Rates of Reaction: Collision Theory and Measuring Rate
Rates of reaction at IGCSE require more than describing collisions qualitatively, calculating the rate of a reaction at a specific moment by drawing a tangent to a graph of volume of gas (or mass) against time and finding its gradient, and using collision theory quantitatively, including how a catalyst provides an alternative reaction pathway with a lower activation energy.
Before you start
Make sure you're comfortable with these topics first:
Method
- State collision theory: for a reaction to occur, particles must collide with sufficient energy, at least the activation energy, and in the correct orientation; anything that increases the frequency of successful collisions increases the rate of reaction.
- Explain each factor using collision theory: increasing concentration/pressure increases the frequency of collisions, since particles are more crowded together; increasing temperature increases both the frequency of collisions and, more importantly, the proportion of particles with energy greater than or equal to the activation energy, so far more collisions are successful; increasing surface area (smaller pieces of a solid) exposes more area to the other reactant, giving more frequent collisions.
- Explain that a catalyst works by providing an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have enough energy to be successful, without the catalyst itself being used up or permanently changed by the reaction.
- To find the average rate over the whole reaction from a graph of volume of gas produced against time, divide the total volume produced by the total time taken: mean rate = change in volume / time taken.
- To find the rate at a specific moment (the instantaneous rate), draw a tangent, a straight line that just touches the curve without crossing it, to the graph at that point, then calculate the gradient of the tangent: gradient = change in y / change in x, using two points far apart on the tangent line for accuracy.
- Remember the rate of reaction is fastest at the very start of a reaction, when reactant concentration is highest, so collisions are most frequent, and decreases as reactants are used up, becoming zero once the graph becomes flat, showing the reaction has finished or a reactant is fully used up.
Worked example
A student measures the volume of hydrogen gas produced when magnesium reacts with excess dilute hydrochloric acid. A tangent is drawn to the graph at 20 seconds. The tangent passes through the points (10 s, 20 cm3) and (30 s, 60 cm3). Calculate the rate of reaction at 20 seconds.
- Identify the two points the tangent passes through: (10, 20) and (30, 60).
- Calculate the change in volume (the y-values): 60 - 20 = 40 cm3.
- Calculate the change in time (the x-values): 30 - 10 = 20 s.
- Calculate the gradient: 40 / 20 = 2.
- Final answer: the rate of reaction at 20 seconds is 2 cm3/s.
Practice questions
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Q1State the two conditions particles must meet for a collision to result in a reaction.Show answer
Answer: They must collide with energy greater than or equal to the activation energy, and in the correct orientation (direction).
Q2Explain, in terms of collision theory, why crushing a solid reactant into a powder increases the rate of reaction.Show answer
Answer: Crushing the solid increases its surface area, exposing more particles to collisions with the other reactant, so the frequency of successful collisions increases.
Q3Explain why increasing temperature has a much bigger effect on rate than increasing concentration by the same proportion.Show answer
Answer: Increasing temperature increases both the frequency of collisions and the proportion of particles with energy greater than or equal to the activation energy, whereas increasing concentration only increases the frequency of collisions, not the energy of each collision.
Q4Explain how a catalyst increases the rate of reaction without being used up.Show answer
Answer: A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react successfully; the catalyst is not chemically changed by the reaction, so it can be reused.
Q5A reaction produces 48 cm3 of gas in the first 60 seconds. Calculate the mean rate of reaction over this time.Show answer
Answer: 0.8 cm3/s (48/60)
Q6A tangent drawn to a rate graph at 15 seconds passes through the points (5 s, 10 cm3) and (25 s, 50 cm3). Calculate the rate of reaction at 15 seconds.Show answer
Answer: 2 cm3/s ((50-10)/(25-5) = 40/20)
Q7State what has happened to the reactants once the graph of volume of gas against time becomes a flat, horizontal line.Show answer
Answer: The reaction has finished, because at least one of the reactants has been completely used up (no more gas is being produced).
Exam-style questions
Written in the style of a IGCSE Science exam paper, with a full mark scheme.
A reaction between marble chips and hydrochloric acid produces 66 cm3 of carbon dioxide gas in the first 30 seconds. Calculate the mean rate of reaction over this time, giving the correct unit.
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A tangent is drawn to a graph of volume of gas against time at 8 seconds. The tangent passes through the points (2 s, 5 cm3) and (14 s, 41 cm3). (a) Calculate the rate of reaction at 8 seconds. (b) State how the value calculated in (a) would compare to the mean rate of reaction over the entire experiment, and explain why.
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Two experiments investigate the effect of temperature on the rate of reaction between magnesium ribbon and excess dilute hydrochloric acid, measuring the volume of hydrogen gas produced over time. Experiment 1 is carried out at 20 degrees C and Experiment 2 is carried out at 40 degrees C, using the same mass of magnesium and the same volume and concentration of acid. Explain, using collision theory, why the graph for Experiment 2 rises more steeply than the graph for Experiment 1, but both graphs eventually reach the same final (maximum) volume of gas.
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Free printable worksheet
Want more practice on paper? Download the rates of reaction: collision theory and measuring rate worksheet pack - 7 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 20 of IGCSE Science Workbook, the whole course as one free printable PDF.
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