What you'll learn
Measuring the rate of a reaction is the experimental half of AQA GCSE Chemistry's rates unit, and it is assessed as much through graphs and calculations as through recall. The question this topic answers is a practical one: given a reaction happening in front of you, how do you put a number on how fast it is going? Everything follows from one principle — you must measure something that changes measurably as the reaction proceeds, and you must measure it against time. By the end of this guide you should be able to select an appropriate method for following a given reaction, describe the two required practicals in detail, calculate a mean rate of reaction with correct units, find the rate at a particular moment by drawing a tangent and calculating its gradient, interpret and compare rate graphs including what their shape and final height tell you, and evaluate the accuracy of a method.
Key terms and definitions
Rate of reaction — how quickly a reactant is used up or a product is formed, measured per unit time
Mean rate — the total change in quantity divided by the total time taken
Instantaneous rate — the rate at one particular moment, found from the gradient of a tangent
Tangent — a straight line touching a curve at exactly one point, used to find the gradient at that point
Gradient — the steepness of a line, calculated as the change in the vertical value divided by the change in the horizontal value
Gas syringe — apparatus for collecting and measuring the volume of gas produced
Turbidity — cloudiness of a solution, used to follow reactions that form a precipitate
Independent variable — the factor deliberately changed in an investigation
Dependent variable — the factor measured as an outcome
Control variable — a factor kept constant so that the result is valid
Anomalous result — a measurement that does not fit the pattern of the others
Core concepts
What can be measured
A reaction can be followed by measuring any property that changes as it proceeds. Three approaches cover everything at GCSE.
Measure the volume of gas produced, where a gas is a product.
Measure the loss in mass, where a gas escapes from an open container.
Measure the formation of a precipitate, where the mixture turns cloudy.
In each case the quantity is recorded at regular time intervals so that a graph can be plotted.
Method 1: collecting gas
The gas produced is collected in a gas syringe, and the volume is read at fixed intervals — every ten or fifteen seconds is typical. Alternatively the gas can be collected in an inverted measuring cylinder full of water, though this is less suitable for gases that dissolve appreciably, such as carbon dioxide.
The advantage of a gas syringe is that it gives a direct, reasonably precise volume reading. The disadvantages are that the syringe may stick, and that some gas escapes in the moment between adding the reactants and inserting the bung — which is why results at the very start are often unreliable.
Rate measured this way has units of cubic centimetres per second.
Method 2: loss of mass
The reaction vessel stands on a balance and the mass is recorded at fixed intervals. As a gas escapes into the air, the total mass falls, and the decrease equals the mass of gas released.
A loose plug of cotton wool is placed in the neck of the flask. It allows the gas to escape while preventing the spray of liquid that effervescence produces from leaving the flask, which would otherwise cause an additional loss of mass and invalidate the results. This detail is examined frequently.
The advantage is that the method is simple and the balance is accurate. The disadvantage is that it is unsuitable for hydrogen, because hydrogen is so light that the mass change is too small to measure reliably.
Rate measured this way has units of grams per second.
Method 3: the disappearing cross
Where a reaction produces a precipitate, the mixture becomes progressively cloudier. A cross is marked on a piece of paper and the flask is stood on top of it. The time taken for the cross to become invisible when viewed from above is recorded.
The classic example is sodium thiosulfate reacting with hydrochloric acid, which produces a fine precipitate of sulfur.
The advantage is that the method requires no specialised apparatus. The disadvantages are significant and often examined: the judgement of when the cross disappears is subjective and varies between observers, and the method gives no continuous data, only a single time for each run. It therefore cannot produce a rate graph in the way the other two methods can.
To improve reliability, the same person should judge every run, viewed from the same distance and in the same lighting, and each concentration should be repeated.
Because a shorter time means a faster reaction, rate here is usually expressed as one divided by the time, in units of one per second.
The required practicals
The first required practical investigates how the concentration of a solution affects the rate of reaction, using the disappearing cross method with sodium thiosulfate and hydrochloric acid. The independent variable is the concentration of sodium thiosulfate, varied by diluting the stock solution with measured volumes of water. The dependent variable is the time for the cross to disappear. The control variables are the total volume of solution, the volume and concentration of the acid, the temperature, and the depth of liquid in the flask.
The second approach investigates the same question by measuring the volume of gas produced, typically from magnesium and hydrochloric acid or from a carbonate and acid. The independent variable is the concentration of the acid; the dependent variable is the volume of gas at fixed time intervals; the control variables are the mass and surface area of the solid, the volume of acid and the temperature.
In both, the total volume must be kept constant when diluting, which is why water is added rather than simply using less solution. This is the control variable candidates most often omit.
Calculating mean rate
The mean rate over a period is the change in quantity divided by the time taken.
If 60 cubic centimetres of gas are produced in 40 seconds, the mean rate is 60 divided by 40, which is 1.5 cubic centimetres per second.
The unit comes directly from the two quantities used, and it must be stated. Rate without units is an incomplete answer.
Mean rate can be calculated over the whole reaction or over any stated interval, and questions often ask for the rate during a specific portion, which requires reading two values off the graph and subtracting.
Reading a rate graph
Plotting quantity of product against time produces a characteristic curve: steep at the start, gradually flattening, then horizontal.
The curve is steepest at the beginning because the concentration of reactants is at its highest, so successful collisions are most frequent.
It flattens progressively because the reactants are being used up, so collisions become less frequent and the rate falls.
It becomes horizontal when the reaction has stopped, which happens when at least one reactant has been completely used up. The height at which the line levels off shows the total amount of product formed, and therefore how much reactant was present.
