NATIONAL 4/5 CHEMISTRY

CHEMICAL CHANGES AND STRUCTURE

LESSON 2

MEASURING AND CALCULATING THE RATE OF REACTION

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.

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

• 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.

Tru e o r false?

Calculating the Rate of Reaction

The rate of reaction can be calculated by measuring the :1. Time taken for the reaction

2. One other variable (something that changes) e.g.

Volume (cm3)Mass (g)

Concentration (moles per litre = moll-1)

and

Rate = Change (in variable)

Time taken

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

Example 2

Use the graph below to calculate the rate of reaction between 5.2 and 10.8 seconds.

• Average rate = (change) / time • 0.4 - 0.2/ (10.8 - 5.2)• 0.2 mol l-1/ 5.6 s

• 0.036 mol l-1 s-1

Rates and Graphs

• These show the increasing amount of product or the decreasing amount of reactant.

Am

ount

of

prod

uct

Time

Am

ount

of

reac

tant

Time

Steep gradient Fast reaction

Shallow gradient Slow reaction

Steep gradient Fast reaction

Shallow gradient Slow reaction

Rate graphs and reactant concentrationsA

mou

nt o

f pr

oduc

t

Time

reactants

product

Reactant Concentration falls

Rate of Reaction falls

All product

All reactant

Mix of reactant

And product

Gradient of graph decreases

Calcium carbonate + hydrochloric acid calcium chloride + carbon dioxide + water CaCO3 + HCL CaCl2 + CO2 + H2O

Time (s) Decrease in mass (g) Concentration of CO2

0 - 4.0030 1.25 2.8660 2.92 2.2590 2.40 1.82

120 3.76 1.49150 3.06 1.22180 3.32 0.98210 3.52 0.80240 3.68 0.65270 3.81 0.54300 3.92 0.44360 4.11 0.27420 4.23 0.15480 4.30 0.09540 4.35 0.04600 4.35 0.04

Time (s) Volume of CO2 (cm3)

0 030 6060 9090 130

120 142150 154180 162210 171240 173270 174300 177360 180420 195480 200540 200600 200

• 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

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:

Rate = Change (in variable)

Time taken

No other variable measured

Rate = 1

Time taken

Units = per second (s-1)

To:

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

0.020.009

0 0.2 0.4 0.6 0.8 1 1.20

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

Graph of concentration of sodium thiosulphate and rate of reaction with hydrochloric acid Ra

te =

(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 = 0.032 S-1 ; Time = 1/0.032 31.25 seconds