Science notes, chapter 5
Thermochemistry
Textbook Science Form 3 (KSSM, Dual Language Programme), pages 146–159
What this chapter covers
- Define thermochemistry, exothermic reactions and endothermic reactions (5.1.1)
- Relate heat released or absorbed in a chemical reaction to temperature change and to exothermic and endothermic reactions (5.1.2)
- Carry out an experiment to compare and contrast exothermic and endothermic reactions (5.1.3)
- Explain, with examples from daily life, exothermic and endothermic reactions (5.1.4)
- Design materials that use exothermic and endothermic reactions to solve problems in daily life (5.1.5)
5.1Endothermic and exothermic reactions
Thermochemistry is the study of the heat changes that occur when chemical reactions take place.
Every chemical reaction is followed by a change in the form of energy. The chemical energy stored in the reactants is converted into other forms, most often heat energy. Heat is either released into the surroundings or absorbed from them, and this divides reactions into two types.
Definitions (5.1.1)
Exothermic reaction
A chemical reaction that releases heat into the surroundings.
Temperature of the products and surroundings rises.
A chemical reaction that releases heat into the surroundings.
Temperature of the products and surroundings rises.
Endothermic reaction
A chemical reaction that absorbs heat from the surroundings.
Temperature of the products and surroundings drops.
A chemical reaction that absorbs heat from the surroundings.
Temperature of the products and surroundings drops.
Word clue: exo = "outside", endo = "inside", thermic = "heat". Exothermic: heat goes out. Endothermic: heat goes in (textbook p. 148).
Thinking that the container, not the reaction, supplies or absorbs the heat. In an exothermic reaction the reaction releases heat and the thermometer reading rises. In an endothermic reaction the reaction takes heat from the water around it, so the reading falls.
Sodium is added to water and the water becomes hot. Is this reaction exothermic or endothermic? Which form of energy is released besides heat?
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Exothermic, because heat is released into the surroundings (the water becomes hot). The reaction also releases light energy (the flame) and sound energy (textbook Photograph 5.1, p. 148).
Detecting heat change with a thermometer (5.1.2)
The simplest way to tell the two types apart is to measure the temperature of the mixture with a thermometer. Remember from Form 2 that heat flows from a hotter object to a colder one until they reach thermal equilibrium.
| Exothermic reaction | Endothermic reaction | |
|---|---|---|
| Heat | Released into the surroundings | Absorbed from the surroundings |
| Thermometer reading | Rises | Drops |
| Temperature of the reaction mixture | Higher than before the reaction | Lower than before the reaction |
| Feels like | Hot | Cold |
| Energy change | Chemical energy converted to heat | Heat converted to chemical energy |
Exothermic28 °C rises to a maximum of 40 °C (a rise of 12 °C), then stays constant.
Endothermic28 °C falls to a minimum of 19 °C (a drop of 9 °C), then stays constant.
Why does the reading stop at a maximum or minimum? When the net rate of heat transfer between the products and the thermometer is zero, they are in thermal equilibrium, so the reading stays steady (textbook p. 151 answers).
Experiment 5.1: compare exothermic and endothermic reactions (5.1.3)
Experiment 5.1 at a glance (textbook pp. 149–151)
Aim: to compare and contrast exothermic and endothermic reactions.Hypothesis: an exothermic reaction releases heat into the surroundings while an endothermic reaction absorbs heat from the surroundings.
Manipulated variable: type of chemical substance. Responding variable: final (maximum or minimum) temperature. Constant variable: volume of water.
Apparatus: polystyrene cup, thermometer, spatula, measuring cylinder.
Materials: sodium hydroxide, ammonium chloride, sodium hydrogen carbonate, 0.1 M sodium hydroxide solution, 0.1 M hydrochloric acid.
Figures 5.1 and 5.2: Science Form 3 (DLP) textbook, p. 150.
- Pour 50 mL of water into a polystyrene cup, wait 2 minutes and record the initial temperature.
- Add two spatulas of the solid, stir until it dissolves, then record the maximum or minimum temperature.
- For the acid reactions, record the initial temperature of 25 mL of hydrochloric acid, add 25 mL of sodium hydroxide solution (or two spatulas of sodium hydrogen carbonate), stir and record the maximum or minimum temperature.
| Reactants | Temperature | Type of reaction |
|---|---|---|
| Sodium hydroxide and water | Rises | Exothermic |
| Ammonium chloride and water | Drops | Endothermic |
| Hydrochloric acid and sodium hydroxide solution | Rises | Exothermic (neutralisation) |
| Hydrochloric acid and sodium hydrogen carbonate | Drops | Endothermic |
Conclusion: the hypothesis is accepted. Exothermic reactions release heat and the temperature rises; endothermic reactions absorb heat and the temperature falls.
