Endothermic & Exothermic Reactions — Drills · Grade 8 Science

Chapter: Chemistry

What these drills covers

By the end of Endothermic & Exothermic Reactions you should be able to:

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

Vocabulary used: exothermic reaction, endothermic reaction, activation energy, model, evidence, system boundary, assumption.

Problem 1

An exothermic reaction
  1. absorbs energy from the surroundings
  2. releases energy to the surroundings
  3. neither absorbs nor releases
  4. destroys energy
Answer and explanation

releases energy to the surroundings

Exothermic reactions release energy, usually warming the surroundings.

Problem 2

A cold pack becomes cold when activated. The reaction inside is
  1. exothermic
  2. endothermic
  3. physical only
  4. neutral
Answer and explanation

endothermic

It absorbs energy from the surroundings, which is why the pack and your hand feel cold.

Problem 3

A mixture's temperature rises from 19.0 °C to 31.5 °C. What is the temperature change in °C?
Answer and explanation

12.5 °C

31.5 − 19.0 = 12.5 °C, a rise, so the reaction is exothermic.

Problem 4

In an endothermic reaction, the energy needed to break bonds is
  1. less than the energy released forming bonds
  2. greater than the energy released forming bonds
  3. exactly equal
  4. zero
Answer and explanation

greater than the energy released forming bonds

More energy goes into breaking than comes out of forming, so the reaction takes energy in overall.

Problem 5

A reaction's temperature falls from 24.0 °C to 18.5 °C. By how many degrees did it fall?
Answer and explanation

5.5 °C

24.0 − 18.5 = 5.5 °C, a fall, indicating an endothermic reaction.

Problem 6

Which is exothermic?
  1. Photosynthesis
  2. Burning fuel
  3. Melting ice
  4. Evaporating water
Answer and explanation

Burning fuel

Combustion releases energy as heat and light; the other three absorb energy.

Problem 7

Explain why breaking bonds always requires energy while a reaction can still release energy overall.
Answer and explanation

Bonds are attractions holding atoms together, so pulling atoms apart always costs energy. A reaction releases energy overall when the new bonds formed in the products release more than was spent breaking the old ones. The overall result is the difference between the two, which is why the direction depends on the whole reaction rather than on any single bond.

Response criteria

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

Problem 8

Two reactions are run in identical insulated cups. One rises 10 °C, the other falls 4 °C. Interpret both and state what the insulation is for.
Answer and explanation

The first is exothermic, releasing energy that warms the mixture; the second is endothermic, absorbing energy from the mixture and cooling it. The insulation reduces energy exchange with the room, so the temperature change measured reflects the reaction rather than heat leaking in or out, which makes the comparison between the two fair.

Response criteria

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

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