1. Kinetics Lesson # 1
Reaction Rates

2. Definitions
Chemical kinetics – branch of chemistry that studies rates of reactions.
Reaction rate – the change in concentration of a reactant or product per unit time.

3. Measuring Reaction Rates
Chemists track the progress of a chemical reaction by observing the appearance of a product or disappearance of a reactant at particular time points. These points are used to determine the rate (think graphing!)
You can also track gas volume, colour, mass, pH and conductivity to determine reaction rates.
For example: Ca (s) + H2O (l) → Ca(OH)2 (aq) + H2 (g)
The easiest way to track this reaction is to measure the quantity of hydrogen gas over time.
You could do this by measuring the change in pressure as the gas is formed in a closed container.

4. Measuring Reaction Rates
For example: ClO- (aq) colourless + I- (aq) colourless →
IO- (aq) yellow + Cl- (aq) colourless
The easiest way to track this reaction is to use a spectrophotometer and measure the wavelengths of light it absorbs. The more yellow the solution, the more product is formed.
For example: CaCO3 (s) + 2 HCl (aq) → CO2 (g) + H2O (l) + CaCl2 (aq)
The easiest way to track this reaction is to measure the mass of all reactants and various points. The mass will decrease due to the formation of a gas. When the mass no longer changes, the reaction is complete, and the rate can be determined.

5. Measuring Reaction Rates
For example: SO3 (g) + H2O (l) → H2SO4 (aq)
The easiest way to track this reaction is to measure the pH, as the product is acidic.
For example: (CH3)3CCl (aq) + H2O (l) → (CH3)3COH (aq) + H+ (aq) + Cl- (aq)
The easiest way to track this reaction is to measure conductivity using a voltmeter. As free ions are created in the products, the solution will be able to conduct more and more as time progresses.

6. Calculating Average Reaction Rates
rateA =

7. Example 1
Using the values to the right, calculate the average rate of appearance of nitric oxide over the first 50 s of the chemical reaction.
Time (±1 s)
[NO (g)] (mol/L) 50
0.0021

8. Example 2
Using the values below, calculate the average rate of disappearance of nitrogen oxide gas over the first 100 s of the reaction.
Time (±1 s)
[NO (g)] (mol/L)
0.0100
100
0.0065

9. Calculating Average Reaction Rates Using Graphical Data
In a graph of reaction concentration versus time, the slope of the line will give you the rate of reaction.
Slope = Δy (rise)/Δx (run)
rateA = Δ[A] = Δy (concentration)
Δ t Δx (time)
Typically reaction progress graphs are a curved line, meaning we need to use the secant (line drawn between two points on a curved line) to determine the slope.

10. Example 3
Use the graph to the right to determine the average reaction rate for the production of oxygen gas between 180 s and 320 s.

11. Instantaneous Reaction Rates
This is the rate of reaction at any single instant in time.
To determine instantaneous rate on a curved line, you must use a tangent (a straight line that touches a curve at a single point and does not cross through the curve).

12. Example 4
Using the graph to the right, calculate the instantaneous rate of appearance of oxygen gas at 250 s.

13. Stoichiometry Rate Relationships
We can assume that a negative slope indicates the rate of consumption of reactants and a positive slope indicates the rate production of produce.
The value for the rate is really an absolute value.

14. Example 5 For the chemical reaction: 2 N2O5 (g) → 4 NO2 (g) + O2 (g),
a) If the rate of disappearance of N2O5 (g) is 5.6x10-2 mol/L.s at 60 s, determine the rate of appearance of oxygen at the same point in time.

15. Example 5 For the chemical reaction: 2 N2O5 (g) → 4 NO2 (g) + O2 (g),
b) Calculate the rate of appearance of NO2 (g) at 90 s if the rate of appearance of oxygen at this time is 2.0x10-2 mol/L.