This gives a powerful method for comparing two runs. A steeper curve means a faster reaction. If two curves level off at the same height, the same total quantity of product was made, so the same amount of limiting reactant was used and only the speed differed. If one levels off higher, more reactant was present.
Finding the rate at a moment: tangents
The mean rate over a period hides the fact that the rate is changing constantly. To find the rate at a particular time, draw a tangent to the curve at that point and calculate its gradient.
The method is to place a ruler so that it touches the curve at the chosen time without crossing it, draw a long straight line, select two points far apart on that line, read their coordinates, and divide the change in the vertical value by the change in the horizontal value.
Two practical points determine accuracy. The tangent must be long, because reading a gradient from a short line magnifies any error. And the two points chosen for the calculation should be as far apart as possible on the tangent, for the same reason.
The gradient carries the same units as the mean rate.
Evaluating a method
Questions frequently ask which method is more suitable for a given reaction, or how a method could be improved.
Choose the gas collection method when a gas is produced and continuous data are needed. Choose the mass loss method when the gas is heavy enough for the change to register, and never for hydrogen. Choose the disappearing cross only when a precipitate forms and only where a single time measurement suffices.
Improvements that gain marks include repeating each run and calculating a mean, identifying and excluding anomalous results before averaging, using a water bath to control temperature precisely, using a data logger to remove human reaction-time error from the timing, and using a measuring instrument of higher resolution.
Worked examples
Example 1: Calculating mean rate with units (3 marks)
A reaction produces 84 cubic centimetres of gas in 70 seconds. Calculate the mean rate of reaction.
The mean rate is the quantity of product formed divided by the time taken, which is 84 divided by 70. This gives 1.2. The quantity was measured in cubic centimetres and the time in seconds, so the units are cubic centimetres per second, and the mean rate is 1.2 cubic centimetres per second.
Example 2: Finding a rate from a tangent (4 marks)
On a graph of gas volume against time, a tangent drawn at 30 seconds passes through the points (10 seconds, 12 cubic centimetres) and (50 seconds, 60 cubic centimetres). Calculate the rate of reaction at 30 seconds.
The gradient of the tangent gives the rate at that moment. The change in volume is 60 minus 12, which is 48 cubic centimetres. The change in time is 50 minus 10, which is 40 seconds. The gradient is 48 divided by 40, which is 1.2 cubic centimetres per second. Note that the two points used are far apart on the tangent, which is what makes the reading reliable.
Example 3: Comparing two curves (4 marks)
Two experiments using the same mass of magnesium ribbon are carried out with acids of different concentration. Curve A rises steeply and levels off at 48 cubic centimetres after 60 seconds. Curve B rises less steeply and levels off at 48 cubic centimetres after 140 seconds. Explain what this shows.
Curve A is steeper throughout, which shows that the reaction using the more concentrated acid proceeded at a faster rate. In the more concentrated solution there are more acid particles in the same volume, so collisions with the magnesium are more frequent and more successful collisions occur each second.
Both curves level off at the same volume of 48 cubic centimetres, which shows that the same total quantity of hydrogen was produced in each experiment. This is expected because the same mass of magnesium was used in both, and the magnesium was the limiting reactant. Concentration therefore changed the rate of the reaction but not the total amount of product formed.
Common mistakes and how to avoid them
The most frequent error is omitting the units from a rate answer. The units follow directly from the quantities used and are worth a mark in most calculations.
Many candidates draw a very short tangent, which makes the gradient reading inaccurate. Draw the line right across the graph area.
Students often confuse the gradient of a tangent with the gradient of the chord between two points on the curve. The tangent touches at one point and gives the instantaneous rate; the chord gives a mean rate over an interval.
In the disappearing cross practical, answers frequently omit that the total volume must be kept constant when diluting. Adding water to compensate is the point of the method.
Another common slip is forgetting the cotton wool in the mass loss method, or misstating its purpose. It allows gas out while preventing liquid spray from escaping.
Finally, candidates sometimes state that a reaction stops because it has run out of time. It stops because at least one reactant has been completely used up.
Exam technique for "Rate of reaction: measuring and factors affecting rate"
Identify what is being produced before choosing a method. A gas means syringe or mass loss; a precipitate means the disappearing cross.
For any calculation, write the relationship, substitute, then give the answer with units. Method marks are available even when the arithmetic slips.
When drawing a tangent under examination conditions, take the time to make it long and to mark clearly the two points you used. Examiners award marks for the construction as well as the value.
When comparing curves, comment on both the steepness and the final height, and say what each tells you. Steepness is about rate; final height is about quantity of reactant.
For evaluation questions, name a specific improvement and say what error it reduces, rather than writing generally about being more careful.
Quick revision summary
Rate is followed by measuring gas volume with a syringe, mass loss on a balance, or the time for a cross to disappear behind a precipitate. Gas collection gives continuous data but loses some gas at the start; mass loss is simple but unusable for hydrogen and needs a cotton wool plug to stop liquid escaping; the disappearing cross is subjective and gives only one time per run. Mean rate is the change in quantity divided by time, with units taken from the quantities used. A rate graph is steepest at the start because reactant concentration is highest, flattens as reactants are used up, and becomes horizontal when a reactant is exhausted; a steeper curve means a faster reaction, and the height at which it levels shows the total product formed. The rate at a moment is the gradient of a tangent, which must be drawn long with widely spaced points for accuracy. In the required practicals the total volume must be kept constant when diluting, and reliability is improved by repeating runs, excluding anomalies, controlling temperature and using data loggers.