Question 1

Which of the following best describes what happens to energy during the formation and breaking of chemical bonds?

Accepted Answer

Energy is absorbed when a bond is broken and released when a bond is formed.

Question 2

Consider the combustion of methane: $$\mathrm{CH_4$(g) + 2\ O_2$(g) \rightarrow CO_2$(g) + 2\ H_2O(g)}. $$Using the bond energies $$\mathrm{C-H = 414\ kJ/mol}$$, $$\mathrm{O=O = 498\ kJ/mol}$$, $$\mathrm{C=O = 799\ kJ/mol}$$, and $$\mathrm{O-H = 464\ kJ/mol}$$, what is the estimated enthalpy change ($$\Delta H_{rxn}) $$for this reaction?

Accepted Answer

$$-802\ \mathrm{kJ/mol}$$

Question 3

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

Accepted Answer

Exothermic reactions release energy to the surroundings.

Question 4

Given the reaction $$2\ \mathrm{H_2O(l) \rightarrow 2\ H_2$(g) + O_2$(g)}$$ and the bond energies $$\mathrm{O-H = 464\ kJ/mol}$$, $$\mathrm{H-H = 436\ kJ/mol}$$, $$\mathrm{O=O = 498\ kJ/mol}$$, calculate the net energy absorbed or released.

Accepted Answer

$$+484\ \mathrm{kJ} ($$energy absorbed)

Question 5

Why do fuels that require more $$\mathrm{O_2$}$$ for combustion generally release more energy per mole?

Accepted Answer

Because the formation of strong $$\mathrm{C=O}$$ and $$\mathrm{O-H}$$ bonds in the products releases more energy.

Question 6

Which of the following best explains why the combustion of glucose ($$\mathrm{C_6H_{12}O_6$}) $$produces less energy per gram than the combustion of oleic acid ($$\mathrm{C$$_{18}$$H_{34}O_2$}$$)?

Accepted Answer

Glucose contains more oxygen atoms, so less $$\mathrm{O_2$} is $$required and less energy is released.

Question 7

A reaction has $$q \u003e 0$$ for the system. Which of the following is true?

Accepted Answer

The system absorbs energy from the surroundings.

Question 8

Which of the following reactions is most likely to be exothermic based on bond energies?

Accepted Answer

$$\mathrm{A-A + B-B \rightarrow 2\ A-B}$$, because forming stronger A-B bonds releases more energy than is required to break A-A and B-B bonds.

Question 9

Which of the following best describes the relationship between bond energy and bond strength?

Accepted Answer

A higher bond energy means a stronger bond.

Question 10

Calculate the energy density (in $$\mathrm{Cal/g}) $$for glucose ($$\mathrm{C_6H_{12}O_6$}) $$given $$\Delta H_{exp} = -2.80 	imes 10^3$\ \mathrm{kJ/mol}$$ and $$M = 180.2\ \mathrm{g/mol}$$. ($$1\ \mathrm{Cal} = 4.184\ \mathrm{kJ})$$

Accepted Answer

$$3.72\ \mathrm{Cal/g}$$

Question 11

Which of the following is a correct statement about the energy profile of an endothermic reaction?

Accepted Answer

The products have higher potential energy than the reactants.

Question 12

Why is the combustion of ethanol ($$\mathrm{C_2H_6O}) $$exothermic, even though the potential energy of ethanol is lower than that of $$\mathrm{CO_2$}$$ and $$\mathrm{H_2O}$$?

Accepted Answer

Because the formation of strong bonds in $$\mathrm{CO_2$}$$ and $$\mathrm{H_2O}$$ releases more energy than is required to break the bonds in ethanol and $$\mathrm{O_2$}.$$

Question 13

Which of the following best explains why 'oxygenated' fuels (fuels that already contain oxygen in their structure) generally produce less energy per mole than hydrocarbons?

Accepted Answer

They require less $$\mathrm{O_2$}$$ reactant, so fewer strong bonds are formed in the products.

Question 14

Given the following bond energies: $$\mathrm{H-H = 436\ kJ/mol}$$, $$\mathrm{Cl-Cl = 240\ kJ/mol}$$, $$\mathrm{H-Cl = 428\ kJ/mol}$$, determine if the reaction $$\mathrm{H_2$ + Cl_2$ \rightarrow 2\ HCl} is $$endothermic or exothermic.

Accepted Answer

Exothermic, because more energy is released in forming $$\mathrm{H-Cl}$$ bonds than is required to break $$\mathrm{H-H}$$ and $$\mathrm{Cl-Cl}$$ bonds.

Question 15

Which of the following is a real-world example of an exothermic process?

Accepted Answer

Combustion of gasoline in a car engine.

Question 16

If a chemical reaction in a laboratory flask feels cold to the touch, what can you infer about the reaction?

Accepted Answer

It is endothermic; the system absorbs heat from the surroundings.

Question 17

Which of the following best describes the main assumption when modeling chemical reactions?

Accepted Answer

Atoms are rearranged, forming particles with different composition and structure.

Question 18

Why is energy density (kcal/g) a more useful quantity than $$\mathrm{kJ/mol}$$ for evaluating fuels?

Accepted Answer

Because it allows direct comparison of the energy content per unit mass, which is important for transportation and storage.

Tracking Energy in Chemical Reactions: Bond Energies and Thermochemistry

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