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
- Draw a line from any convenient point on the straight line (from P to M)
- Calculate vertical distance between N and M.
- 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.
Factors Affecting reaction rates
- Temperature of a reaction
- Concentration of a solution (pressure of a gas)
- Surface area of a solid
- Light (in some reactions, i.e. photosynthesis)
This is the end of topic!
Drafted by Cherry (Chemistry)