RATES OF REACTION

1

Learning Goals:To be able to calculate rates of reaction and explain how rates of reaction can be affected.

Rate of Reaction2

Obtained by measuring the rate at which a product is formed or the rate at which a reactant is consumed over a period of time. Reaction rates can be measured as changes in mass, volume,

pH, colour or concentration of a chemical substance.

The units can be expressed as mol/s, mL/min, g/s, mol/L/h, etc..

timeelapsedionconcentratin change ratereaction average

tc r

Determining Average Reaction Rate usingthe Secant of Graphs

3

The plot of the concentration verses time of reaction can yield much information about the rate of reaction. Draw a secant that covers the appropriate time period

and determine the slope of that line.

Con

cent

ratio

n (m

ol/L

)

Time (s)

0.5

1.0

1.5

2.0

20 40 60

Average rate of reaction4

Con

cent

ratio

n (m

ol/L

)

Time (s)

0.5

1.0

1.5

2.0

20 40 60

Rateavg = slope of secant

= 1.75 mol/l – 0 65 s – 0= 0.027 mol/L s

Determining Instantaneous Reaction Rateusing the Tangent of Graphs

5

Draw a tangent to the curve and determine the slope of the tangent. The instantaneous rate of reaction, usually the initial

rate of reaction, is determined by calculating the slope of the tangent (straight line touching at one point) at the point of interest.

Initial (Instantaneous) rate of reaction6

Con

cent

ratio

n (m

ol/L

)

Time (s)

0.5

1.0

1.5

2.0

20 40 60

Rateinst = slope of tangent

= 2.0 mol/l – 0 20 s – 0= 0.1 mol/L s

Example7

Example: What is the average rate of production of ammonia for the system between 1 and 4 mins if the concentration of ammonia is 3.5M after 1 min and 6.2M after 4 min?

Example 28

Example: Determine the average rate of depletion of HCl from 1-3 mins based on the data in the graph

Assignment9

Pg 361 # 1,2Pg 364 # 3, 4, 6Pg 365 # 8, 11Pg 366 # 1,2