# 6.1 - Expressing Reaction Rates

## Presentation on theme: "6.1 - Expressing Reaction Rates"— Presentation transcript:

6.1 - Expressing Reaction Rates
Reaction Rate – The change in amount of reactants or products over time. Rate of Reaction = Amount of Afinal – Amount of Ainitial tfinal – tinitial Where, Amount is in moles and time (t) in seconds Can also be expressed: Rate of Reaction = ∆Amount of A , units are mol/s ∆t

If rxn is b/n gases or in solution
Rate of Reaction = Conc. of Afinal – Conc. of Ainitial tfinal – tinitial Rate of Reaction = ∆[A] OR r = ∆c ∆t ∆t Where, concentration is measured in mol/L and time in s Therefore, rate of reaction is measured in mol/(Ls) Note: Reaction rates are always positive values Note: Square brackets [] = concentration Practice: Pg. 361 #1 and 2

Average Rate of Reaction
The rate of a reaction is not usually constant See figure 3 (Pg. 362) Average Rate of Reaction – the average change in concentration per unit time over a given time interval. Slope of a line between 2 points on the curve of a Conc. Vs. Time graph

Instantaneous Rate of Reaction
The rate of a reaction at a particular moment or instant in time. (ie. the rate at 10.0 seconds) The slope of a tangent to the curve on a Conc. Vs. Time Graph (Figure 4: Pg. 362) Tangent – touches the line at only one point.

Why does Rate of Rxn Change?
At the start,  [reactants] therefore more successful collisions occur and more product is formed. As the [reactants]  , there are fewer successful collisions occur so the rate of product formation .

Calculating Rate of Reaction Using Molar Ratios
Given the following rxn: 2N2O5(g)  4NO2(g) + O2(g) Rate of Rxn can be expressed as: Rate of disappearance of N2O5(g) Rate of production of NO2(g) Rate of production of O2(g) If we know the rate for one reactant/product we can use the molar ratio to calculate the others

Ex. Calculating Rates using Molar Ratios
Given: 2N2O5(g)  4NO2(g) + O2(g) 4NO2(g) is produced at 5.0 x 10-6 mol/(Ls) Find rate of disappearance of N2O5 and rate of formation of O2 Rate of disappearance of N2O5: 2 mol N2O5 x 5.0 x 10-6 mol NO2/Ls = 2.5 X 10-6 mol N2O5/(Ls) 4 mol NO2

Ex. Calculating Rates using Molar Ratios
Rate of formation of O2: 1 mol O2 x 5.0 x 10-6 mol NO2/Ls = 1.2 X 10-6 mol O2/(Ls) 4 mol NO2 Therefore, the rate of disappearance of N2O5 is 2.5 X 10-6 mol N2O5/(Ls) The rate of formation of O2 is, 1.2 X 10-6 mol O2/(Ls)

Practice Problems Pg. 364 # 3, 4, 6

Methods for Measuring Rate of Rxn
Decrease in mass If a gas is produced...you must use an open container Change in pH Reactions involving acids and bases Conductivity Reactions involving ionic compounds in solution Pressure Changes Gases must be produced Colour Change – increase or decrease over time Volume – of a gas produced Practice: Pg. 365 #8

6.2 - Factors Affecting Rate of Reaction
Temperature (Temp =  Rate of Rxn) [Reactants] ([Reactants] =  Rate of Rxn) The presence of a catalyst  Rate of Rxn Surface Area of Reactants (Surface Area = Rate of Rxn) The type of reactants Read Pg. 367 – 370 Answer # 1 – 4, 6