Chemical Energy & Thermodynamics — Problem set, Grade 11

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

Review the key ideas

Energy in Chemical Reactions

A reaction energy account needs a system, defined states and amount. Bond breaking requires energy, product formation can offset it, and reaction enthalpy is distinct from activation barrier and rate. Calorimetry uses a complete heat-capacity/boundary account.

Calorimetry, Hess’s Law & Enthalpy Calculations

Choose a system, preserve energy signs, and scale enthalpy with the reaction equation.

Entropy, Free Energy & Reaction Feasibility

Free energy connects enthalpy and entropy; kinetics determines how quickly change can occur.

Problem 1

A chemical hand-warmer becomes hot once you activate it. Is the reaction inside exothermic or endothermic, and what is the sign of ΔH?
Answer and explanation

Exothermic; ΔH is negative

The pack releases heat into its surroundings (your hands), which is the definition of an exothermic reaction. An exothermic reaction loses energy, so its ΔH is negative.

Problem 2

How much heat (in J) is released when 150. g of water cools from 60.0°C to 40.0°C? (c = 4.18 J/g°C)
Answer and explanation

12,540 J (about 12.5 kJ) released

The water cools by ΔT = 60.0 − 40.0 = 20.0°C, so q = mcΔT = 150 × 4.18 × 20.0 = 12,540 J of heat released.

Problem 3

For a reaction, breaking all the reactant bonds absorbs 620 kJ and forming all the product bonds releases 790 kJ. Calculate ΔH and state whether the reaction is exothermic or endothermic.
Answer and explanation

ΔH = −170 kJ; exothermic

ΔH = (energy to break bonds) − (energy to form bonds) = 620 − 790 = −170 kJ. The negative sign means energy is released overall, so the reaction is exothermic.

Problem 4

Breaking costs 500kJ and forming releases 650kJ. Find modeled ΔH.
Answer and explanation

ΔH=500−650=−150kJ for the represented amount; exothermic. Average bond values are approximate.

Response criteria

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

Problem 5

Chapter review: 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 6

A reaction releases 3.00 kJ while .050 mol reacts. Find ΔH per mole.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

-60 kJ/mol

−3/.050 =−60.

Problem 7

How does neglected heat loss affect an exothermic cup estimate?
Answer and explanation

Less heat reaches the measured solution, so the estimated release magnitude is typically too small.

Response criteria

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

Problem 8

Chapter review: 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 9

Why convert entropy units before calculating ΔG?
Answer and explanation

TΔS must have the same energy-per-mole units as ΔH.

Response criteria

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

Problem 10

ΔH=30 kJ/mol, ΔS=.050 kJ/mol·K. Find crossover T.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

600 K

30/.050=600.

Problem 11

Chapter review: 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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