Rates of Reaction

•Section 6.1

Rate of Reaction The rate of reaction indicates how fast

reactants are being converted to products during a chemical reaction

It is the rate of formation of a product, or the rate of consumption of a reactant, divided by the corresponding coefficient in the stoichiometric equation

Rate has units of mol dm-3 s-1

Rate Rate will equal the change in concentration

divided by the change in time R → P, rate = Δ[P] = – Δ[R]_ Δt Δt Notice the negative sign for the reactants

because the concentration decrease with time whereas the concentration of the products increases with time (by convention, rate is a + value)

Stoichiometric Consideration You have to consider the amount of each

substance (number of moles) MNO4

-(aq) + 8H+

(aq) + 5Fe2+(aq) → Mn2+

(aq) + 4H2O(l) + 5Fe3+(aq)

The rate of appearance of Fe3+ is five times as great as the the rate at which MNO4

- is consumed

The rate is usually considered to apply to a product that has a coefficient of 1

Continued Rate = - Δ[MNO4

-] = 1 Δ[Fe3+]

Δt 5 Δt A general way to write this for reaction A→B

Rate = 1 Δ[B] = - 1 Δ[A] b Δt a Δt

Graphs Any property that differs between the

reactants and the products can be used to measure the rate of the reaction

The graph is drawn of that property against time

The rate of reaction is proportional to the slope (gradient) of the curve or line (ignoring the sign)

Graphs Changes in the gradient illustrate the effect of

changing conditions on the rate of reaction Usually the rate of reaction decreases with

time because the concentration of the reactants decreases with time (the reaction rate usually depends on the reactant concentration)

Graphs It is common to compare initial rates (gradient

of the tangent to the curve at t = 0)

Measuring Rates Basically any property that changes between

the start and end of the reaction can be used Best if the property changes by a large amount Easier to use a characteristic that is directly

proportional to the concentration of one or more of the components

Measuring Rates For instance, it is not recommended to use pH

because it is a logarithmic scale Keep in mind that the units for a reaction rate

are mol dm-3 s-1 It is important to keep the reaction mixture at

a constant temperature because temperature affects the reaction rate

Usually a water bath is used

Techniques for Measuring Rates Titration Collection of an evolved gas or increase in gas

pressure Measurement of the mass of the reaction

mixture Light absorption Electrical conductivity Clock techniques

Titration Remove small samples from the reaction

mixture at different times Titrate the sample to determine the

concentration of either a reactant or a product Only good for very slow reactions because the

titration takes so much time A graph is made of concentration against time

Example of Titration Reaction H2O2(aq) + 2H+

(aq) + 2I-(aq) → 2H2O(l) + I2(aq)

The amount of iodine produced can be measured by titrating the mixture with aqueous sodium thiosulfate

Collection of an Evolved Gas/Increase in Gas Pressure

The gas is collected in a gas syringe or in a graduated cylinder over water

The volume collected at different times is recorded

The gas must not be water soluble if it is collected over water

Could monitor the gas pressure in a container of fixed volume

Example Reactions Zn(s) + 2H+

(aq) → Zn2+(aq) + H2(g)

Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)

Measurement of the Mass of the Reaction Mixture

The total mass of the reaction mixture will only vary if a gas is evolved

The gas should have a high molar mass for this technique to be effective (not H2)

CaCO3(s) + 2H+(aq) → Ca2+

(aq) + H2O(l) + CO2(g)

Light Absorption Sometimes a reaction produces a precipitate

which “clouds” the reaction mixture A mark can be made on a piece of paper that

is then viewed through the reaction mixture When the mark is obscured, the reaction is

complete Keep the depth of the liquid constant

Example S2O3

2-(aq) + 2H+

(aq) → H2O(l) + SO2(g) + S(s)

The yellow suspended sulfur will obscure the mark

Light Absorption If a colored reactant or product is involved, the

intensity of the color can be used to monitor the concentration of the species

You could compare the color with your eyes against a know set of standard solutions

Could use a colorimeter or a spectrophotometer

Example Reaction between propanone and iodine to

form iodopropanone The yellow-brown iodine is the only colored

species involved CH3COCH3(aq)+ I2(aq)→ CH3COCH2I(aq)+ H+

(aq) + I-(aq)

Use the complementary color of blue The intensity of blue light passing through the

solution will increase with time as the iodine concentration falls

Electrical Conductivity The presence of ions allows a solution to

conduct electricity If there is a large change in the concentration

of ions during a reaction, the reaction rate can be found from the change in conductivity

Use an instrument that measures A.C. resistance by placing two electrodes in the solution

Example for Electrical Conductivity PCl3(aq) + 3H2O(l) → H2PO3

-(aq) + 4H+

(aq) + 3Cl-(aq)

Electrical conductivity will increase as the number of ions increases (products)

Clock Techniques Some reactions occur in which the product

produced can be further reacted with another substance to form another product

The formation of the 2nd product can usually be measured through a color change

The time taken for the 2nd product to appear is inversely proportional to the rate of the original reaction

Example H2O2(l) + 2H+

(aq) + 2I-(aq) → 2H2O(l) + I2(aq)

2S2O32-

(aq) + I2(aq) → S4O62-

(aq) + 2I-(aq)

Blue color of the iodine-starch complex appears when all of the thiosulfate has been consumed

This is inversely proportional to the rate