Rate of reaction graphs

A rate of reaction graph plots the amount of product made (or reactant used up) against time. The steeper the line, the faster the reaction. A typical curve is steepest at the start, gets less steep as the reactants are used up, and goes flat when the reaction stops. Comparing curves shows how temperature, concentration, surface area and catalysts change the rate.

What a rate of reaction graph shows

AQA's GCSE specification asks you to draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time. Time always goes along the bottom. Up the side goes whatever you measured.

• The gradient (steepness) is the rate. A steep section means a lot of product is made each second.
• It is steepest at the start, when there are the most reactant particles to collide.
• It curves over as reactants are used up and collisions become less frequent.
• A flat line means the reaction has stopped: one reactant, the limiting reactant, has run out.
• The height of the flat part shows the total amount of product, which depends only on how much limiting reactant there was.

Shapes of rate graphs

Whichever way the line goes, the rule is the same: steep means fast, flat means stopped.

Comparing rate of reaction graphs

The figure shows three runs of a reaction that gives off a gas. Curve A is the original.

• B is faster. It is steeper at the start and levels off sooner, but at the same height as A, because the amount of limiting reactant is the same. This is what you get from a higher temperature, a higher concentration of an excess reactant, a larger surface area, or a catalyst.
• C makes half as much gas. It levels off at 24 cm³ instead of 48 cm³ because it had half as much limiting reactant. With less reactant it also starts a little less steeply.

Collision theory explains the first four: they make successful collisions more frequent, either by bringing particles together more often or, for temperature and catalysts, by making more collisions reach the activation energy. The reaction rate simulator lets you change each one and watch.

Reading values from a rate graph

Mean rate between two times

On curve A, the volume is 27.1 cm³ at 10 s and 38.9 cm³ at 20 s. The mean rate between those times is (38.9 − 27.1) ÷ (20 − 10) = 1.18 cm³/s. Over the first 10 s it is 27.1 ÷ 10 = 2.71 cm³/s, faster, as the steeper start shows.

Rate at one moment

For the rate at a particular time, draw a tangent to the curve at that time and find its gradient. AQA asks all students to draw tangents and use their slope as a measure of rate; calculating the gradient is Higher tier. The rate of reaction formula guide works through a tangent calculation step by step.

Time for the reaction to finish

Read where the curve first becomes flat. Curve B is almost flat by 30 s, while A is still rising slowly at 45 s.

Try one: answer a question about a rate graph, then press Mark to check your answer in the Reaction Kinetics course.

How to sketch a new curve

1. Start at the same point as the original, usually the origin.
2. If the reaction is faster, make the new curve steeper at the start, so it sits above the original.
3. Decide the final height: the same as the original if the limiting reactant is unchanged, lower or higher if it has changed.
4. Level it off sooner if it is faster, later if it is slower.
5. Label it clearly so the examiner can tell which curve is which.

Common mistakes

• Raising the final level for a catalyst or higher temperature. They make the reaction faster, not make more product.
• Drawing straight lines. Rate graphs are curves; only the start is roughly straight.
• Reading the rate from the height. The rate is the gradient, not how high the line is.
• Ending the faster curve higher. With the same limiting reactant, both curves level off at the same height.
• Forgetting units. Rate from a gas volume graph is in cm³/s; from a mass graph, g/s.

Reading rate graphs is part of the Reaction Kinetics course. For practice in a short timed paper, try today's GCSE Chemistry Higher or GCSE Combined Science Higher paper.

Read next

Chemistry · Rates and equilibriumRate of reaction formula: mean rate, units and tangentsThe rate of reaction formula: mean rate from reactant used or product formed, units g/s, cm³/s and mol/s, rate from a tangent, the practical and examples.