Measuring rates of reaction

In a nutshell

The rate of reaction shows the speed of reactions and how they change over time. There are several different ways in which the rate of reaction can be followed.

What does reaction rate show?

The reaction rate shows the change in the amount of reactants or products per unit time, typically seconds.

How to follow the rate of reaction

There are lots of different ways a reaction can be followed, however they may not be applicable for every reaction. If no gas is produced, the gas volume method will not produce a reaction rate.

Continuous monitoring is used to monitor the rate of reaction. Continuous monitoring means measurements are taken over the duration of the reaction. 

Gas volume

If a gas is given off, it can be collected in a gas syringe and the amount of gas present at regular time intervals is recorded. This works for a reaction where a gas is produced.

To calculate the amount of gas present in the gas syringe, use the ideal gas equation to calculate the number of moles.

Example

Reacting an acid with a metal carbonate will lead to the production of carbon dioxide. The volume of the gas produced can be measured and plotted over time. 

​$$2HCl\ +\ MgCO_3\ \rightarrow\ MgCl_2\ +\ H_2O\ +\ CO_2$$​​

Change in mass

If a gas is given off in a reaction, it will cause for the mass of the reactants to decrease. The mass can be recorded at regular intervals. The moles of gas can be calculated for carbon dioxide, which enables you to work out the remaining moles of your reactant(s). 

Colour change

The progress of a reaction can lead to a colour change. A machine called a colorimeter can measure the absorbance of a solution. The greater the absorption, the greater the concentration of the solution. The absorbance can be plotted over time and the rate can be calculated by comparing to a calibration curve.

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Change in pH

A reaction may produce/use up an acid/base, so the pH can be monitored at regular time intervals and therefore the concentration of $$H^+$$ or $$OH^-$$ ions.​

Titration

Small samples of a reaction can be taken at regular time intervals and titrate them using a standard solution. The rate can be calculated from measuring the change in concentration of the products over time.

Electrical conductivity

If the number of ions in solution changes over time, the conductivity of the solution will be affected. Conductivity can be measured over time.

Calculating reaction rate

By taking regular readings during the course of a reaction, the rate of reaction can be calculated.

By plotting the time on the x-axis and and the concentration of the reactant on the y-axis, the gradient of the graph can be used to calculate the rate of reaction. If the graph is linear you can calculate the rate of reaction at any point. If the graph is a curve, a tangent will have to be drawn and then the gradient of the tangent can be calculated.

To calculate the gradient the below equation is used:

​$$gradient\ =\ \frac{change\ in \ y}{change\ in \ x}$$​​

Colorimeters

Colorimeters measure the absorbance of a particular wavelength of light by a solution. Normally, the wavelength of light used would be absorbed by either the products but not the reactants, or vice versa.

To convert the absorbance to a concentration, a calibration curve is used. Calibration curves measure the absorbance of standard solutions. A graph is then plotted with the absorbance values on the y-axis and the concentration of solution on x-axis. The concentration of the solution can then be obtained from the absorbance.