Rates of Reaction
Rates of reaction, in the separate GCSE Chemistry course, is treated in more depth than in Combined Science: precise collision theory, how each factor changes the frequency or the energy of collisions, calculating rate from the gradient of a graph, using a tangent for the rate at one specific moment rather than just a mean rate, and the two required practicals used to measure how a variable affects rate, the gas-volume method and the colour-change (disappearing cross) method.
Before you start
Make sure you're comfortable with these topics first:
Method
- State collision theory precisely: for a reaction to happen, particles must collide with each other with at least the activation energy; increasing the frequency of collisions and/or the energy of collisions increases the rate of reaction.
- Explain each factor in terms of collision frequency or collision energy: increasing temperature increases both the frequency of collisions and the proportion of particles that collide with energy greater than or equal to the activation energy; increasing concentration or pressure increases only the frequency of collisions, by increasing the number of particles in a given volume; increasing surface area increases only the frequency of collisions, by exposing more particles of a solid to the other reactant.
- Find the rate of reaction at one specific moment, rather than the mean rate over the whole reaction, by drawing a tangent to the curve on a graph of quantity against time at that point, then calculating the gradient of the tangent as (change in the y-axis quantity) / (change in time), using two points on the tangent line.
- For the required practical using the gas-volume method, know the apparatus (a conical flask, bung and delivery tube leading to a gas syringe), name a suitable independent variable such as the temperature of the acid, and identify the main source of error as gas escaping while the bung is being fitted, which is reduced by fitting the bung as quickly and consistently as possible each time.
- For the required practical using the colour-change (disappearing cross) method, know the apparatus (a conical flask of sodium thiosulfate and dilute hydrochloric acid, placed over a piece of paper marked with a black cross, and a stopwatch), that the dependent variable is the time taken for the cross to no longer be visible through the increasingly cloudy sulfur precipitate, and that the main source of error is that judging the exact moment the cross disappears is subjective.
- State the improvement for the disappearing cross method: use a light sensor connected to a data logger to measure light transmission through the mixture objectively, giving a precise, repeatable end point instead of relying on judging by eye.
- When comparing the rate of the same reaction at two different times, compare the two calculated gradients directly and explain any difference using the change in concentration of a reactant as the reaction proceeds.
Worked example
The graph for the reaction between magnesium and excess dilute hydrochloric acid shows the volume of hydrogen gas produced against time. A tangent is drawn to the curve at 20 seconds. This tangent passes through the points (10 s, 10 cm3) and (30 s, 50 cm3). Calculate the rate of reaction at 20 seconds.
- Identify two points on the tangent line: (10 s, 10 cm3) and (30 s, 50 cm3).
- Calculate the change in volume (the change in y): 50 - 10 = 40 cm3.
- Calculate the change in time (the change in x): 30 - 10 = 20 s.
- Calculate the gradient of the tangent: 40 / 20.
- Gradient = 2 cm3/s.
- Final answer: the rate of reaction at 20 seconds is 2 cm3/s.
Practice questions
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Q1State the condition, in terms of energy, that colliding particles must meet for a reaction to occur.Show answer
Answer: The particles must collide with energy greater than or equal to the activation energy.
Q2Explain why increasing temperature increases the rate of a reaction, referring to both the frequency and the energy of collisions.Show answer
Answer: Increasing temperature increases the speed and kinetic energy of the particles, so they collide more frequently, and a greater proportion of these collisions have energy greater than or equal to the activation energy, so more collisions are successful.
Q3Explain why increasing the surface area of a solid reactant increases the rate of a reaction.Show answer
Answer: A larger surface area exposes more particles of the solid to the other reactant at any one time, so collisions between reacting particles happen more frequently.
Q4Explain how a catalyst increases the rate of a 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 are successful; the catalyst itself is not chemically changed by the reaction, so it can be used again.
Q5A tangent drawn to a volume-time graph at 15 seconds passes through the points (5 s, 6 cm3) and (25 s, 66 cm3). Calculate the rate of reaction at 15 seconds.Show answer
Answer: 3 cm3/s ((66 - 6) / (25 - 5) = 60 / 20 = 3).
Q6In the disappearing cross required practical, state the dependent variable that is measured.Show answer
Answer: The time taken for the cross to no longer be visible through the mixture.
Q7In the gas-volume required practical measuring how concentration affects rate, state one variable that should be controlled.Show answer
Answer: Any one of: the mass or surface area of the solid reactant (for example marble chips), the volume of acid used, or the temperature.
Q8Explain why using a light sensor and data logger is more reliable than timing by eye in the disappearing cross practical.Show answer
Answer: It removes the subjectivity of a person judging the exact moment the cross disappears, giving more precise, objective and repeatable readings of the end point.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
In the disappearing cross required practical, a student repeats the experiment at five different temperatures, keeping the volume and concentration of the sodium thiosulfate and the hydrochloric acid the same each time. State the independent variable, the dependent variable, and one variable that must be controlled in this investigation.
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Hydrogen peroxide decomposes into water and oxygen, catalysed by manganese dioxide: 2H2O2 -> 2H2O + O2. A graph of the volume of oxygen produced against time is drawn for this reaction. A tangent to the curve at 5 seconds passes through the points (0 s, 4 cm3) and (10 s, 44 cm3). A second tangent, at 40 seconds, passes through the points (30 s, 78 cm3) and (50 s, 82 cm3). (a) Calculate the rate of reaction at 5 seconds. (b) Calculate the rate of reaction at 40 seconds. (c) Explain why the rate of reaction is slower at 40 seconds than at 5 seconds.
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Explain, using collision theory, how each of the following changes increases the rate of a chemical reaction: (a) increasing the temperature of the reaction mixture, (b) increasing the concentration of a reactant in solution, (c) increasing the surface area of a solid reactant by using smaller pieces, (d) adding a suitable catalyst.
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Free printable worksheet
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This topic is chapter 16 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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