1. Expressing and Measuring Reaction Rates SCH 4U

2. Expressing Reaction Rates Understanding the rate of a reaction can be very important to understanding the reaction and its effects on the surroundings Reaction rates are expressed as the change in the amount of reactants or products over time. Chemists typically express the amount of a reactant or product in moles.

3. Expressing Rates Reaction rate is the change in the amount of reactants or products over time Consider an activity and in that activity we determined the rate of transfer of water from one graduated cylinder to another by dividing the volume transferred over a period of time

4. Expressing Reaction Rates In chemistry we can described the rate of a reaction based on the amount of product produced or reactant used up. Rate of reaction = Amount of A final - Amount of A initial (mol) time final - time initial = Δ amount of A Δt A = amount of the compound of interest in units of mol or mol/L

5. Units of Reaction Rates The amount of the compound of interest is expressed in units of mol when we are talking about a solid, or in units of mol/L when we are studying a solution phase or gaseous phase reaction. Rate of reaction has units of mol/s or mol/L. s Rates of reactions are always positive.

6. Reaction Rates Chemical reactions proceed at different rates, rates are influenced by factors such as: Temperature Pressure/ Concentration Surface area State or phase of the reactants Nature of the reactants Catalyst

7. Average and Instantaneous Rates of Reactions Chemical reactions do not always proceed at a constant rate as a result chemist consider average or instantaneous rates of reaction.

8. Average Rates How would you determine the average rate from your data table? How does this translate to determining the average rate given graphical information

9. Average Rate

10. Determining Average Rate The average rate of reaction is determined graphically by determining the slope of a secant (a secant is a line drawn between two points)

11. Instantaneous Rates How would you determine the rate at a particular moment? How would you do this graphically?

12. Instantaneous Rate

13. Determining Instantaneous Rate The instantaneous rate is determined by determining the slope of the tangent to curve at a particular point.

14. Reaction Rates We can express the rate of reaction with respect to the decrease in reactants or increase in products

15. 2N 2 O 5(g)  4NO 2 + O (g) For the above reaction we can express the rate of reaction in one of the following ways: a) In terms of the disappearance of N 2 O 5 b) In terms of the production of O 2 c) In terms of the production of NO 2

16. 2N 2 O 5(g)  4NO 2 + O (g) We can relate the different rates through Stoichiometry Δ[O 2 ] = 1 Δ[NO 2 ] Δt 4 Δt or Δ[O 2 ] = -1 Δ[N 2 O 5 ] Δt 2 Δt

17. Practice Problem Ammonia NH 3,reacts with oxygen to produce nitric oxide, and water vapour At a specific time in the reaction, ammonia is disappearing at a rate of 0.068 mol/L. s What is the corresponding rate of production of water?

18. Methods for Measuring Rate Mass, pH, conductivity e.g. Mg (s) + 2HCl (aq)  MgCl 2(aq) + H 2(g) Pressure e.g. 2N 2 O 5(g)  4NO 2(g) + 2O 2(g) Colour – spectrometer (UV-visible) iodine clock lab