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Unit 8: Rates of reaction

Measuring Rates Of Reaction — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 10 2 quizzes

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

Measuring rates of reaction — HIA INTERNATIONAL notes: Define rate of reaction, measure it by gas volume, mass loss or time, and read rate from a graph.Rate of reaction = how fast reactants are used up or products are made.Rate = amount of product formed ÷ time — or amount of rea…

  • A reaction slows as it proceeds
  • Magnesium ribbon + sulfuric acid
  • Volume of gas
  • Loss of mass
  • Time for a solution to go cloudy
  • The gradient (slope) of the line shows the rate

Section 1 of 9Rate of reaction

1Rate of reaction
Rate of reaction how fast reactants are used up or products are made.
2Rate
Rate amount of product formed ÷ time
  • or amount of reactant used ÷ time, in units such as cm3/s or g/s.
3A reaction slows as it proceeds
  • A reaction slows as it proceeds bubbles come off quickly at first, then fewer, and finally none when it has stopped.
4Magnesium ribbon + sulfuric acid
  • Magnesium ribbon + sulfuric acid the easiest way to follow it is to measure the volume of hydrogen made.

Section 2 of 9Methods of measuring the rate

1Volume of gas
  • Volume of gas collect it in a gas syringe or by collecting over water in an upturned measuring cylinder and read it at regular times.
2Loss of mass
  • Loss of mass weigh the flask on a balance as the gas escapes, for example calcium carbonate + acid;
    • cotton wool in the neck lets gas out but stops spray.
3Time for a solution to go cloudy
  • Time for a solution to go cloudy the disappearing cross with sodium thiosulfate and acid;
    • a shorter time means a faster rate.
4Time to collect a fixed volume of gas
  • Time to collect a fixed volume of gas for example 25 cm3 of carbon dioxide.

Section 3 of 9Measuring rate by collecting gas

1
Set up a conical flask connected to a gas syringe or a measuring cylinder over water.
2
Add a measured volume and concentration of acid to the flask.
3
Add the solid, insert the bung quickly and start the stopwatch.
4
Record the gas volume every 30 seconds until three readings are the same.
5
Plot volume against time and compare the gradients.

Section 4 of 9Using graphs to find the rate

The gradient (slope) of the line shows the rate
  • The gradient (slope) of the line shows the rate the steeper the slope, the faster the reaction.
The line is steepest at the start
  • The line is steepest at the start the reaction is fastest then.
The slope becomes less steep
  • The slope becomes less steep the reaction is slowing down.
The line levels off
  • The line levels off no more product is formed and the reaction has finished because a reactant is used up.
Average rate between two times
  • Average rate between two times the volume of gas produced between them divided by the time taken.
If 20 cm3 is produced between 10 s and 30 s, the average rate is 20 cm3 in 20 s, which is 1 cm3/s.

Section 5 of 9Why the rate changes during a reaction

Why the rate changes during a reaction
For a reaction the particles must collide with enough energy
  • For a reaction the particles must collide with enough energy at the start there are many unreacted particles, so collisions are frequent.
As particles react
  • As particles react fewer unreacted particles remain and collisions become less frequent.
Eventually all of one reactant has reacted
  • Eventually all of one reactant has reacted there are no more collisions that make the product and the reaction stops.

Section 6 of 9Good practice in rate experiments

1Fair test
  • Fair test change only the independent variable, measure the dependent variable and keep the others controlled.
2Repeat readings
  • Repeat readings this improves reliability.
3Anomalous result
  • Anomalous result a point that does not fit the pattern of the graph; repeat that reading to check it.
4A graph shows the pattern clearly
  • A graph shows the pattern clearly choose a suitable scale, label the axes with units and draw a smooth line through the points.

Section 7 of 9Safety

Safety
Wear safety glasses.
Do not collect more gas than the syringe holds
  • Do not collect more gas than the syringe holds a bigger volume needs more suitable apparatus or smaller amounts of reactants.
Hydrogen is flammable.

Section 8 of 9Key terms

TermMeaning
Rate of reactionHow fast reactants are used up or products are formed.
GradientThe slope of a line on a graph, which shows the rate.
Anomalous resultA result that does not fit the pattern.
Gas syringeApparatus that collects and measures the volume of a gas.
Collecting over waterCollecting a gas in a measuring cylinder full of water.
CollisionWhen particles meet; reactions need collisions with enough energy.
ReliableGiving similar results when the investigation is repeated.
Dependent variableThe variable that is measured in an investigation.

Section 9 of 9Exam technique

1
Saying a steeper line on a graph means a slower reaction — it means a faster reaction.
2
Reading a flat line as a fast reaction — it means the reaction has stopped.
3
Measuring the rate without a time reading — rate always involves time.
4
Changing more than one variable in a rate experiment — only one variable may change.

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