Expressing and Measuring Reaction Rates

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 Afinal - Amount of Ainitial (mol) timefinal - timeinitial =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. 2N2O5(g)  4NO2 + O(g) • For the above reaction we can express the rate of reaction in one of the following ways: • In terms of the disappearance of N2O5 • In terms of the production of O2 • In terms of the production of NO2

16. 2N2O5(g)  4NO2 + O(g) • We can relate the different rates through StoichiometryΔ[O2] = 1 Δ[NO2] Δt 4 Δt or Δ[O2] = -1 Δ[N2O5] Δt 2 Δt

17. Practice Problem Ammonia NH3 ,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, conductivitye.g. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) • Pressuree.g. 2N2O5(g)  4NO2(g) + 2O2(g) • Colour – spectrometer (UV-visible)iodine clock lab