Energy in Chemical Reactions — Practice · Grade 11 Science

Chapter: Chemical Energy & Thermodynamics

What this practice covers

By the end of Energy in Chemical Reactions you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: enthalpy change (ΔH), exothermic reaction, endothermic reaction, bond energy, activation energy (Ea), specific heat (c).

Problem 1

In an endothermic reaction, the sign of ΔH is…
  1. negative
  2. positive
  3. always zero
  4. impossible to determine
Answer and explanation

positive

An endothermic reaction absorbs energy into the system, so ΔH is positive.

Problem 2

How much heat (in J) is absorbed when 50.0 g of water is warmed from 15.0°C to 35.0°C? (c = 4.18 J/g°C)
Answer and explanation

4180 J

ΔT = 35.0 − 15.0 = 20.0°C, so q = mcΔT = 50 × 4.18 × 20 = 4180 J.

Problem 3

On a potential-energy diagram, the activation energy is the difference in energy between…
  1. the two dashed helper lines
  2. the reactants and the products
  3. the reactants and the peak (the activated complex)
  4. the products and the x-axis
Answer and explanation

the reactants and the peak (the activated complex)

Activation energy is the height of the peak above the reactant energy level — the energy barrier the reaction must climb.

Problem 4

In an insulated, constant-pressure reaction calorimeter, the measured solution temperature falls. Assume negligible heat loss and no other process. Is the reaction endothermic or exothermic, what is the sign of ΔH, and why?
Answer and explanation

Under the stated assumptions, the reaction absorbs heat from the solution, so it is endothermic and ΔH is positive. A temperature fall without these conditions would not uniquely prove reaction enthalpy; other energy transfers or processes could contribute.

Problem 5

A reaction releases 6270 J of heat into 100. g of water (c = 4.18 J/g°C). By how many °C does the water's temperature rise?
Answer and explanation

15 °C

Rearranging q = mcΔT gives ΔT = q ÷ (m × c) = 6270 ÷ (100 × 4.18) = 6270 ÷ 418 = 15°C.

Problem 6

If ΔH=−50 kJ for one reaction amount, find ΔH for twice that amount. Show the reasoning, not just the conclusion.
Hints

Do not omit the reaction-amount basis when scaling.

Answer and explanation

−100 kJ under identical conditions.

Problem 7

Why does bond-energy estimation subtract energy released in forming product bonds from energy used breaking reactant bonds?
Hints

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

Answer and explanation

Net reaction energy balances input to break bonds against release from forming new bonds; breaking alone is not the energy source.

Problem 8

Reaction enthalpy belongs to the equation’s stated amount and directionReaction enthalpy belongs to the equation’s stated amount and directionA → B ΔH = −40 kJ per equation amount2A → 2B ΔH = −80 kJB → A ΔH = +40 kJScale coefficients and energy together; reversal changes the sign.
Original hypothetical transformation values, not real thermochemical data or an assertion about rate.
Interpret the supplied visual. What is wrong with reversing A→B while keeping the same negative ΔH sign?
Answer and explanation

It reverses the physical energy change without reversing the account’s sign; the corresponding B→A enthalpy is +40 kJ for the displayed basis.

Enthalpy is a state function, and a thermochemical reaction change is associated with a defined transformation and amount basis. Multiplying every coefficient scales the enthalpy change by the same factor; reversing the reaction reverses its sign. A number labeled kJ/mol must specify mol of what—product, reactant or reaction extent. Phase labels also matter because liquid and vapor have different enthalpy states. Keep these details when using a reference value in stoichiometry.

It reverses the physical energy change without reversing the account’s sign; the corresponding B→A enthalpy is +40 kJ for the displayed basis.

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 9

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 10

OpenSciEd magnification template linking the observable heater to two particle-scale views before and after adding water; it is a model-building prompt, not a completed reaction diagram.
OpenSciEd’s empty observable/particle-scale heater template is a model-building prompt, not a completed reaction or a calorimeter record. Flameless-heater model at visible and particle scales · OpenSciEd · 7.2 Chemical Reactions & Energy, Lesson 1, source, CC BY 4.0. Figure excerpt rendered from the PDF and resized for the web.
What would a complete explanation need to add to the empty source heater template? Distinguish system boundary, reaction species, bond-energy account and thermometer response, and name one conclusion the template alone cannot establish.
Answer and explanation

Define the selected reaction system and surroundings, identify species from appropriate evidence, conserve material and account for bond breaking plus formation. Connect transferred energy with heat capacity and measured temperature under stated conditions. The empty template alone cannot identify the reaction, quantify enthalpy or prove a construction method.

Identify the source’s visible labels, scales and unfinished or omitted information.

Use the local complete explanations to connect the observation with an explicit mechanism.

State assumptions and a limit before extending the model.

Response criteria

  • Interpret the source symbols and boundary correctly.
  • Explain the relevant mechanism using quantities/species consistently.
  • State a meaningful omission, assumption or evidence limitation.

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