CHEM Topic 6 + 16 Chemical Kinetics

chemical kinetics

the study of rates of chemical reactions and the mechanisms by which a chemical reaction takes place

rate of reaction

the speed at which reactants are used up or products are formed

reaction rate

change in concentration over time

collision theory

1. the reactant particles must collide with each other

2. the collisions must be of enough energy to overcome the activation energy

3. the collisions must occur with correct geometrical alignment/orientation (= steric factor)

activation energy

the minimum energy required by the reactants to bring about a chemical reaction

factors to change reaction rate

• increase number of collisions between reactant particles

• increase energy of collisions

• decrease activation energy

catalyst

substance that accelerates the rate of reaction by lowering the activation energy without being consumed/used up

homogenous catalyst

acts in the same state as reactant

heterogenous catalyst

acts in different state to reactant

measuring changes in concentration

• changes in pH (acid-base reactions)

• changes in conductivity (in electrolytes)

• changes in mass/volume of solids or gases

• changes in colour (transition metals or coloured compounds)

• graph change in concentration/time (slope of tangent)

• measure rate of gas produced

• measure time for colour to change

• measure how fast reactants used up

electrical conductivity

• presence of ions allows a solution to conduct

• reaction rate can be found from changes in conductivity

• conductivity probe

titration

allows measuring the change in volume as reaction progresses

titration apparatus

pH

• not ideal method because dealing with small changes in concentration over pH scale

• useful if change in [H+] or [OH-]

• pH probe

light absorption/change in colour

* if reaction produced precipitate: able to time how long it takes formation of product to cloud view of mark made on piece of paper placed under reaction vessel

spectrophotometer

* measures wavelength of colour forming solution

colorimeter

filters out a specific colour

Br2

brown

I2(s)

violet

I2(g)

light yellow

KMnO4

dark purple

individual order

the power to which initial concentration is raised in the rate equation

overall order

the sum of all the individual orders in the rate equation

contact process

industrial method for the production of sulfuric acid using a catalyst

half life of a reaction

the time required for half of the reactants to react

0 order half life

decreases

1st order half life

constant

2nd order half life

increases

1st order units of rate constant (k)

s-1

2nd order units of rate constant (k)

s-1 mol-1 dm3

3rd order units of rate constant (k)

s-1 mol-2 dm6

unimolecular

single molecule involved in an elementary step

bimolecular

two molecules or atoms involved in collision in an elementary step

termolecular

three molecules or atoms involved in collision in an elementary step

intermediate

a species that is formed from the reactants in a chemical reaction which then goes on to react further to form the products

transition state/activated complex

• highest energy state = indicates point at which new bonds are being formed + old bonds being broken

• highly unstable

rate determining step

• step with the highest activation energy

• slow step