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Expressing and Measuring Expressing and Measuring Reaction RatesReaction RatesSCH 4U
Expressing Reaction RatesExpressing Reaction RatesUnderstanding 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.
Expressing RatesExpressing RatesReaction 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
Expressing Reaction RatesExpressing 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
Units of Reaction RatesUnits 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.
Reaction RatesReaction RatesChemical reactions proceed at
different rates, rates are influenced by factors such as:
TemperaturePressure/ ConcentrationSurface areaState or phase of the reactantsNature of the reactantsCatalyst
Average and Instantaneous Average and Instantaneous Rates of ReactionsRates of ReactionsChemical reactions do not always
proceed at a constant rate as a result chemist consider average or instantaneous rates of reaction.
Average RatesAverage RatesHow would you determine the
average rate from your data table?
How does this translate to determining the average rate given graphical information
Average RateAverage Rate
Determining Average RateDetermining Average RateThe average rate of reaction is
determined graphically by determining the slope of a secant (a secant is a line drawn between two points)
Instantaneous RatesInstantaneous RatesHow would you determine the
rate at a particular moment?
How would you do this graphically?
Instantaneous RateInstantaneous Rate
Determining Instantaneous Determining Instantaneous RateRateThe instantaneous rate is
determined by determining the slope of the tangent to curve at a particular point.
Reaction RatesReaction RatesWe can express the rate of
reaction with respect to the decrease in reactants or increase in products
2N2N22OO5(g) 5(g) 4NO 4NO2 2 + O + O(g)(g)
For the above reaction we can express the rate of reaction in one of the following ways:
a)In terms of the disappearance of N2O5
b)In terms of the production of O2
c)In terms of the production of NO2
2N2N22OO5(g) 5(g) 4NO 4NO2 2 + O + O(g)(g)
We can relate the different rates through Stoichiometry
Δ[O2] = 1 Δ[NO2] Δt 4 Δt or
Δ[O2] = -1 Δ[N2O5] Δt 2 Δt
Practice ProblemPractice 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?
Methods for Measuring Methods for Measuring RateRateMass, pH, conductivity
e.g. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Pressuree.g. 2N2O5(g) 4NO2(g) + 2O2(g)
Colour – spectrometer (UV-visible)
iodine clock lab