Topic 6 Kinetics

• Rates of reaction• Collision theory

6.1 Reaction rate

• Definition• Experimental procedures• Analyse data• Rate = (conc. of A at time t2 - conc. of A at

time t1) / t2-t1 = D[A]/Dt

• Unit: M/s = mol*dm-3*s-1

• The rate of which products are formed = the rate of which reactants are consumed.

• But the value can vary according the number of moles involved:

MnO4- + 8 H+ + 5 Fe2+ Mn2+ + 4 H2O + 5 Fe3+

• The formation of Fe3+ is five time faster than the consumption of MnO4

-.

Measurement of reaction rate

• Any property that differ between a product or a reactant can be used. e.g. absorption of light, electrical conductivity, gas production, removal of samples for chemical studies and so on

• What property can be measured in this reaction:

Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g)

Examples of measurements

• Mass or volume change for gaseous reactions• Change in pH for acid-base reactions• Change in conductivity for reactions with

electrolytes• Use of a spectrophotometer for reactions

involving colour change

• A graph is normally drawn : property against time. The rate of the reaction is proportional to the gradient of the curve

• Change in reaction conditions => change in gradient. Often no need to convert the “property” to M/s

Changes in mass against time

Marble CaCO3 + acid = carbondioxide + CaCl2

Changes in mass and volume against time

Marble CaCO3 + acid = carbondioxide + CaCl2

Changes in concentration against time

• Activation energy (Ea)• Kinetic theory• Collision theory• Factors affecting kinetics• Maxwell-Boltmann• Catalysts

6.2 Collision theory

Activation energy (Ea)

• When two particles going to react they must have energy enough to overcome the repulsive force in their negatively charged electron clouds.

• They must also have enough energy to break the required bonds

• The amount of activation energy needed for a reaction differ widely from reaction to reaction.

e.g. • H+ + OH- H2O reacts immediately at room

temperature • Wood + O2 CO2 + H2O doesn’t react at room

temperature

Collision theory

• In order to react, the two particles involved must:

1. Collide with each other2. The collision must be

energetic enough to overcome the activation energy.

3. The collision must bring the reactive parts of the molecule into contact in the correct way. (Hard if large organic molecules )

Maxwell-Boltzmann energy distribution curve

• The average kinetic energy of a collection of molecules is proportional to their absolute temperature.

• Lots of collisions between them give a wide range of different molecular speeds.

• The maximum on the curve: represent the kinetic energy possessed by the largest fraction of the molecules. Most probably kinetic energy. The average kinetic energy occurs at a higher value.

• For a reaction the kinetic energy must overcome the Ea. At a higher temperature a bigger fraction of molecules have kin. energy > Ea => faster reaction.

Factors that affect the reaction rate

Factors mainly affecting collision rate

Factors mainly affecting Ea

Concentration/pressure Temperature

Surface area Catalyst

Catalyst• If a catalyst is used the reaction take an other

path with a lower Ea => the Ea shift to left in the graph => bigger fraction of molecules have kin. energy > Ea => faster reaction

• Concentration: Zinc reacts faster with concentrated than diluted acid

• Surface area: Zinc reacts faster with zinc powder than a big lump.

• Temperature: Number of particles with E > Ea is higher at a higher temperature (recall Maxwell-

Boltzmann energy distribution curve)

Also a small effect on the collision rate.• Catalyst: A compound that increases the

reaction rate without being consumed. Catalyst reduce the Ea by taking an other reaction path.

Exercises

• Page 116