How do we define the speed of chemical changes if there is no distance traveled? Instead of a distance, chemists measure a change in the concentration of one of the reactants or products over time. The following table shows how this changes the definition and the formula used to calculate it .
Physics Definition
Chemistry Definition
Rate of change of distance over time
Rate of change of concentration over time
Formula
Formula
Speed =
Distance
   
Time
Speed (rate) = [Reactant/ Product]

 
Time
  • Note that the formulas are very similar , and with them chemists can use graphs of concentration vs time to calculate average and instantaneous rates of reactions just as is done in physics. In the following assignment you are asked to calculate the average and instantaneous rates of a chemical reactions.
Calculating average Vs Instantaneous rates
I) Example:
Examine the following data for the reaction
CH4 (g) + O2 (g) ----> CO2 (g) + H2O (g)
Time (seconds)
[CH4 (g) ]
10
2.40 mol/l
20
1.20 mol/l
30
0.80 mol/l
40
0.60 mol/l
a) The average speed of the reaction between 10 and 20 seconds is calculated as;
 
[ Reactant ]
( 1.20- 2.40 ) mol/l
  
Average Speed
= - 0.12 mol/*s
 
time
20- 10 seconds
  
b) Calculate the average speed from 20 to 40 seconds. Answer
c) To calculate the instantaneous speed at 30 seconds we would need to graph the information and take the slope of the tangent line to our graph at 30 seconds . Answer
ii) Activity: Perform the following activity to test your understanding of calculating rates
Assignment #2 : Average and Instantaneous Rates of Chemical Reactions