Name: _____________________________
Date: _____________________
Calculating Average Rates of Reaction
You are required to show work for all calculations.
1. During the combustion of methane, CH4, shown by the reaction
CH4(g) + O2(g)  CO2(g) + H2O(g)
The concentration of methane was measured at various time intervals and the following
results were obtained:
Time
(s)
10
20
30
40
[CH4]
(mol  l-1)
2.40
1.20
0.80
0.60
Calculate the average rate of loss of methane during the 10 to 40 second time period.
Solution:
D[CH 4 ]
Dt
[0.60 - 2.4]mol
=
(40 -10)s × L
-1.8mol
=
30s × L
mol
= -0.06
L×s
rate =
Note: the answer is negative because we are calculating the rate of loss of methane.
Calculating Reaction Rates using Stoichiometry
Example:
In the following decomposition reaction,
2 N2O5 → 4 NO2 + O2
Unit II: Kinetics
Assignment 1
1
Name: _____________________________
Date: _____________________
Oxygen gas is produced at the average rate of 9.1 × 10-4 mol/(L*s). Over the same period, what
is the average rate of the following?

The production of nitrogen dioxide
Rate of production of NO2 = Rate of production of O2 x (molar ratio)
+
D[N 2O5 ] 9.1´10-4 molO2 4molN 2O5
=
´
L×s
1mol0 2
Dt
= 3.6 × 10-3

mol
L×s
The loss of nitrogen pentoxide
-
D[N 2O5 ] 9.1´10-4 molO2 2molN 2O5
=
´
L×s
1mol0 2
Dt
= -1.8 × 10-3
mol
L×s
Practice Problems:
1. Consider the following reaction:
N2(g) + 3 H2(g) → 2 NH3(g)
If the rate of loss of hydrogen gas is 0.03 mol/(L*s), what is the rate of production of ammonia?
Solution:
D[H 2 ]
mol
= 0.03
Dt
L×s
D[NH 3 ] 0.03molH 2 2molNH 3
+
=
´
Dt
L×s
3molH 2
mol
= 0.02
L×s
-
Unit II: Kinetics
Assignment 1
2
Name: _____________________________
Date: _____________________
3. Measurements taken during the reaction CO(g) + NO2(g)  CO2(g) + NO(g)
showed a concentration of carbon monoxide of 0.019 mol/L at 27 min and of 0.013 mol/L at
45 min. Calculate the average rate, in mol/(L*Min) over this 18 min period, of each of the
following:
a) the loss of carbon monoxide, CO
Solution:
D[CO] [0.013- 0.019]mol
=
Dt
18min× L
mol
= -3.33´10-4
L × min
Note: The answer is negative because the reactant is being used up
b) the gain of carbon dioxide, CO2
Solution:
D[CO2 ] 3.33´10-4 molCO 1molCO2
molar ratio
=
´
Dt
L×s
1molCO
D[CO2 ] 3.33´10-4 molCO 1molCO2
=
´
Dt
L×s
1molCO
mol
= 3.33´10-4
L × min
Note: The answer is positive because the product is being produced
4. In the following reaction the average rate of loss of carbon monoxide, over a set period, is
0.15 mol/(L*s).
2 CO(g)  CO2(g) + C(s)
What is the average rate of production of carbon dioxide during the same period.
Solution:
D[CO2 ] D[CO]
=
´molar ratio
Dt
Dt
D[CO2 ] 0.15molCO 1molCO2
=
´
Dt
L×s
2molCO
mol
= 0.075
L×s
Unit II: Kinetics
Assignment 1
3
Name: _____________________________
Date: _____________________
Ethanal vapour undergoes thermal decomposition in the reaction
C2H4O(g)  CH4(g) + CO(g)
The following data was recorded:
Table 1: Concentration of Ethanal During Thermal Decomposition
[C2H4O] (mol/L)
Time (s)
0.360
0
0.290
100
0.250
185
0.200
270
0.180
420
0.150
575
0.130
730
0.110
950
0.090
1250
0.080
1440
(a) Use the data in Table 1 to plot a graph of the concentration of ethanal vapour vs. time.
(b) From the graph, determine the average rate of reaction between the times t= 0 s and t =
420 s. (draw a secant)
(d) Determine the instantaneous rates of decomposition of ethanal when its concentrations
are
i. 0.20 mol/L
ii. 0.10 mol/L.
(draw tangents at those points and find the slope of the tangent)
.
Unit II: Kinetics
Assignment 1
4