bond enthalpies, what does this indicate about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

Question 10

Which of the following is a limitation of using average bond enthalpies to estimate reaction enthalpy?

Accepted Answer

Bond enthalpies are average values and may not reflect the exact energy change for specific molecules.

Question 11

Calculate the enthalpy change for the reaction $$\mathrm{H_2$ + Cl_2$ \rightarrow 2HCl}$$ using bond enthalpies from Table 8.4. (H–H = 436 kJ/mol, Cl–Cl = 243 kJ/mol, H–Cl = 431 kJ/mol)

Accepted Answer

$$\Delta H_{rxn} = [436 + 243] - [2 	imes 431] = -183\ \mathrm{kJ/mol}$$

Question 12

Which bond would require the most energy to break, based on Table 8.4?

Accepted Answer

C≡N

Question 13

A student calculates $$\Delta H_{rxn}$$ for a reaction and gets a positive value. Which of the following best explains what happened at the molecular level?

Accepted Answer

More energy was required to break the bonds in the reactants than was released when forming the bonds in the products.

Question 14

Which of the following reactions would you expect to be highly exothermic based on the bond enthalpies in Table 8.4?

Accepted Answer

Formation of water from hydrogen and oxygen: $$2H_2 + O_2$ \rightarrow 2H_2O$$

Question 15

Which of the following best describes why bond enthalpy calculations are only estimates for $$\Delta H_{rxn}$$?

Accepted Answer

Bond enthalpies are averaged over many compounds and do not account for molecular environment.

Estimating Reaction Enthalpy Using Bond Energies

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