Group One
Average Rate of a Chemical Reaction
Reaction rate is a measure of how fast a reactant is used up or how fast a product is
formed. Since these measurements are made over a period of time, we are actually
dealing with an average rate. This is comparable to measuring the average speed of
travel, in which we determine the distance covered and divide by time.
For chemical reactions we can determine the average reaction rate in a similar manner.
Instead of using the distance covered within a specific period, we now use the amount of
product formed in a specified period of time or the amount of reactant used up in that
time. The amount of product formed, or reactant used, is determined by measuring their
concentrations at various times during the reaction. In the following table there are some
sample data for the general reaction A B. The table contains the initial reactants and
product concentrations as well as the additional measurements after the reaction had gone
on for 3 minutes, and then again at 6 minutes.
Variation in Concentration of A and B with Time for the reaction: A B
Time (min)
[A] mol/L
[B] mol/L
0.0
1.00
0.00
3.0
0.40
0.60
6.0
0.25
0.75
Average rate = change in concentration =  [products or reactants]
Time elapsed
time elapsed
Note that if the rate is fast the time to finish will be short - see graph page 238
Chemistry a Second Course
Example:
Using the values in the above table, calculate the average rate of loss of reactant A during
the first six minutes of the reaction:
Soln:
Rate of loss of A = [A]/ Time
= 1.00 – 0.25 mol/L / 6 min
= 0.75 mol/L / 6min
= 0.13 mol/L / min
Demo:
1. Slow Reaction:
Materials:
- Apple
- Fine Steel Wool
- Water
Demonstration:
- Cut an apple and place some fine steel wool in water. Observe the
slow formation of a brown colour as they are oxidized.
2. Fast reaction
The Creature from the Black Lagoon
Materials:
15g of sucrose
15mL of concentrated sulfuric acid
200mL beaker
Fume Hood
Safety: Experiment must be completed in the fume hood, as fumes are choking
Caution: concentrated sulfuric acid is very nasty stuff. Do not let it come into contact
with skin or clothes.
Procedure :
-
In a fume hood mix 15 g of sucrose and place in a heavy 200 ml
beaker.
Add 15 ml of concentrated sulfuric acid.
Explanation: The acid dehydrates the sucrose. The reaction is very exothermic and
actually changes the water to steam. What is left is elemental carbon.
The idea of black carbon being in white sugar is interesting to students, could it be that
the properties like color change as bonds between atoms are made? Another thought is
that sugars like sucrose look like C bonded to water molecules in the formula since the
ratio of H to O is 2 to 1 -- C12H22O11. That is why they are called carbohydrates,
because scientists thought (incorrectly) in the beginning that they were combinations of C
and water.
Sample Questions:
1. When can we equate the rate of formation of a product with the rate of
decomposition of a reactant?
2. Measurements taken during the reaction CO(g) + NO2(g)  CO2(g) + NO(g)
showed that 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, over this 18 minute of each of
the following:
a. The loss of carbon monoxide
b. The gain of carbon dioxide.
3. In the following reaction the average rate of loss of carbon monoxide, over a set
period, is 0.15mol/L(s)
2CO(g)  CO2(g) + C(s)
What is the average rate of production of carbon dioxide during the same period?
4. At high temperatures, ammonia reacts with oxygen to produce nitrogen monoxide
and steam:
4NH3(g) + 5O2(g)  4NO(g) + 6 H2O(g)
In one experiment the average rate of decomposition of ammonia was found to be 4.5
x 10-2 mol/L(s). For this same time interval calculate the following:
a. The rate of production of water
b. The rate of production of nitrogen monoxide
c. The rate at which the oxygen is consumed.
5. Using a balanced equation for the complete combustion of octane (C8H10) answer
the following question:
Octane is being consumed at a rate of 4.0L/min at SATP. What is the rate of
production of CO2 in mol/min?
N2(g) + 3 H2(g) = 2NH3(g)
6. If ammonia is produced at a rate of 20.0 g/min, what is the rate of consumption of
H2 in L/min at SATP?
7. If N2 is being consumed at 100.0L/min (SATP), at what rate is hydrogen being
consumed at in g/min?