Rate of Reaction

• Is the change in concentration of a reactant or product per unit time.
• Rate= change in concentration/time
• Units: moldm^-3s^-1
• Rate is fastest at the start of a reaction since each reactant has its greatest concentration.
• Rate slows down as the reaction proceeds since the concentration of the reactant decreases.
• Rate becomes zero when the reaction stops, i.e. when one of the reactants has been used up.

Measuring Rate of Reaction

• Change in gas volume: in a reaction in which gas is formed, the volume of gas can be recorded using a gas syringe at various times.

Mg (s) + 2HCl (aq) ➔  MgCl₂ (aq) + H₂

• Change in gas pressure: reactions between gases involve a change in the number of moles of gas. The change in pressure (at constant volume) at various times can be followed using a manometer.

PCl₅ (g) ➔ PCl₃ (g) + Cl₂ (g)

• Change in Colour: the intensity of the colour of the iodine can be monitored over time by using a colorimeter and hence its change in concentration can be measured.

CH₃COCH₃ (aq) + I₂ ➔ CH₃COCH₂I (aq) + HI (aq)

• Sampling: Samples of the reaction mixture are removed at various times. The reaction in each sample taken is slowed down significantly (quenched) by diluting in ice-cold water. Each sample is titrated against standard alkali and the concentration of ethanoic acid is calculated.

CH₃CO₂C₂H₅  (aq) + H₂0 (l) ➔ CH₃CO₂H (aq) + C₂H₅OH (aq)

Calculating initial rates of reaction

Is done by measuring the concentration of a reactant over a period of time. The results obtained are plotted to give a graph. To find the initial rate, it is necessary to find the initial slope (gradient) of the line. The graph will always be a straight line to begin with.

Rate= change in concentration/time

1. Draw a line from any convenient point on the straight line (from P to M)
2. Calculate vertical distance between N and M.
3. N-M/P-M = rate of reaction

• To find out the relationship between the initial rate and the initial concentrations of the reactants, a series of the experiments in which only the concentration of one reactant is changed at a time must be performed. The results are then compared.
• If the initial rate of reaction is independent of the initial concentration, then no matter what change in concentration the rate of reaction will remain constant and will not be affected.
• If the initial rate of reaction is directly proportional to the initial concentration then when concentration is doubled, rate of reaction will also be doubled.

Do you remember the content of AS/A-level Chemistry regarding collision theory? The effect of the factors below are explained by collision theory! 👩‍🏫

Factors Affecting reaction rates

• Temperature of a reaction
• Concentration of a solution (pressure of a gas) 
• Surface area of a solid
• Catalyst
• Light (in some reactions, i.e. photosynthesis) 

Reference:

https://getrevising.co.uk/resources/wjec_as_chemistry_kinetics

This is the end of topic!

CLICK HERE TO LEARN MORE ABOUT OUR A-LEVEL CHEMISTRY COURSES

Drafted by Cherry (Chemistry)