1. NATIONAL 4/5 CHEMISTRY
CHEMICAL CHANGES AND STRUCTURE LESSON 2 MEASURING AND CALCULATING THE RATE OF REACTION

2. Previous Learning from S3:
A working knowledge of the factors affecting rates of reaction is required for this Course. To compare rates of chemical reactions, changes in mass, volume and other quantities can be measured. Graphs can then be drawn to help this comparison.
Reactions can be followed by measuring changes in concentration, mass and volume of reactants and products.
The average rate of a reaction, or stage in a reaction, can be calculated from initial and final quantities and the time interval.
The rate of a reaction, or stage in a reaction, is proportional to the reciprocal of the time taken, ie if the rate is high the time taken will be small, and vice versa.
National 4 Chemistry:
Rates of reaction
Reactions monitored and graphs interpreted and rate calculated from a plotted graph
Previous Learning from S3:
Through experimentation, I can identify indicators of chemical reactions having occurred. I can describe ways of controlling the rate of reactions and can relate my findings to the world around me. SCN 3-19a

3. True or false?

4. Calculating the Rate of Reaction
The rate of reaction can be calculated by measuring the :
1. Time taken for the reaction
and
2. One other variable (something that changes) e.g.
Volume (cm3)
Mass (g)
Concentration (moles per litre = moll-1)
Rate = Change (in variable)
Time taken

5. Example 1
Use the graph below to calculate the rate of reaction for the first 20 seconds.
Average rate = (change) / time
20 cm3/ 20 s
1 cm3 S-1

6. Example 2
Use the graph below to calculate the rate of reaction between 5.2 and 10.8 seconds.
Average rate = (change) / time
/ ( )
0.2 mol l-1/ 5.6 s
0.036 mol l-1 s-1

7. Rates and Graphs
These show the increasing amount of product or the decreasing amount of reactant.
Amount of product
Time
Amount of reactant
Time
Steep gradient Fast reaction
Shallow gradient Slow reaction
Steep gradient Fast reaction
Shallow gradient Slow reaction

8. Rate graphs and reactant concentrations
Amount of product
Time
All product
Mix of reactant
And product
Reactant Concentration falls
Rate of Reaction falls
reactants
product
Gradient of graph decreases
All reactant

9. Calcium carbonate + hydrochloric acid  calcium chloride + carbon dioxide + water
CaCO3 + HCL  CaCl2 + CO2 + H2O
Time (s)
Decrease in mass (g)
Concentration of CO2 -
4.00 30
1.25
2.86 60
2.92
2.25 90
2.40
1.82
120
3.76
1.49
150
3.06
1.22
180
3.32
0.98
210
3.52
0.80
240
3.68
0.65
270
3.81
0.54
300
3.92
0.44
360
4.11
0.27
420
4.23
0.15
480
4.30
0.09
540
4.35
0.04
600
Time (s)
Volume of CO2 (cm3) 30 60 90
130
120
142
150
154
180
162
210
171
240
173
270
174
300
177
360
420
195
480
200
540
600
Activity 1 plot either of these graphs on graph paper and work out the average rate for the first 120 seconds and then between 300 – 600 seconds

10. Rate and Time
For some reactions, a colour change will indicate the end of the reaction.
The only measurement carried out during this type of experiment is time. Therefore the rate equation is slightly changed from:
No other variable measured
Rate = Change (in variable)
Time taken
To:
Rate =
Time taken
Units = per second (s-1)

11. Concentration of Potassium Iodide (KI) Mol/l
Concentration of sodium thiosulphate and the time taken for the cross to disappear
Concentration of Potassium Iodide (KI) Mol/l
Time (S)
Rate (1/t) S-1
1.0 23
0.8 29
0.6 39
0.4 60
0.2
111
Activity 2 - Plot a Time vs Rate Graph for this reaction and identify the relationship between concentration and rate of reaction
0.0435
0.0345
0.027
0.02
0.009

12. Rate = (1/t) S-1
Concentration of sodium thiosulphate (mol/l)
Q) How long did it take for the cross to be obscured when the concentration of sodium thiosulphate was 0.7 mol / l?
A) Rate = 1/t… so we must rearrange the equation to Time = 1/rate and obtain the rate from the graph
0.7 mol l = S-1 ; Time = 1/0.032  seconds