O Level / IGCSE Chemistry · Chemical reactions. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
The rate of reaction is how fast reactants are used up or products are formed. It can be followed by measuring the volume of gas collected, or the loss in mass as a gas escapes, at regular times.
Collision theory explains rate. Particles react only when they collide with at least the activation energy. Anything that makes these successful collisions happen more often makes the reaction faster.
On a graph of volume or mass against time, a steeper line means a faster reaction. The line is steepest at the start, when the reactants are most concentrated, and becomes flat when one reactant is used up.
Students say a faster reaction makes more product. If the amounts of reactants are the same, the graph is steeper but levels off at the same height.
Answer: 2.4 cm3/s at first, then 0.6 cm3/s. The reaction slows down because the acid becomes less concentrated, so collisions are less frequent.
Try each one, then open the answer.
Answer: B. A 20 °C rise is two steps of 10 °C, so the rate doubles twice and becomes four times faster. The time is therefore 80 ÷ 4 = 20 s.
Answer: C. A catalyst speeds up a reaction by providing a route with a lower activation energy and is chemically unchanged at the end.
Answer: D. A larger surface area exposes more particles to the acid, increasing the frequency of successful collisions.
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