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Expressing and Measuring Reaction Rates - PowerPoint PPT Presentation


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Expressing and Measuring Reaction Rates. SCH 4U. Expressing Reaction Rates. Understanding the rate of a reaction can be very important to understanding the reaction and its effects on the surroundings

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Presentation Transcript
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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.
slide3

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
slide4

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

slide5

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.

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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
slide7

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.
slide8

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
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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)
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Instantaneous Rates

  • How would you determine the rate at a particular moment?
  • How would you do this graphically?
slide13

Determining Instantaneous Rate

  • The instantaneous rate is determined by determining the slope of the tangent to curve at a particular point.
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Reaction Rates

  • We can express the rate of reaction with respect to the decrease in reactants or increase in products
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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
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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

slide17

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?

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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