Q: Which formula is used to calculate the enthalpy of reaction ($$\Delta H_{\text{rxn}}) $$when given bond energies?

$$\Delta H_{\text{rxn}} = \text{Reactants} - \text{Products}$$

Q: If the bond enthalpy for S-H is 347 kJ/mol, F-H is 565 kJ/mol, and F-F is 159 kJ/mol, what is the total energy required to break all the bonds in 1 mole of $$\mathrm{H_2S}$$ and 3 moles of $$\mathrm{F_2$}$$?

$$(2 \times 347) + (3 \times 159) = 694 + 477 = 1171 kJ$$

Q: If the bond enthalpy for C=O is 745 kJ/mol and for O-H is 467 kJ/mol, what is the total energy released when 2 moles of C=O and 2 moles of O-H bonds are formed?

$$(2 \times 745) + (2 \times 467) = 1490 + 934 = 2424 kJ$$

Q: Which of the following is a correct application of the enthalpy of reaction formula using bond energies?

$$\Delta H_{\text{rxn}} = \sum \text{Bond energies of bonds broken} - \sum \text{Bond energies of bonds formed}$$

Q: If a reaction requires breaking 4 moles of F-F bonds ($$159 kJ/$$mol) and forming 4 moles of F-H bonds ($$565 kJ/$$mol), what is the net enthalpy change?

$$(4 \times 159) - (4 \times 565) = 636 - 2260 = -1624 kJ$$

Question 1

Which of the following best describes an endothermic process?

Accepted Answer

Energy is absorbed to break a bond and the enthalpy change ($$\Delta H) $$has a positive sign.

Question 2

Given the reaction: $$\mathrm{N_2$} + 3\mathrm{H_2$} ightarrow 2\mathrm{NH_3$}$$, and the bond enthalpies $$\mathrm{N\equiv N} = 945 kJ/$$mol, $$\mathrm{H-H} = 432 kJ/$$mol, and $$\mathrm{N-H} = 391 kJ/$$mol, what is the enthalpy change ($$\Delta H_{	ext{rxn}}) $$for the reaction?

Accepted Answer

$$-92 kJ$$

Question 3

Which formula is used to calculate the enthalpy of reaction ($$\Delta H_{	ext{rxn}}) $$when given bond energies?

Accepted Answer

$$\Delta H_{	ext{rxn}} = 	ext{Reactants} - 	ext{Products}$$

Question 4

In the reaction $$\mathrm{H_2S} + 3\mathrm{F_2$} \rightarrow \mathrm{SF_4$} + 2\mathrm{HF}$$, which bond is formed in the products that is not present in the reactants?

Accepted Answer

S-F

Question 5

If the bond enthalpy for S-H is 347 kJ/mol, F-H is 565 kJ/mol, and F-F is 159 kJ/mol, what is the total energy required to break all the bonds in 1 mole of $$\mathrm{H_2S}$$ and 3 moles of $$\mathrm{F_2$}$$?

Accepted Answer

$$(2 	imes 347) + (3 	imes 159) = 694 + 477 = 1171 kJ$$

Question 6

Which of the following statements about bond energy is correct?

Accepted Answer

Bond energy is the amount of energy stored in a bond between atoms in a molecule.

Question 7

For the combustion of acetylene: $$2\mathrm{C_2H_2} + 5\mathrm{O_2$} \rightarrow 4\mathrm{CO_2$} + 2\mathrm{H_2O}$$, which bond is broken in the reactants but not present in the products?

Accepted Answer

C\equiv C

Question 8

A reaction has $$\Delta H_{	ext{rxn}} = -1301 kJ. $$What does this indicate about the reaction?

Accepted Answer

The reaction is exothermic and releases energy.

Question 9

Which step is necessary before calculating the enthalpy of reaction using bond energies?

Accepted Answer

Check if the chemical equation is balanced.

Question 10

If the bond enthalpy for C=O is 745 kJ/mol and for O-H is 467 kJ/mol, what is the total energy released when 2 moles of C=O and 2 moles of O-H bonds are formed?

Accepted Answer

$$(2 	imes 745) + (2 	imes 467) = 1490 + 934 = 2424 kJ$$

Question 11

Which of the following is a correct application of the enthalpy of reaction formula using bond energies?

Accepted Answer

$$\Delta H_{	ext{rxn}} = \sum 	ext{Bond energies of bonds broken} - \sum 	ext{Bond energies of bonds formed}$$

Question 12

In a medical scenario, a patient is exposed to a chemical reaction that is highly exothermic. What is the most likely effect on the surrounding tissue?

Accepted Answer

The tissue will release energy and heat up.

Question 13

If a reaction requires breaking 4 moles of F-F bonds ($$159 kJ/$$mol) and forming 4 moles of F-H bonds ($$565 kJ/$$mol), what is the net enthalpy change?

Accepted Answer

$$(4 	imes 159) - (4 	imes 565) = 636 - 2260 = -1624 kJ$$

Question 14

Which bond has the highest bond enthalpy according to the provided data?

Accepted Answer

C\equiv C

Question 15

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

Accepted Answer

The reaction is endothermic and absorbs energy.

Question 16

In the reaction $$2\mathrm{C_2H_2} + 5\mathrm{O_2$} \rightarrow 4\mathrm{CO_2$} + 2\mathrm{H_2O}$$, which bonds are formed in the products?

Accepted Answer

C=O and O-H

Question 17

Why is it important to multiply the coefficients of each bond type by its bond enthalpy when calculating $$\Delta H_{	ext{rxn}}$$?

Accepted Answer

To account for the number of moles of each bond broken or formed in the reaction.

Question 18

A chemist wants to estimate the enthalpy of reaction for a process but only has bond enthalpy data. Which formula should they use?

Accepted Answer

$$\Delta H_{	ext{rxn}} = \sum 	ext{Bond energies of bonds broken} - \sum 	ext{Bond energies of bonds formed}$$

Bond Enthalpy and Enthalpy of Reaction Formulas in GOB Chemistry

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