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Unit 8: Energy changes and reactions of metals

Exothermic reactions — HIA International - Grade 7 Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 12 3 quizzes

📚 HIA International - Grade 7 Hub / Topical Revision
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Lesson summary

8.1 Exothermic reactions — HIA INTERNATIONAL notes: Describe burning and other reactions that give out thermal energy, and plan an investigation of one.Burning = a chemical reaction in which a substance combines with oxygen; also called combustion.Fuel = the substance that reacts…

  • Energy change
  • Carbon burning
  • Oxidation reaction
  • Potassium and water
  • Same energy
  • Questions to test

Section 1 of 9Burning

Burning
Burning a chemical reaction in which a substance combines with oxygen; also called combustion.
Fuel
Fuel the substance that reacts with oxygen, such as charcoal, wood, coal, natural gas and oil.
Needs
  • Needs oxygen, fuel and heat (thermal energy).
Energy change
  • Energy change the chemical energy of the fuel is changed to thermal, light and sound energy that spreads out into the surroundings.

Section 2 of 9Exothermic reactions

Exothermic reaction
Exothermic reaction a chemical reaction in which thermal energy is given out.
Evidence
  • Evidence the temperature of the surroundings rises.
Why
  • Why atoms rearrange into new bonds and chemical energy is changed to thermal energy.
Carbon burning
  • Carbon burning carbon + oxygen gives carbon dioxide.

Section 3 of 9Examples of exothermic reactions

ReactionWord equationObservation
Hydrogen burninghydrogen + oxygen gives waterEnergy given out as kinetic, thermal, sound and light energy; can propel a model rocket
Magnesium burningmagnesium + oxygen gives magnesium oxideHeat and bright white light
Potassium and waterpotassium + water gives potassium hydroxide + hydrogenSo much heat that the hydrogen burns
Magnesium and acidmagnesium + hydrochloric acid gives magnesium chloride + hydrogenThe test tube gets hot

Section 4 of 9Oxidation

Oxide
Oxide a new substance formed when a substance combines with oxygen.
Oxidation reaction
Oxidation reaction any reaction in which a substance combines with oxygen.

Section 5 of 9Reactions with water and acid

Potassium and water
  • Potassium and water bonds between oxygen and hydrogen atoms in water break and the atoms rearrange to form potassium hydroxide and hydrogen.
Energy
  • Energy stored chemical energy changes to thermal energy that dissipates into the environment.
Dissipate
Dissipate spread out.

Section 6 of 9Measuring the temperature rise

1
Measure 10 cm3 of dilute hydrochloric acid into a test tube.
2
Measure the starting temperature.
3
Add an identical piece of magnesium ribbon.
4
When the reaction stops, measure the final temperature.
5
Temperature rise = final temperature − starting temperature.
Same energy
  • Same energy equal amounts of the same reagents change the same amount of chemical energy to thermal energy, whatever the final temperature.
Safety
  • Safety wear safety glasses.

Section 7 of 9Planning an investigation of magnesium and acid

Questions to test
  • Questions to test does more magnesium, a different metal or a different acid give a bigger temperature change.
Preliminary work
  • Preliminary work practise measuring temperature and find out how big a change in the variable is needed to give a measurable change.
Heat loss
  • Heat loss thermal energy spreads out, so use insulation such as a polystyrene cup to reduce it.
Plan
  • Plan equipment list, results table, risk assessment, and teacher check before starting.
Changes
  • Changes record any changes made to the plan and why.

Section 8 of 9Key terms

TermMeaning
Exothermic reactionA reaction that gives out thermal energy.
CombustionBurning; a reaction with oxygen.
FuelA substance that burns to release energy.
OxideA compound of an element with oxygen.
OxidationA reaction in which a substance combines with oxygen.
DissipateTo spread out.
Thermal energyHeat energy.
Preliminary workEarly trials done before the main investigation.

Section 9 of 9Exam technique

1
Saying burning removes energy — it releases energy.
2
Saying a higher end temperature means more energy was released — start temperature matters.
3
Saying burning does not need oxygen.
4
Forgetting that heat is lost to the surroundings in measurements.

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