Calorimetry, Hess’s Law & Enthalpy Calculations — Practice · Grade 11 Science

Chapter: Chemical Energy & Thermodynamics

What this practice covers

By the end of Calorimetry, Hess’s Law & Enthalpy Calculations you should be able to:

8 questions, numeric and written answer, each with a worked explanation.

Vocabulary used: calorimeter heat capacity, state function, Hess’s law.

Problem 1

Reaction warms 50 g water by 4°C; c=4.18. Find heat gained by water.
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

836 J

50×4.18×4.

Problem 2

Why reverse ΔH when reversing a reaction?
Answer and explanation

The same state difference is traversed in the opposite direction.

Response criteria

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

Problem 3

For 40 g water warming 5°C with c=4.18, find water heat gain.
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

836 J

40×4.18×5=836.

Problem 4

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 5

A reaction loses 3.0 kJ to water and 0.5 kJ to its container. Find its total heat change. Show the reasoning, not just the conclusion.
Hints

System and surroundings have opposite signs in this boundary.

Answer and explanation

−3.5 kJ, assuming no other transfers.

Problem 6

Two steps have ΔH=+20 and −55 kJ and add to a target reaction. Find target ΔH.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

20−55=−35 kJ by Hess's law with the given orientations and scales.

Problem 7

Hess’s law combines state changes while canceling intermediatesHess’s law combines state changes while canceling intermediatesA → B ΔH₁ = −30 kJB → C ΔH₂ = +12 kJA → C ΔH = −18 kJCancel intermediates and add the corresponding energy changes.
Original hypothetical two-step enthalpy cycle with stated values and compatible species states assumed.
Interpret the supplied visual. Why can B cancel while the +12 kJ second step must still be included?
Answer and explanation

B is an intermediate in the summed transformation. Its associated state-change energy remains part of the overall endpoint balance.

Hess’s law uses the state-function character of enthalpy: the overall change depends on initial and final states rather than which conceptual route is added. Scale or reverse component equations as necessary, cancel shared intermediates and apply the same operations to their enthalpies. Intermediate B can disappear from the net equation without implying it was never involved in a real mechanism. The algebraic route need not be the actual laboratory pathway, and phases must match when canceling a species.

B is an intermediate in the summed transformation. Its associated state-change energy remains part of the overall endpoint balance.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 8

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.

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