Chemistry 9th Chap 8 : Topic 9 - Maryam Publications

Chemistry Chapter 8 : Topic 9

Enthalpy Change Calculations

1 / 15

In a chemical reaction, energy is required to break bonds in the reactants, and energy is released when new bonds are formed in the products. Which statement correctly explains the role of bond energies in calculating enthalpy change?

A) Enthalpy change is unrelated to bond energies

B) Enthalpy change is calculated as energy absorbed to break bonds minus energy released in forming bonds

C) Enthalpy change depends only on energy released during bond breaking

D) Enthalpy change depends only on energy required to form bonds

2 / 15

During bond breaking in a reaction, energy is always:

A) Neither absorbed nor released

B) Absorbed because bonds must be overcome

C) Released because bonds become stable

D) Converted into mass

3 / 15

Consider the reaction:
H₂ + Cl₂ → 2HCl
Bond energies (kJ / mol): H-H=436, Cl-Cl=242, H-Cl=431
What is the correct enthalpy change?

A) - 242 kJ mol⁻¹

B) - 183 kJ mol⁻¹

C) + 242 kJ mol⁻¹

D) + 183 kJ mol⁻¹

4 / 15

Which situation will always result in an exothermic reaction when using bond energies?

A) No bonds are involved.

B) Energy required to break bonds is greater than energy released in forming bonds.

C) Energy required to break bonds equals energy released.

D) Energy released in forming bonds is greater than energy required to break bonds.

5 / 15

A reaction involves breaking 2 H-H bonds and 1 O=O bond and forming 4 O-H bonds. Which expression correctly represents enthalpy change?

A) ΔH= sum of reactant masses - product masses

B) ΔH= energy of bonds broken - energy of bonds formed

C) ΔH= bond energy × temperature

D) ΔH= energy of bonds formed - energy of bonds broken

6 / 15

In bond energy calculations, which factor is most important for accuracy?

A) Physical state only

B) Number of atoms in reactants

C) Color of substances

D) Type and strength of bonds broken and formed

7 / 15

A student calculates (ΔH) as positive for a reaction. This means:

A) Reaction is impossible

B) Reaction is endothermic

C) Reaction is exothermic

D) No energy change occurs

8 / 15

Which of the following correctly compares bond breaking and bond formation?

A) Both processes absorb energy

B) Bond breaking releases energy, bond formation absorbs energy

C) Both processes release energy

D) Bond breaking absorbs energy, bond formation releases energy

9 / 15

A reaction has very strong bonds in the reactants and weak bonds in the products. What will likely be the enthalpy change?

A) Cannot be determined

B) Zero

C) Positive (endothermic)

D) Negative (exothermic)

10 / 15

Which of the following best describes bond energy?

A) Energy required to change state of matter

B) Energy required to break one mole of bonds in gaseous state

C) Energy required to melt a substance

D) Energy released during freezing

11 / 15

For the reaction:
N₂ + 3H₂ → 2NH₃
If breaking bonds requires more energy than is released in forming bonds, then the reaction is:

A) Spontaneous without energy change

B) Exothermic

C) Neutral

D) Endothermic

12 / 15

Which step is NOT included in bond energy calculations of enthalpy change?

A) Using bond energy values from tables

B) Measuring temperature change directly

C) Forming bonds in products

D) Breaking bonds in reactants

13 / 15

If ΔH=- 250 kJ/mol for a reaction, it indicates:

A) 250 kJ energy is absorbed

B) No energy change

C) 250 kJ energy is released

D) Bonds are not involved

14 / 15

A reaction absorbs 400 kJ to break bonds and releases 550 kJ when forming bonds. What is ΔH?

A) - 950 kJ mol⁻¹

B) + 150 kJ mol⁻¹

C) + 950 kJ mol⁻¹

D) - 150 kJ mol⁻¹

15 / 15

Why are average bond energies used in calculations?

A) Because energy is always constant

B) Because bond energies vary slightly in different molecules

C) Because reactions do not involve bonds

D) Because all bonds have identical strength

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