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Physical chemistry

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Sample exercises

Grades 9–12 · Physical chemistry

Which sign normally describes ΔH for an exothermic reaction?

  1. Always zero.
  2. Negative.
  3. Positive.
Answer and explanation

Negative.

The reacting system transfers energy to the surroundings.

Grades 9–12 · Physical chemistry

A bond-enthalpy model requires 450 kJ/mol to break bonds and releases 610 kJ/mol forming bonds. Estimate ΔH.

Answer and explanation

-160 kJ/mol

450 − 610 = −160 kJ/mol.

Grades 9–12 · Physical chemistry

Does breaking a chemical bond itself release energy?

  1. No; energy is required to separate the bonded atoms.
  2. Yes, every broken bond releases energy.
  3. Only because atoms vanish.
Answer and explanation

No; energy is required to separate the bonded atoms.

Energy release is associated with forming favorable new bonds.

Grades 9–12 · Physical chemistry

Why can the surroundings warm while the reaction enthalpy is negative?

Answer and explanation

Energy leaving the reacting system enters the surroundings.

The sign refers to the system, not the thermometer reading alone.

Grades 9–12 · Physical chemistry

A process absorbs 75 kJ from its surroundings. What is its heat transfer q for the system under the stated sign convention?

Answer and explanation

75 kJ

Heat entering the system is positive.

Grades 9–12 · Physical chemistry

Why are bond-enthalpy estimates approximate?

  1. Atoms do not conserve mass.
  2. Tabulated values average over molecular environments.
  3. The sign is random.
Answer and explanation

Tabulated values average over molecular environments.

Actual bond energies depend on their environment.

Study notes

pOH at 25 °C

The sum pH + pOH is approximately 14 for the stated dilute aqueous model at 25 °C; it changes with temperature.

Grades 10–12

Ideal-gas volume

Use absolute temperature in kelvins. With R = 8.314 kPa·L/(mol·K), pressure in kPa gives volume in litres.

Grades 10–12

Bond-enthalpy estimate

Bond breaking requires energy; bond formation releases it. Average bond energies give an approximate reaction enthalpy.

Grades 10–12

Simple equilibrium ratio

For the stated A ⇌ B model, use equilibrium concentrations. Other equations require different powers and expressions.

Grades 10–12

Average disappearance rate

Reactant concentration falls; report its positive average disappearance rate as (initial − final)/time.

Grades 10–12

Standard cell potential

Subtract the anode reduction potential from the cathode reduction potential. Do not multiply potentials by equation coefficients.

Grades 11–12