Presentation on theme: "6.1 - Expressing Reaction Rates"— Presentation transcript:
1 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 Ainitialtfinal – tinitialWhere, Amount is in moles and time (t) in secondsCan also be expressed:Rate of Reaction = ∆Amount of A , units are mol/s∆t
2 If rxn is b/n gases or in solution Rate of Reaction = Conc. of Afinal – Conc. of Ainitialtfinal – tinitialRate of Reaction = ∆[A] OR r = ∆c∆t ∆tWhere, concentration is measured in mol/L and time in sTherefore, rate of reaction is measured in mol/(Ls)Note: Reaction rates are always positive valuesNote: Square brackets  = concentrationPractice: Pg. 361 #1 and 2
3 Average Rate of Reaction The rate of a reaction is not usually constantSee 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
4 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.
5 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 .
6 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
7 Ex. Calculating Rates using Molar Ratios Given:2N2O5(g) 4NO2(g) + O2(g)4NO2(g) is produced at 5.0 x 10-6 mol/(Ls)Find rate of disappearance of N2O5 and rate of formation of O2Rate of disappearance of N2O5:2 mol N2O5 x 5.0 x 10-6 mol NO2/Ls = 2.5 X 10-6 mol N2O5/(Ls)4 mol NO2
8 Ex. Calculating Rates using Molar Ratios Rate of formation of O2:1 mol O2 x 5.0 x 10-6 mol NO2/Ls = 1.2 X 10-6 mol O2/(Ls)4 mol NO2Therefore, the rate of disappearance of N2O5 is 2.5 X 10-6 mol N2O5/(Ls)The rate of formation of O2 is, 1.2 X 10-6 mol O2/(Ls)
10 Methods for Measuring Rate of Rxn Decrease in massIf a gas is produced...you must use an open containerChange in pHReactions involving acids and basesConductivityReactions involving ionic compounds in solutionPressure ChangesGases must be producedColour Change – increase or decrease over timeVolume – of a gas producedPractice: Pg. 365 #8
11 6.2 - Factors Affecting Rate of Reaction Temperature (Temp = Rate of Rxn)[Reactants] ([Reactants] = Rate of Rxn)The presence of a catalyst Rate of RxnSurface Area of Reactants(Surface Area = Rate of Rxn)The type of reactantsRead Pg. 367 – 370 Answer # 1 – 4, 6