CHEMISTRY · Stage 9 · Chapter 12: Chemical Reactions 3 and Rates
Chemistry 12.9 Rates of reaction — Measuring Gas Volume and Mass Loss Over Time
5 min read 6 parts Lesson 9 of 13
Lesson summary
12.9 Rates of reaction — Measuring Gas Volume and Mass Loss Over Time — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define rate of reaction, describe how to follow the rate of a reaction that makes a gas and interpret rate graphs.Rate of reaction = a measure of…
- Fast and slow reactions
- Why it matters
- Finding the rate
- Calculation
- Limitation
- Choosing a method
Part 1 of 6What rate means
- Rate of reaction a measure of how quickly a reactant is used up, or how quickly a product forms.
- Fast and slow reactionsreactions in fireworks and explosions are fast, whereas rusting is slow and takes years.
- Why it matterschemists want to slow down some reactions such as rusting and speed up reactions that make useful products such as soap, fertilisers or medicines.
- Finding the rateyou cannot tell how quickly a reaction happens from its equation, so you must do an experiment.
- Calculationrate = amount of product formed ÷ time taken, or amount of reactant used up ÷ time.
Part 2 of 6Measuring gas volume
1Apparatus
— collect the gas in a gas syringe or an upturned measuring cylinder over water.
2Method1 point
— start a stopwatch when the reactants are mixed and record the total volume at regular intervals, such as every 10 or 20 seconds or every minute.
- Bung the flask quickly so that gas does not escape before it can be measured.
3Example1 point
— Vijay dropped magnesium into hydrochloric acid, and the hydrogen went into a gas syringe, pushing the plunger out.
- Mg + 2HCl → MgCl2 + H2.
Part 3 of 6Measuring mass loss
- Method — if the gas escapes, the flask on a balance gets lighter, so record its mass at regular intervals.
- Cotton wool in the neck of the flask stops acid spray leaving.
- Result — the loss in mass equals the mass of gas released.
- Limitation — it works best for dense gases like carbon dioxide, because hydrogen is so light that the mass change is tiny.
- Choosing a method — a colorimeter tracks a coloured product, the cloudiness of a precipitate can be timed with a cross, and data logging records automatically.
Part 4 of 6Presenting results
- Table — put the independent variable (time) in the left column and the dependent variable (volume of gas) in the large right column.
- Reliability — repeat the investigation three times and calculate average values to reduce error.
- Line graph — plot a line graph because both variables are continuous.
- Put the independent variable on the x-axis and the dependent variable on the y-axis, and space the numbers evenly.
- Draw a cross for each point and a smooth curve of best fit, with equal numbers of points above and below the line.
- Anomalies — points that do not fit the curve should be rechecked and ignored when calculating means.
Part 5 of 6Reading a rate graph
- Start — the graph rises steeply because the reactants are at high concentration, so gas forms quickly and the rate is fast.
- Middle — the slope gets less steep as the reaction slows down.
- End — the line becomes horizontal when no more gas is made, which shows that the reaction has finished because a reactant has been used up.
- In Vijay's experiment the graph stopped rising from the sixth minute, because all the magnesium had been used up.
- Steepness — the steeper the line, the faster the rate.
- Total product — the final plateau shows the total amount of product.
- Rate at a point — draw a tangent and calculate the gradient = change in y ÷ change in x.
- Average rate — total volume ÷ total time.
- Half-way time — the time to reach half of the final volume is shorter for faster reactions.
Part 6 of 6Limiting reactant
- Stoppinga reaction may stop because a reactant is used up rather than because the rate has decreased.
- Final volumeit depends on the amount of the limiting reactant.
- Shape of the curveit depends on factors that change the rate.
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