CalculateQuestion 1 · Recall
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By the end you can describe how a reaction's rate can be followed experimentally, and calculate a rate from tabulated data or from the gradient of a concentration–time curve.
To measure a rate you need to measure a quantity that changes as the reaction proceeds, at known times.
| What you measure | Typical method | Good for |
|---|---|---|
| Volume of gas | Gas syringe, or inverted burette over water | Reactions producing a gas, such as metal + acid |
| Loss in mass | Balance | Reactions giving off a gas, when the gas can escape |
| Colour intensity | Colorimeter (absorbance), or timing until a cross disappears | Reactions with a coloured reactant or product |
| Amount of acid or alkali | Titrate samples taken at intervals (quench each sample first) | Slow reactions in solution |
| Electrical conductivity | Conductivity meter | Reactions where the number of ions changes |
Whichever method you choose, keep the temperature constant unless temperature is the variable you are testing. Changing temperature changes the rate for a different reason.
The mean rate over an interval is the change in concentration divided by the time taken:
That is an average. It hides the fact that reactions usually start fast and slow down as reactants are used up.
The rate at a particular instant is the gradient of the concentration–time curve at that point. To find it by hand, draw a tangent to the curve and calculate the gradient of the tangent.
The initial rate is the rate at : the gradient of the tangent drawn at the start of the curve. Initial rate is popular in experiments because the concentrations of the reactants are known exactly at that moment.
Worked example: Mean rate from concentration data
The concentration of a reactant falls from at to at . Calculate the mean rate of the reaction over this interval.
Worked example: Rate from a tangent
On the concentration–time curve for a reactant, the tangent drawn at passes through the points and , with concentration in and time in seconds. Calculate the initial rate.
The mean rate over the whole reaction is the same as the initial rate.
Reactions slow down as reactants are used up, so the mean rate is smaller than the initial rate. Only the initial rate is measured at a known, unchanging set of concentrations.
A steeper curve means more product is formed.
The gradient gives the speed. How much product you get depends on the amounts of reactants, not on how steep the curve is.
The gradient of a concentration–time graph is negative, so the rate is negative.
The negative gradient shows the reactant's concentration is decreasing. Rate is reported as a positive value; use the magnitude.
You can measure the rate of a gas-producing reaction in an open flask by mass loss, whatever the gas.
Mass loss works only if the gas escapes. If the gas is very soluble, or the apparatus traps it, the balance will not show the loss. Choose a method that matches the reaction.