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Chemistry · Practice book · Senior 4 · Unit 17 of 17

Energy changes and energy profile diagrams for chemical reactions

Senior 4 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 17 Energy changes and energy profile diagrams for chemical reactions

Key unit competence: Apply the concept of energy changes to construct energy profile diagrams for exothermic and endothermic reactions.

Key ideas

  • A system is the part being studied; everything else is the surroundings. Open systems exchange energy and matter; closed systems exchange energy only; isolated systems exchange neither.
  • Internal energy (U) is the total kinetic and potential energy of the particles in a system. First law of thermodynamics: energy is conserved; ΔU = q + w (heat added to the system plus work done on it).
  • Enthalpy change (ΔH) is the heat energy change at constant pressure. Exothermic reactions release heat (ΔH negative, temperature rises); endothermic reactions absorb heat (ΔH positive, temperature falls).
  • Bond breaking is endothermic; bond making is exothermic. ΔH = energy to break bonds − energy released forming bonds.
  • Energy profile diagrams plot energy against reaction progress: reactants, a peak (transition state) and products. Activation energy (Ea) is the energy difference between reactants and the peak. In exothermic reactions the products are lower than the reactants; in endothermic reactions they are higher.
  • A catalyst lowers Ea by providing an alternative pathway but does not change ΔH. Heat measured in experiments: q = mcΔT (c of water = 4.18 J/g/°C); ΔH = −q ÷ moles.

Common misconceptions

  • "Breaking bonds releases energy." No: Breaking bonds always requires energy; energy is released when bonds form.
  • "A catalyst changes the value of ΔH." No: A catalyst lowers the activation energy only; ΔH is unchanged.
  • "An exothermic reaction has a positive ΔH because heat is given out." No: Exothermic ΔH is negative: the system loses energy to the surroundings.

Worked examples

Example 1. Draw and label an energy profile diagram for an exothermic reaction with and without a catalyst.

Solution

Axes: energy (y) against progress of reaction (x).

Reactants on the left at a higher level; products on the right at a lower level; downward arrow between them labelled ΔH (negative).

A curve rising to a peak (transition state); the height from reactants to peak is Ea.

A second, lower peak (dashed) for the catalysed path with smaller Ea; ΔH is unchanged.

Example 2. Calculate the enthalpy change for CH4 + 2O2 → CO2 + 2H2O using bond energies: C–H 413, O=O 498, C=O 805, O–H 464 kJ/mol.

Solution

Bonds broken: 4 C–H + 2 O=O = 4(413) + 2(498) = 1652 + 996 = 2648 kJ.

Bonds formed: 2 C=O + 4 O–H = 2(805) + 4(464) = 1610 + 1856 = 3466 kJ.

ΔH = 2648 − 3466 = −818 kJ/mol (exothermic).

Practice questions

  1. Define an exothermic reaction.

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    A reaction that releases heat to the surroundings; ΔH is negative.

    Common mistake: Saying ΔH is positive.

  2. Distinguish between open, closed and isolated systems.

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    Open: exchanges matter and energy; closed: energy only; isolated: neither.

    Common mistake: Mixing up closed and isolated.

  3. State the first law of thermodynamics.

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    Energy is neither created nor destroyed; ΔU = q + w.

    Common mistake: Saying entropy increases.

  4. What is activation energy?

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    The minimum energy colliding particles need to react.

    Common mistake: Saying it is ΔH.

  5. Calculate q when 200 g of water warms by 5.0 °C.

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    200 × 4.18 × 5.0 = 4180 J.

    Common mistake: Forgetting to multiply by mass.

  6. Give one exothermic and one endothermic process from daily life.

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    Exothermic: burning charcoal; endothermic: cooking an egg / photosynthesis / evaporation of sweat.

    Common mistake: Calling melting ice exothermic.

  7. Explain why bond making is exothermic.

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    Atoms become more stable when they bond, releasing energy.

    Common mistake: Saying energy is needed to make bonds.

  8. Does a catalyst change ΔH? Explain.

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    No; it changes only the pathway and Ea; reactant and product energies are unchanged.

    Common mistake: Saying a catalyst makes the reaction more exothermic.

  9. Sketch (describe) the energy profile of an endothermic reaction.

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    Products higher than reactants; ΔH arrow up (positive); hump showing Ea above reactants.

    Common mistake: Drawing products lower.

  10. If 1.0 g of ethanol (Mr 46) releases 29.7 kJ, find ΔH of combustion.

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    n = 0.0217 mol; ΔH = −29.7 ÷ 0.0217 = −1367 kJ/mol.

    Common mistake: Forgetting the negative sign.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Which process is endothermic?a. an exothermic reaction
    2. Which process is exothermic?b. Photosynthesis
    3. Hand warmers that get hot when activated use:c. the minimum energy needed for a reaction to occur
    4. Activation energy is:d. Combustion of ethanol
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    1 → b (Photosynthesis) · 2 → d (Combustion of ethanol) · 3 → a (an exothermic reaction) · 4 → c (the minimum energy needed for a reaction to occur)

    • Absorbs light energy.
    • Burning releases heat.
    • e.g. iron oxidation or crystallisation.
    • The energy barrier.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    lower in energy than the reactants  ·  transition state (activated complex)  ·  kinetic and potential energies  ·  catalyst
    1. Internal energy is the sum of the particles' .
    2. The highest point on an energy profile represents the .
    3. On an energy profile for an exothermic reaction, the products are .
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    1: kinetic and potential energies · 2: transition state (activated complex) · 3: lower in energy than the reactants

    • Total energy of the system.
    • Bonds partly broken and formed.
    • Energy is released.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. In an exothermic reaction, ΔH is:a. negative
    2. ΔH for a reaction = energy to break bonds − energy released forming bonds. If bond forming releases more, the reaction is:b. endothermic
    3. The first law of thermodynamics states that:c. exothermic
    4. Bond breaking is:d. energy cannot be created or destroyed
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    1 → a (negative) · 2 → c (exothermic) · 3 → d (energy cannot be created or destroyed) · 4 → b (endothermic)

    • Heat is lost by the system.
    • ΔH negative.
    • Conservation of energy.
    • Energy is needed to separate atoms.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    endothermic  ·  falls  ·  exothermic  ·  lowering the activation energy
    1. A catalyst affects an energy profile by .
    2. Dissolving ammonium nitrate in water is .
    3. In an endothermic reaction, the temperature of the surroundings .
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    1: lowering the activation energy · 2: endothermic · 3: falls

    • Alternative pathway.
    • The solution gets cold.
    • Heat is absorbed.

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