Bond Energies and the Overall Energy Change · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Break, make and take the difference

Science · Matter & Materials · ages 15-16
Name ______________________   Date ____________

  1. Making the bonds in the products: energy in or out?

    • Energy is taken in
    • No energy moves at all
    • Energy is given out

  2. Breaking the bonds in the reactants: energy in or out?

    • Energy is taken in
    • Energy is given out
    • No energy moves at all

  3. The overall change equals the break total minus the make total.

    Circle one:   True   False

  4. Breaking totals 1800 and making totals 2600. What is the overall energy change?

    Answer: ______________

  5. In H2 + Br2 -> 2HBr, how many H-Br bonds form on the right?

    • 1
    • 3
    • 2

  6. An overall change of -800 means what?

    • It is endothermic and cools the surroundings
    • It is exothermic and warms the surroundings
    • It cannot happen at all

  7. A table-based value differs from the school measurement. Which reason fits?

    • The reaction created extra atoms
    • Tables hold averages and heat escapes in school labs
    • Bond energies change with the weather

  8. Why does burning a hydrocarbon in oxygen release so much energy?

    • Weak bonds break and far stronger C=O and O-H bonds form
    • No bonds break at all during burning
    • Only single bonds ever form in the products
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Break, make and take the difference W1-mt_VI0_1ogIOy-s1

  1. Energy is given out · Forming bonds releases energy, so making is exothermic.
  2. Energy is taken in · Pulling atoms apart costs energy, so breaking is endothermic.
  3. True · Pay the breaking cost first, then subtract the making payback.
  4. -800 · 1800 minus 2600 is -800, an exothermic result.
  5. 2 · Two HBr molecules each hold one H-Br bond, so two form.
  6. It is exothermic and warms the surroundings · A negative value means making paid back more than breaking cost.
  7. Tables hold averages and heat escapes in school labs · Averages are approximate and real flasks leak heat to the room.
  8. Weak bonds break and far stronger C=O and O-H bonds form · Strong product bonds pay back far more than breaking cost.
Worksheet · LightMySky