Need for Speed

Four factors, two levers

Four things change how fast a reaction goes: concentration, or pressure for gases; the surface area of a solid; temperature; and a catalyst. Most students can list them, and listing them is worth almost nothing. Every question on this topic wants the mechanism, and there is a reason that is learnable rather than something to memorise four times over. All four work through the same two levers. Particles have to collide for a reaction to happen, and the collision has to carry enough energy, called the activation energy. So a factor speeds a reaction up by making collisions more frequent, or by making more of them energetic enough, or both. Concentration, pressure and surface area pull the frequency lever. Temperature pulls both, which is why it has such a large effect and why it is the one people half-explain. A catalyst works on the energy threshold, and that is the next module on this topic. Four factors, two levers, and one sentence pattern that answers any of them.

Words for rate

Five terms, and the last two are the ones the mark scheme is looking for.

Work out the mean rate

A student measures the gas given off by a reaction. In total, 48 cubic centimetres of gas are collected over 60 seconds. Work out the mean rate of reaction, in cubic centimetres per second.

Complete the gas-volume curve

This graph shows the total volume of gas collected against time for the same reaction, in cubic centimetres against seconds. Six points are already plotted. Plot the missing point at 20 seconds, where the total volume collected is 32 cubic centimetres.

Why the curve flattens

The curve you have just completed is steep at the start and almost flat by the end. What is happening?

• The reactants are being used up, so collisions become less frequent and the rate falls
• The mixture is cooling down, so the particles have less energy
• The reaction stops partway through and then restarts more slowly
• The gas syringe is full, so no more gas can be collected

Frequency or energy

Every factor pulls one of these two levers, or both. Knowing which one a factor pulls is what turns naming into explaining.

Match the factor to its lever

Which explanation earns the mark

A question asks why increasing the concentration of an acid speeds up its reaction with a metal. Select the TWO answers that would gain credit.

• There are more acid particles in the same volume, so collisions with the metal happen more frequently
• More frequent collisions means more successful collisions each second, so the rate increases
• The acid is stronger, so the reaction is faster
• The particles have more energy, so more collisions exceed the activation energy

Naming it is not explaining it

This is the single most reliable way to lose marks on this topic, and it is worth more than any extra content. Questions here almost always say explain, and an explanation has to reach the collisions. Writing that a reaction is faster because the acid is more concentrated repeats the question back. Writing that there are more acid particles in the same volume, so collisions are more frequent, so there are more successful collisions each second, answers it. Keep a sentence pattern ready and fill it in: what changed, what that does to the collisions, and what that does to the rate. For temperature you need both halves, because raising the temperature makes particles move faster so they collide more often AND makes more collisions exceed the activation energy. Leaving out the second half of the temperature answer is the commonest half-mark in the topic.

Spell out the temperature answer

When the temperature is raised, the particles gain _____ energy and move faster. That means they collide more _____. It also means a greater proportion of collisions have energy above the _____ energy. Both effects increase the number of _____ collisions each second, so the rate _____. Temperature is the only factor here that moves both levers at once.

The method in order for RP5

The required practical investigates how one variable changes the rate. Put the stages of a run in order. This version measures the gas given off; the other permitted method times how long a marked cross takes to disappear.

• Decide which single variable you are changing, and what you will measure to show the rate
• Measure out the reactants, keeping everything except the chosen variable the same
• Add the reactants together and start timing at the same moment
• Record the quantity collected at regular time intervals
• Repeat the run and compare, then change only the chosen variable and start again

A rate answer, annotated

A question gives the data you have been working with and asks two things: calculate the mean rate, and explain why the rate falls as the reaction proceeds. Here is an answer that would score full marks. The mean rate is the total volume divided by the total time, so 48 divided by 60, which is 0.8 cubic centimetres per second. The rate falls because the reactants are being used up, so there are fewer reactant particles in the same volume. Fewer particles means collisions happen less frequently, so there are fewer successful collisions each second and the rate decreases. Notice three things. The calculation shows the working and carries the unit. The explanation names what changed, what it did to the collisions, and what that did to the rate, in that order. And nothing is said about temperature, because nothing in the question changed it. One more point about the practical itself: the two permitted methods measure different things. Collecting gas gives you a quantity building up over time, which is what this curve shows. The disappearing-cross method gives you a single time to a fixed endpoint, so a shorter time means a faster reaction. Do not mix the two up in an answer.

The answer that stops too soon

Four student answers to explain questions. Select the ONE that names a factor without ever reaching the collisions.

• The powder has a larger surface area, so more particles are exposed and collisions are more frequent.
• At a higher pressure the gas particles are closer together, so they collide more often.
• The reaction is faster because the acid used was more concentrated than before.
• At a higher temperature more collisions have energy above the activation energy, so more are successful.

Rate over the first ten seconds

Using the same graph, 20 cubic centimetres of gas had been collected after 10 seconds. Work out the rate over that first 10 seconds, in cubic centimetres per second, and compare it with the mean rate of 0.8 you found earlier. At Higher tier the same idea is taken further by drawing a tangent to find the rate at a single instant.

Running the rate investigation

You are carrying out the required practical on rate. Take the decisions in order.

• You are investigating how concentration affects rate. What must stay the same between runs?
• You are using the disappearing-cross method. What does a shorter time mean?
• One run gives a result far away from the other two at the same concentration. What do you do?
• You are asked to explain your results. What must the explanation reach?

Design and explain

A student wants to find out how the surface area of a solid affects the rate of its reaction with an acid. Describe how they should do it and explain what they would find.

• State the variable being changed and the variable being measured
• Name two things that must be kept the same, and say why each matters
• Say how the rate would be worked out from the measurements taken
• Predict what happens to the rate as the surface area increases
• Explain that prediction in terms of collisions, reaching the frequency of successful collisions
• Explain why the rate would fall as the reaction goes on, even with nothing changed