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Exothermic and Endothermic Reactions — Answers

Answer sheet with model answers and success criteria for the Year 10 mixtures & chemical reactions worksheet.

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Success TutoringWorksheetsAnswer sheetYear 10 · Science · Mixtures & Chemical Reactions

Exothermic and Endothermic Reactions — Answers

Part A: Releasing or absorbing energy?

Classify each reaction.

In an exothermic reaction, energy is released to the surroundings, usually as heat, so the temperature of the surroundings rises. Combustion, respiration and neutralisation are exothermic.

In an endothermic reaction, energy is absorbed from the surroundings, so the temperature falls (or energy must be supplied continuously). Photosynthesis and thermal decomposition are endothermic.

1.Burning natural gas on a stove
  1. AExothermic
  2. BEndothermic

Answer: Exothermic

Combustion releases energy as heat and light.

2.Photosynthesis in a leaf
  1. AExothermic
  2. BEndothermic

Answer: Endothermic

Plants absorb light energy to make glucose from carbon dioxide and water.

3.Cellular respiration in muscle cells
  1. AExothermic
  2. BEndothermic

Answer: Exothermic

Respiration releases energy from glucose; that is why your body warms up during exercise.

4.Heating calcium carbonate strongly until it breaks down into calcium oxide and carbon dioxide
  1. AExothermic
  2. BEndothermic

Answer: Endothermic

Thermal decomposition needs a continuous supply of heat energy, so it is endothermic.

5.Adding an acid to an alkali, which makes the solution warmer
  1. AExothermic
  2. BEndothermic

Answer: Exothermic

Neutralisation releases energy to the surroundings, so the temperature rises.

6.In an endothermic reaction, the reaction mixture feels cold because energy is taken in from the surroundings.

Answer: True

Energy moves from the surroundings (including your hand and the beaker) into the chemicals, so the temperature of the surroundings falls.

Part B: Measuring energy changes

Students measured the starting and highest (or lowest) temperature of two reactions in a foam cup. Reaction 1: magnesium ribbon added to dilute hydrochloric acid: 21 °C → 35 °C. Reaction 2: citric acid solution mixed with sodium hydrogen carbonate (baking soda): 20 °C → 12 °C.

Temperature change = final temperature − initial temperature. A positive change means exothermic; a negative change means endothermic.

7.Calculate the temperature change for reaction 1 and state whether it is exothermic or endothermic.

Answer: +14 °C, exothermic

  1. Change = final − initial = 35 − 21 = +14 °C
  2. The temperature rose, so energy was released: exothermic
8.Calculate the temperature change for reaction 2 and classify it.

Answer: −8 °C, endothermic

  1. Change = 12 − 20 = −8 °C
  2. The temperature fell, so energy was absorbed: endothermic
9.Reaction 1 was repeated three times, giving temperature rises of 13 °C, 15 °C and 14 °C. Calculate the mean.

Answer: 14.0 °C

  1. (13 + 15 + 14) ÷ 3 = 42 ÷ 3 = 14.0 °C
10.Suggest one way to reduce heat loss to the surroundings and make the results more accurate.

Answer: Use a polystyrene (foam) cup instead of a glass beaker, and put a lid on it, so less heat escapes or enters during the reaction.

Part C: Bonds and energy profiles

In a reaction, bonds in the reactants are broken, which needs energy, and new bonds in the products are made, which releases energy. If more energy is released by making bonds than is taken in to break bonds, the reaction is exothermic. If less is released, it is endothermic.

An energy profile shows the energy of the reactants and products, with a hump between them: the activation energy.

11.Is breaking a chemical bond exothermic or endothermic? What about forming a bond?

Answer: Breaking bonds is endothermic (energy is taken in). Forming bonds is exothermic (energy is released).

12.In a reaction, 2000 kJ is needed to break the bonds in the reactants and 2500 kJ is released when the bonds in the products form. Is the reaction exothermic or endothermic, and what is the overall energy change?

Answer: Exothermic; 500 kJ released

  1. More energy is released than taken in
  2. 2500 − 2000 = 500 kJ released (exothermic)
13.In another reaction, breaking bonds takes in 1200 kJ and making bonds releases 900 kJ. Classify the reaction and calculate the overall energy change.

Answer: Endothermic; 300 kJ absorbed

  1. 1200 − 900 = 300 kJ absorbed
  2. More energy is needed than is released, so it is endothermic
14.On an energy profile for an exothermic reaction, where are the products compared with the reactants?
  1. AHigher in energy
  2. BLower in energy
  3. CAt the same energy
  4. DAbove the peak of the activation energy hump

Answer: Lower in energy

The products have less stored chemical energy than the reactants because the difference has been released to the surroundings.

15.What is activation energy?

Answer: The minimum energy that reacting particles need to collide with for a reaction to start; it is the energy needed to begin breaking bonds (the hump on an energy profile).

Part D: Apply

16.Instant cold packs used for sports injuries and single-use hand warmers both contain chemicals that are kept separate or sealed until needed. Explain the energy change in each product, why the temperature of the pack changes, and why each is suited to its use.[4 marks]

Model answer: When an instant cold pack is squeezed, an inner bag breaks and the chemicals mix in an endothermic process that absorbs energy from the surroundings. Energy moves from the injured area and the pack's surroundings into the chemicals, so the pack feels cold, which helps reduce swelling and pain. A hand warmer contains iron powder that reacts with oxygen from the air when the packet is opened, an exothermic oxidation reaction. Energy is released to the surroundings, so the pack warms up, keeping hands warm for several hours. Both are single-use because the chemicals are used up in the reaction.

Success criteria:

  • Cold pack: endothermic, absorbs energy from surroundings
  • Hand warmer: exothermic, releases energy to surroundings
  • Explains the direction of energy transfer causing the temperature change
  • Links each to its use
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