Chemical kinetics is the study of the rate at which reactions occur and how to speed up or slow down the reaction.
If we compare power generated by the combustion reaction of carbon with power generated by the decomposition of nitroglycerin, it will become obvious why the rates of reactions are important:
C(s) + O2(g) → CO2(g), slow reaction with ΔH = -394 kJ
P = E/t = 394 kJ/large = small power (not scary)
C3H5(NO2)3(l) → 3CO2(g) + ½NO2(g) + 5/2H2O(g) + 5/2N2(g), fast reaction with ΔH = -190 kJ
P = E/t = 190 kJ/small = large power (very, very scary)
Average Rate & Instantaneous Rate
Reaction rate is the change in the concentration of a reactant or a product per unit time (rate = Δ[R]/Δt or rate = Δ[P]/Δt).
- Reaction rate has units which can be written as mol/L/s or M/s or molL-1s-1or Ms-1.
Average reaction rate can be calculated over a given time interval.
This can be determined in two ways - using a concentration versus time graph, determine the slope of a line joining two points on the curve (a sectant line) or calculate as below.
![The diagram on the left is a [R ] vs t graph and the diagram on the right is a [P ] vs t graph.](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjJoKwigwpRlIV0PTiOaQYcSxfC2cEEWrlmmzx1ALcjXOb94UtWM-6sS-M2MZgFcmlvWOky2vnBf_1AhojZgPeG6Tjm7zL4CSwWg75YQEycT2Pve9YqQ4SlePSLBuraXA8wT45wzspi4XI/s2022/IMG_E7458%255B1%255D.JPG "The diagram on the left is a [R ] vs t graph and the diagram on the right is a [P ] vs t graph.") |
| The graph to the left represents the change in reactant concentration over time. The graph to the right represents the change in product concentration over time. |
ex. What is the rate of consumption of each reactant when the rate of production of ammonia is 4.0 x 10-3 Ms-1? N2(g) + 3H2(g) → 2NH3(g)
👉 set up a "rate ratio" which is similar to a mole ratio - use the coefficients in front of each substance to create the ratio
- rate(N2)÷1 = rate(NH3)÷2
👉 plug in the known value, given in the question and solve for the unknown value
rate(N2) = -4.0 x 10-3 Ms-1÷2
rate(N2) = - 2.0 x 10-3 Ms-1
∴ the rate of consumption of nitrogen gas is 2.0 x 10-3 Ms-1
Note: We have to make one side of the equation negative because consumption and production are not the same (they are opposite of each other). Thus, we would not be able to use the equal sign. By making consumption = -production, we can circumvent this issue).
- 1/3 rate(H2) = ½ rate(NH3)
rate(H2) = - 3/2 (4.0 x 10-3 Ms-1)
rate(H2) = - 6.0 x 10-3 Ms-1
∴ the rate of consumption of hydrogen gas is 6.0 x 10-3 Ms-1
Instantaneous reaction rate is calculated for an instant in time - determine the slope of a tangent line at the point on the curve for the instant in time.
Measuring Reaction Rates
Reactions that Produce a Gas
- for a reaction that produces a gas, such as the reaction of zinc with hydrochloric acid
- the experimenter can collect and measure the volume and/or pressure of the produced gas to follow the rate of the reaction
Reactions that Involve Ions
- for a reaction that produces ions
- the experimenter can measure the conductivity of a solution to follow the rate of the reaction
Reactions that Change Colour
- for a reaction in which a coloured reactant disappears or a coloured product is produced
- the experimenter can measure colour intensity using a spectrophotometer
Homework #1-6
Homework and Answer Keys can be found here.
