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

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