Two similar reactions, two types. Hydrochloric acid reacting with sodium carbonate is exothermic, but hydrochloric acid reacting with sodium hydrogen carbonate is endothermic (textbook p. 157, Summative Practice 5, question 5).
Operational definitions. Release of heat: the thermometer reading increases. Absorption of heat: the thermometer reading decreases. To make the maximum or minimum reading more accurate, wrap the polystyrene cup with cotton wool or felt and cover it with a lid, which is a heat insulator and cuts heat loss to (or gain from) the surroundings.
In Experiment 5.1 the temperature of 50 mL of water is 28 °C before 2 spatulas of ammonium chloride are dissolved and 19 °C afterwards. State the type of reaction and the temperature change.
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Endothermic. The temperature drops by 28 − 19 = 9 °C because heat is absorbed from the water.
Examples in daily life (5.1.4)
| Exothermic reactions (heat released) | Endothermic reactions (heat absorbed) |
|---|---|
| Burning of paper, wood, candle and petrol (combustion) | Photosynthesis in green plants |
| Fireworks display and bomb explosion | Baking a cake or making bread |
| Respiration in humans and animals | Extraction of iron from iron ore |
| Neutralisation of an acid with an alkali | Dissolving an ammonium salt (e.g. ammonium chloride, ammonium nitrate) in water |
| Rusting of iron; sodium reacting with water | Heating calcium carbonate (thermal decomposition) |
| Thermite reaction; dissolving sodium hydroxide or calcium chloride in water | Hydrochloric acid reacting with sodium hydrogen carbonate |
Examples: textbook Photograph 5.2 (p. 152), Summary (p. 154) and Summative Practice (pp. 155–158).
Why this matters
- Body temperature: respiration is exothermic. During vigorous exercise the rate of respiration increases, so more heat is released and body temperature rises.
- Global warming: burning fossil fuels is exothermic and releases heat and carbon dioxide. Burning less fossil fuel reduces the problem. Replanting trees raises the rate of photosynthesis, which absorbs heat (endothermic), so the surroundings become cooler.
- Thermite reaction: heating iron(II) oxide, aluminium powder and magnesium tape releases so much heat that the iron melts. The molten iron is used to repair or join broken railway tracks.
- Cooking: baking a cake needs a continuous supply of heat because the reaction absorbs heat (endothermic).
Thinking that "heating" always means an exothermic reaction. Baking a cake or heating calcium carbonate needs heat to be supplied and absorbed, so these are endothermic. Burning a fuel needs a flame only to start; once going it releases heat, so it is exothermic.
Heating calcium carbonate in a boiling tube turns limewater cloudy. Is the reaction exothermic or endothermic? Explain.
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Endothermic. The calcium carbonate must keep being heated: heat is absorbed from the surroundings to decompose it into calcium oxide and carbon dioxide (which turns limewater cloudy).
Designing materials for daily life (5.1.5)
| Instant hot pack | Instant cold pack |
|---|---|
| Contains a salt such as calcium chloride and a small bag of water. Dissolving the salt is exothermic. | Contains a salt such as ammonium nitrate and a small bag of water. Dissolving the salt is endothermic. |
| Releases heat to relieve muscle cramp and to widen the lumen of blood capillaries so that blood flows faster. | Absorbs heat to reduce swelling and inflammation, and to narrow the lumen of blood capillaries so that blood flows more slowly, which helps to stop bleeding. |
To make a pack: put the powder in a large plastic bag and the water in a small thin bag inside it. Pressing or pricking the small bag releases the water, which mixes with the powder and starts the reaction. Other designs from Activity 5.1 (p. 153) are an emergency lamp for a power failure and a container that keeps a high or low temperature.
Exam rule: always name the reaction type and give the heat direction and the temperature effect, e.g. "exothermic, because heat is released into the surroundings and the temperature rises".
A hospital needs a pack to stop bleeding from a cut. Which pack and which salt would you choose? Explain.
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An instant cold pack with ammonium nitrate. Dissolving it in water is endothermic, so heat is absorbed, the pack becomes cold, the lumen of the blood capillaries narrows and the blood flows more slowly, which helps to stop bleeding.