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6.1 - Expressing Reaction Rates Reaction Rate – The change in amount of reactants or products over time. Rate of Reaction = Amount of A final – Amount of A initial t final – t initial 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

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If rxn is b/n gases or in solution Rate of Reaction = Conc. of A final – Conc. of A initial t final – t initial Rate of Reaction = [A] ORr = 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

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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

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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.

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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.

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Calculating Rate of Reaction Using Molar Ratios Given the following rxn: 2N 2 O 5(g) 4NO 2(g) + O 2(g) Rate of Rxn can be expressed as: – Rate of disappearance of N 2 O 5(g) – Rate of production of NO 2(g) – Rate of production of O 2(g) If we know the rate for one reactant/product we can use the molar ratio to calculate the others

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Ex. Calculating Rates using Molar Ratios Given: 2N 2 O 5(g) 4NO 2(g) + O 2(g) 4NO 2(g) is produced at 5.0 x 10 -6 mol/(L s) Find rate of disappearance of N 2 O 5 and rate of formation of O 2 Rate of disappearance of N 2 O 5 : 2 mol N 2 O 5 x 5.0 x 10 -6 mol NO 2 /L s = 2.5 X 10 -6 mol N 2 O 5 /(L s) 4 mol NO 2

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Ex. Calculating Rates using Molar Ratios Rate of formation of O 2 : 1 mol O 2 x 5.0 x 10 -6 mol NO 2 /L s = 1.2 X 10 -6 mol O 2 /(L s) 4 mol NO 2 Therefore, the rate of disappearance of N 2 O 5 is 2.5 X 10 -6 mol N 2 O 5 /(L s) The rate of formation of O 2 is, 1.2 X 10 -6 mol O 2 /(L s)

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Practice Problems Pg. 364 # 3, 4, 6

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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

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6.2 - Factors Affecting Rate of Reaction 1.Temperature ( Temp = Rate of Rxn) 2.[Reactants] ( [Reactants] = Rate of Rxn) 3.The presence of a catalyst Rate of Rxn 4.Surface Area of Reactants ( Surface Area = Rate of Rxn) 5.The type of reactants Read Pg. 367 – 370 Answer # 1 – 4, 6

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