The rate of a chemical reaction can be defined as the time taken for a known amount of reactants to form known amount of products.
Some reactions are too slow to be determined e.g weathering others are instantaneous
The SI unit of time is seconds. Minutes and hours are also common .
Time is determined using a stop watch/clock
Candidates using stop watch/clock should learn to:
- Press start button concurrently with starting off determination of a reaction using one hand each.
- Press stop button when the reaction is over.
- Record all times in seconds unless specified.
- Press reset button to begin another timing
- Ignore time beyond seconds for stop clock/watch beyond this accuracy
- Avoid accidental pressing of any button before recording
It can be very frustrating repeating a whole procedure.
The following factors theoretically and practically alter/influence/affect/determine the rate of a chemical reaction:
An increase in concentration increases the rate the rate of reaction by reducing the time taken to completion.
Theoretically, increase in concentration is a decrease in distance between reacting particles which increases their collision frequency.
Practically decreasing concentration is diluting/adding water.
To demonstrate the effect of concentration on reaction rate
You are provided with
- sodium thiosulphate containing 40gdm -3 solution labeled A
- 2M hydrochloric acid labeled solution B
You are required to determine the rate of reaction between solution A and B
- Measure 40cm3 of solution A into 100 cm3 glass beaker.
- Place it on top of a pen-mark “X”.
- Measure another 40cm3 of solution B.
- Simultaneously put solution B into solution A and start off a stop watch/clock.
- Determine the time taken for the pen-mark “X” to be invisible/obscurred from above.
- Repeat the procedure by measuring 35cm3 of solution B and adding 5cm3 of water.
- Complete the table 1 below by using other values os solution B and water.
Volume of solution A (cm3)
Volume of solution B (cm3)
Volume of water (cm3)
Time taken for x to be invisible(seconds)
Calculate 1/t in each case
Plot a graph of 1/t (y-axis) against volume of solution B.
Explain the shape of the graph
(Straight line graph from the origin)
Decrease in concentration decreases the rate of reaction.
The higher the concentration of solution B the less time taken for mark x to be obscurred/invisible due to increased collision frequency between the reacting particles.
From the graph determine the time taken for the mark to be invisible at 37cm3
At 37cm3 then 1/t => 1/37 = 0.027
From a well plotted graph:
1/t = 0.027 => 16.2602 seconds
From graph determine the volume of solution B at 100 seconds
100 seconds => 1/t = 1/1000 = 0.01
From a well plotted graph:
At 1/t = 0.01 => the volume of B = 17.0cm3
State another factor that would alter the rate of the above reaction.
State another factor that would not alter the rate of the above reaction.
An increase in temperature increases the rate of reaction.
An increase of 10 oC/10K practically doubles the rate of a chemical reaction/reduces time of completion by 1/2.
An increase in temperature increase the kinetic energy of reacting particles increasing their collision frequency
Practically ,increase in temperature involves heating the reactants The results and presentation should be as in the effect of concentration.
Increased temperature reverses the table I time results
i.e less time as temperature increases.