Rate is how quickly a reaction gets through its reactants
Definition
Rate of reaction
How quickly a reactant is used up or a product is formed.
- A fast reaction uses up its reactants and forms its products in a short time.
- Measuring a rate means following one quantity as the reaction proceeds.
- Either a reactant being used up or a product being formed can be followed.
- The mean rate over a period is found from the change and the time: mean rate=change in the quantitytime taken\text{mean rate} = \frac{\text{change in the quantity}}{\text{time taken}}mean rate=time takenchange in the quantity
- The unit follows the quantity measured, such as cm3/s\text{cm}^3/\text{s}cm3/s for a gas.
Key Idea
Any quantity that changes measurably as the reaction runs can be used to follow its rate.
Choosing what to measure
- A reaction that gives off a gas can be followed by the volume of gas collected.
- The same reaction can be followed by the loss in mass as the gas escapes.
- A reaction that forms a cloudy product can be followed by how long it takes to hide a mark.
- A reaction that changes colour can be followed by how quickly the colour appears or fades.
- The choice depends on what the reaction visibly or measurably changes.
Example
- Marble chips and acid: collect the carbon dioxide in a gas syringe.
- The same reaction on a balance: record the falling mass as the gas leaves.
- Thiosulfate and acid: time how long a cross takes to disappear.
How the measurements are taken
- A gas syringe gives the volume directly, provided the apparatus is gas-tight.
- Collecting gas over water works too, though a soluble gas dissolves and is lost.
- A balance records mass loss, which suits a gas too dense or too soluble to collect.
- Readings are taken at regular intervals and timed from the moment of mixing.
- Repeating the run and taking a mean reduces the effect of a single poor reading.
Practical
- Marble chips and acid: measure a fixed volume of dilute hydrochloric acid into a flask, add a fixed mass of chips, fit the bung at once and record the gas volume at set intervals.
- Changing one thing: use different acid concentrations, or the same mass of differently sized chips, keeping everything else the same.
- Thiosulfate and acid: mix fixed volumes over a paper cross and time how long the cross takes to disappear, repeating at several temperatures.
- Relative rate: 1/time1/\text{time}1/time is used as a measure of rate, because the same endpoint means the same amount of sulfur has formed.
- Control the rest: volumes, concentrations, temperature and the observer are kept the same, or the comparison means nothing.
Reading a graph of the reaction
- Plotting the quantity against time gives a curve that rises and then flattens.
- The curve is steepest at the start, when the reactants are at their most concentrated.
- It grows less steep as the reactants are used up and the reaction slows.
- It becomes flat when the reaction has finished and nothing more is changing.
- The gradient at any point gives the rate at that moment.
Common Mistake
- A flat line means the reaction has stopped, not that it is running at a steady rate.
- The height the curve levels off at is the total amount formed, which is a different question from the rate.
Comparing two curves on the same axes
- The curve that is steeper at the start belongs to the faster reaction.
- The curve that flattens sooner belongs to the reaction that finished first.
- Two reactions can end at the same height while one gets there much faster.
- The same final height means the same amount of limiting reactant was used in both.
- A curve ending lower means less product formed, which is a difference in amount rather than in rate.
Self review
- Give two quantities that can be measured to follow a reaction.
- Why does a balance suit a reaction that gives off carbon dioxide?
- Why is a rate curve steepest at the beginning?
- What does it mean when a rate curve becomes flat?
- Two curves level off at the same height but one rises more steeply. What does that tell you?
