12.9Chapter 12: Chemical Reactions 3 and Rates

Rates of reaction — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 13 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision

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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

Section 1 of 8What rate means

Rate of reaction

Rate of reaction a measure of how quickly a reactant is used up, or how quickly a product forms.

Fast and slow reactions

Fast and slow reactions reactions in fireworks and explosions are fast, whereas rusting is slow and takes years.

Why it matters

Why it matters chemists 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 rate

Finding the rate you cannot tell how quickly a reaction happens from its equation, so you must do an experiment.

Calculation

Calculation rate = amount of product formed ÷ time taken, or amount of reactant used up ÷ time.

Section 2 of 8Measuring gas volume

Apparatus

Apparatus collect the gas in a gas syringe or an upturned measuring cylinder over water.

Method

Method 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.

Example

Vijay dropped magnesium into hydrochloric acid, and the hydrogen went into a gas syringe, pushing the plunger out.
Mg + 2HCl → MgCl₂ + H₂.

Section 3 of 8Measuring mass loss

Method

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

Result the loss in mass equals the mass of gas released.

Limitation

Limitation it works best for dense gases like carbon dioxide, because hydrogen is so light that the mass change is tiny.

Choosing a method

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.

Section 4 of 8Presenting results

Table

Table put the independent variable (time) in the left column and the dependent variable (volume of gas) in the large right column.

Reliability

Reliability repeat the investigation three times and calculate average values to reduce error.

Line graph

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

Anomalies points that do not fit the curve should be rechecked and ignored when calculating means.

Section 5 of 8Reading a rate graph

1Start

Start the graph rises steeply because the reactants are at high concentration, so gas forms quickly and the rate is fast.

2Middle

Middle the slope gets less steep as the reaction slows down.

3End

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.

4Steepness

Steepness the steeper the line, the faster the rate.

5Total product

Total product the final plateau shows the total amount of product.

6Rate at a point

Rate at a point draw a tangent and calculate the gradient = change in y ÷ change in x.

7Average rate

Average rate total volume ÷ total time.

8Half-way time

Half-way time the time to reach half of the final volume is shorter for faster reactions.

Section 6 of 8Limiting reactant

Stopping

Stopping a reaction may stop because a reactant is used up rather than because the rate has decreased.

Final volume

Final volume it depends on the amount of the limiting reactant.

Shape of the curve

Shape of the curve it depends on factors that change the rate.

Section 7 of 8Key terms

Term	Meaning
Rate of reaction	how fast reactants are used up or products form.
Gas syringe	apparatus to measure the volume of a gas.
Tangent	a straight line touching a curve at one point.
Gradient	the steepness of a line, change in y divided by change in x.
Limiting reactant	the reactant that is used up first and stops the reaction.
Line of best fit	a smooth line drawn through the trend of the points.