6.1 - Expressing Reaction Rates

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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 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

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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

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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
• 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

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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

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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)

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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

• 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