Chemistry 3.5 - chemical kinetics

What is chemical kinetics?

The study of rate, or speed, of chemical reactions

What is the rate of a reaction?

The rate of change in concentration of a reactant (or product) per unit time

Rate = change in concentration of reactants or products/time for the change to take place

What are the units of rate?

moldm^-3s^-1

What are some methods of continuous monitoring to determine the rate of a reaction?

• monitoring gas collection using a syringe or inverted measuring cylinder

• monitoring changes in pressure caused by changes in the amount of gas present

• monitoring mass loss by using a balance

• monitoring intensity of light passing through a solution using a colorimeter

• monitoring concentration of reactants or products though sampling and quenching

What is colorimetry?

A method of determining the concentration of a substance by shining light through the solution in a special glass tube called a cuvet

Describe the process of colorimetry

• light is shined through the solution in a cuvet

• amount of light absorbed is measured

• a calibration graph is used to find the concentration

• the instrument by which the measurements are made is called a colorimeter

• a coloured filter is used so that the wavelength used are colours that the solution absorbed strongly e.g. using a yellow filter in measuring the concentration of a blue coloured solution like copper (I) sulphate

What is sampling and quenching?

Most complicated to monitor rates of reaction so only used when other methods are not possible

Describe the process of sampling and quenching

• Small samples of the reaction mixture are removed (sampling)

• they are added to ice cold water which decreases the concentration and lowers the temperature to stop the reaction (quenching)

• samples are taken ar various times throughout the reaction and the concentration of either a reactant or product in each sample is determined e.g. by titration

What graphs are usually used when measuring rates?

Concentration-time graphs

• often show the concentration of reactants falling with time

How do you measure the reaction rate from concentration-time curve graphs?

• measure gradient of the curve by drawing a tangent

• gradient = change in y/change in x

When does a reaction occur?

When reactant particles collide with energy greater than the activation energy

What does the rate of a reaction depend on?

The frequency of successful collisions

What is the order of a reaction?

Shows how the reaction rate is affected by changes in concentration of the reactant

What are the possible orders of a reaction?

• 0 order - doubling concentration has no effect on rate

• 1st order - doubling concentration doubles rate

• 2nd order - doubling concentration quadruples rate

What does the rate equation show?

The relationship between the rate of a reaction and the concentration of reactants

• shows the order of the reaction with respect to each reactant

• can only be determined from experiments unlike K_c (can be determined from the stoichiometric equation)

How do you write out each order with respect to the reactant?

• 0 order with respect to A = [A]

• 1st order with respect to B = [B]¹

• 2nd order with respect to C = [C]²

Rate equation

What is the overall order of a reaction?

The sum of the individual orders

Describe the rate - concentration graph for 0th order

• Reactant has no effect on the rate

• Rate continues at a steady pace

• Rate = K

Describe the rate - concentration graph for 1st order

• As concentration doubles the rate also doubles (as concentration triples the rate triples etc)

• The rate and concentration are indirectly proportional to each other

• Rate = k[A]

Describe the rate - concentration graph for 2nd order

• As concentration doubles the rate quadruples (as concentration triples the rate increases by 9 etc)

• The rate increases by the square of the concentration

• Rate = k[A]²