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Unit 8: Rates of reaction

GRADE 8 – Unit 8: Rates of reaction — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 1 of 10 1 quiz

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

GRADE 8 - Unit 8: Rates of reaction — HIA INTERNATIONAL notes: Summarise neutralisation, signs of chemical reactions and how temperature, surface area, concentration and catalysts change the rate of reaction.Neutralisation — the reaction between an acid and a base or alkali: acid…

  • The pH moves towards 7
  • Universal indicator shows the end point
  • Everyday uses
  • In a chemical reaction the reactants are rearranged into products
  • Signs of a reaction
  • A word equation

Part 1 of 7Neutralisation

Neutralisation
Neutralisation
  • Neutralisation the reaction between an acid and a base or alkali: acid + alkali → salt + water.
The pH moves towards 7
  • The pH moves towards 7 and the solution is neutral only when exact amounts react.
Example: hydrochloric acid + sodium hydroxide → sodium chloride + water.
Universal indicator shows the end point
  • Universal indicator shows the end point green means neutral, pH 7.
Everyday uses
  • Everyday uses antacids for indigestion, toothpaste, lime on acidic soil and alkali in acidic lakes.

Part 2 of 7Chemical reactions and their signs

1In a chemical reaction the reactants are rearranged into products
  • In a chemical reaction the reactants are rearranged into products atoms are rearranged and the total mass is unchanged.
2Signs of a reaction
  • Signs of a reaction gas given off (fizzing), colour change, temperature change, precipitate, reactant disappearing or a pH change.
3A word equation
  • A word equation reactants → products, for example magnesium + oxygen → magnesium oxide.
4Precipitate
Precipitate an insoluble solid formed when solutions react; it makes the mixture cloudy.

Part 3 of 7Tests for gases

GasTestResult
HydrogenLighted splintSqueaky pop
OxygenGlowing splintRelights
Carbon dioxideLimewaterTurns cloudy

Part 4 of 7Rate of reaction and how it is measured

Rate of reaction and how it is measured
Rate of reaction
Rate of reaction how quickly reactants are used up or products are made.
Rate
Rate amount of product formed ÷ time
  • with units such as cm3/s or g/s.
Volume of gas collected in a gas syringe or over water at regular intervals.
  • Mass lost on a balance as a gas escapes.
On a graph of product against time
  • On a graph of product against time a steeper line means a faster rate and a flat line means the reaction has finished.
The reaction is fastest at the start
  • The reaction is fastest at the start because the reactants are at their highest concentration.

Part 5 of 7Collision theory

Collision theory
Particles must collide with enough energy to react
  • Particles must collide with enough energy to react the minimum energy needed is the activation energy.
A reaction goes faster when
  • A reaction goes faster when collisions are more frequent, or more collisions have enough energy to be successful.
These factors change the speed of a reaction
  • These factors change the speed of a reaction they do not change the total amount of product.

Part 6 of 7Temperature and surface area

Higher temperature
  • Higher temperature particles have more kinetic energy, collide more often and a larger proportion of collisions are successful.
A rise of about 10 °C roughly doubles the rate of many reactions.
Larger surface area
  • Larger surface area smaller pieces expose more particles, so collisions are more frequent.
A 3 cm cube cut into 27 cubes of 1 cm has a surface area of 54 cm2 before and 162 cm2 after, three times larger.

Part 7 of 7Concentration and catalysts

1Higher concentration
  • Higher concentration more particles in the same volume, so more frequent collisions and a faster rate.
2Catalyst
Catalyst a substance that speeds up a reaction without being used up.
3
Manganese dioxide speeds up the decomposition of hydrogen peroxide, and the catalyst can be used again.
  • Enzymes such as catalase are biological catalysts.
4Catalysts are used in industry and in car exhausts
  • Catalysts are used in industry and in car exhausts they are cheaper than raising temperature, concentration or surface area.

Key terms

TermMeaning
Rate of reactionHow fast reactants are used up or products are formed.
Activation energyThe minimum energy particles need for a collision to cause a reaction.
CatalystA substance that speeds up a reaction without being used up.
Surface areaThe total area of a solid exposed to react.
ConcentrationThe amount of a substance in a given volume of solution.
NeutralisationThe reaction of an acid with a base or alkali to form a salt and water.
PrecipitateAn insoluble solid formed when solutions react.
Collision theoryThe idea that reactions happen when particles collide with enough energy.

Avoid these mistakes

1
Saying a catalyst is used up — it is not used up.
2
Saying temperature makes particles bigger — particles move faster with more energy.
3
Reading a flat line on a graph as the reaction speeding up — it means the reaction has stopped.
4
Saying bigger pieces have more surface area — smaller pieces have a larger total surface area.

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