7.1 - Rates of reaction
7.1.1: Define the term rate of reaction and describe the measurement of reaction rates. Rate of reaction can be defined as the decrease in the concentration of reactants per unit time or the increase in the concentration of product per unit time.
The nature of a chemical reaction
A chemical reaction is a process in which a new substance or substances is/are formed
During the course of a chemical reaction the starting materials chemically change to become the products.
This process is not instantaneous, the new substances created are formed at a specific rate.
How can the speed of a chemical reaction be measured?
In the same way that a cars speed can be measured by the cars progress with time so the rate of a chemical reaction can be followed by describing the appearance of product (or disappearance of reactant) with time.
To be able to describe the rate we must first define what we mean by the term 'amount' of a substance. The quantity of matter can be measured by mass, volume, moles or some factor involving these quantities.
As many of the chemical reactions studied involve solutions, it is convenient to talk about the concentration of the solution in moles of solute per volume (usually litres) of solvent. We can then study how the concentrations of the products (or the reactants) vary with time.
This allows us to define the rate of a reaction as:
The rate of change of concentration of a reactant or product = change in concentration / time
change in concentration
If the concentration of reactants changes from 2.0 moles per litre to 1.6 moles per litre in 20 seconds
then the change in concentration = 2.0 - 1.6 = 0.4
the time taken = 20 seconds
therefore the rate of reaction = 0.4 /20 = 0.02 moles per litre per second
Experimental measurement techniques
If we are to measure the rate of the reaction then we must be able to measure something that tells us the concentration of either the reactants or products.
THE EASIEST REACTIONS TO STUDY ARE ONES IN WHICH GAS IS EVOLVED
Gas evolution means that two things can be measured
- The decrease in mass of the reaction mixture
- The volume of gas produced
Measuring the mass loss with time
This simply involves carrying out the reaction on a top pan balance and taking readings every predetermined period of time.
Measuring the volume of gas produced with time
Although both of these methods are basically straightforward a slight complication arises at the time of mixing the reactants. A divided flask (test-tube) must be used to ensure that the reaction does not start until all the apparatus is assembled and gas tight.
Why do we study the rate of chemical reactions?
By studying how fast processes occur when the reaction conditions are varied we can obtain information about the very nature of chemical reactions, about HOW they occur.
NOTE: The rate of chemical reactions CAN ONLY be found by experimentation. There are no theoretical or mathematical ways to calculate the rate of a reaction. IT IS A PURELY EXPERIMENTAL SCIENCE
7.1.2: Analyse data from rate experiments. Graphs of changes in concentration, volume or mass against time should be interpreted qualitatively.