Chemical Energy & Thermodynamics — Chapter quiz · Physical Science: Chemistry, Grade 11

Use calorimetry, Hess’s law, entropy, and free energy with explicit systems and signs.

Problem 1

The enthalpy change ΔH of an exothermic reaction is…
  1. negative
  2. always zero
  3. always exactly 1
  4. positive
Answer and explanation

negative

An exothermic reaction releases energy to its surroundings, so the system loses energy and ΔH is negative.

Problem 2

How much heat (in J) is needed to raise the temperature of 100. g of water from 20.0°C to 45.0°C? (c = 4.18 J/g°C)
Answer and explanation

10450 J

ΔT = 45.0 − 20.0 = 25.0°C, so q = mcΔT = 100 × 4.18 × 25 = 10,450 J.

Problem 3

Explain the following claim and supply a valid example or counterexample: Breaking a bond costs energy; the overall reaction can release more when product bonds form.
Answer and explanation

ΔH=500−650=−150kJ for the represented amount; exothermic. Average bond values are approximate. Breaking a bond costs energy; the overall reaction can release more when product bonds form.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 4

Independent chapter check: A selected reaction absorbs 720 kJ in the bond-breaking account and releases 880 kJ on product formation. Estimate net enthalpy, then predict the corresponding value when the entire equation is doubled.
Answer and explanation

ΔH≈720−880=−160 kJ for the selected equation amount. Doubling all coefficients gives approximately −320 kJ. Average bond energies are an approximate full-reaction estimate, not proof about spontaneity or rate.

ΔH≈720−880=−160 kJ for the selected equation amount. Doubling all coefficients gives approximately −320 kJ. Average bond energies are an approximate full-reaction estimate, not proof about spontaneity or rate.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 5

Two steps have ΔH −20 and +8 kJ. Find total.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

-12 kJ

−20+8=−12.

Problem 6

Why are average bond-energy estimates approximate?
Answer and explanation

Actual bond environments and phases differ from the tabulated averages.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 7

Independent chapter check: A reaction heats 50.0 g water by 8.00°C. Use c=4.18 J/(g·°C), neglect apparatus/losses, and assume 0.0200 mol reacts. Estimate reaction heat and molar enthalpy on that basis.
Answer and explanation

qwater=50.0×4.18×8.00=1672 J. qreaction≈−1.672 kJ. Dividing by 0.0200 mol gives approximately −83.6 kJ/mol on the specified reacting-amount basis, subject to the neglected terms.

qwater=50.0×4.18×8.00=1672 J. qreaction≈−1.672 kJ. Dividing by 0.0200 mol gives approximately −83.6 kJ/mol on the specified reacting-amount basis, subject to the neglected terms.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 8

ΔH=10kJ/mol, ΔS=.020kJ/mol·K, T=400 K. Find ΔG.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

2 kJ/mol

10−8=2.

Problem 9

Does a catalyst make positive ΔG negative?
Answer and explanation

No. It changes reaction rate pathways, not the overall state-function difference.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 10

Independent chapter check: For a selected process use ΔH=+30.0 kJ/mol and ΔS=+0.150 kJ/(mol·K), treated constant. Find ΔG at 250 K, the modeled crossover temperature and one limit of the prediction.
Answer and explanation

At 250 K, ΔG=30.0−250(0.150)=−7.50 kJ/mol. The modeled crossover is 30.0/0.150=200 K. Actual ΔH/ΔS can vary with temperature and phases, and the calculation does not establish a rate.

At 250 K, ΔG=30.0−250(0.150)=−7.50 kJ/mol. The modeled crossover is 30.0/0.150=200 K. Actual ΔH/ΔS can vary with temperature and phases, and the calculation does not establish a rate.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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