Chemistry - R2.2 How fast? The rate of chemical change

Rates of reaction

Rate of reaction

Change in concentration of a particular reactant/product per time

Instantaneous reaction rate

V_inst = |dc|/dt

Initial reaction rate

Instantaneous rate measured at t = 0 (draw tangent line)

Collision theory

Allows the particles to collide non-elastically and undergo chemical changes

Collision theory and KE

KE of most particles is insufficient for breaking chemical bonds

A collision between two fast-moving particles might be violent enough to break or rearrange chemical bonds and transform the reactants into products

Orientation of colliding particles is also important

Reaction of species

Species react as a result of collisions of sufficient energy and proper orientation

For a chemical reaction to occur:

Two or more particles must collide with each other

Colliding particles must have the correct mutual orientation

The sum of KE of particles must be sufficient to initiate the reaction

Factors affecting reaction rate

Concentration, Temperature, Surface area, pressure(gas), catalyst

Activation energy

The minimum energy that colliding particles need for a successful collision leading to a reaction.

Difference in energy between the reactants and transition state

KE > Ea → successful collision

Transition state

Least stable arrangement of atoms in reacting species

Maxwell-Boltzman energy distribution curve

The plots of statistical distribution of the energies at various temperature

Catalyst

Increase the rate of reaction by providing an alternative reaction pathway with lower Ea (reduces Ea).

Reacts with a reactant to form an unstable compound(intermediate)

Intermediate goes through further chemical changes.

Catalysed reactions

Proceed in different steps → each step requires less energy than uncatalysed reaction

Reaction mechanism

Many reactions occur in a series of chemical changes across multiple intermediates and transition states.

A sequence of these changes are called reaction pathway

Multistep reaction

Involves one or more elementary step

Molecularity

Determined by the number of species involved in an elementary step

Unimolecular = involves a single species

Bimolecular = involves two colliding species

Termolecular = involves three species colliding at same time

Rate-determining step

Slowest step

Limits the overall rate of reaction for any given concentrations of reactants

Has the highest Ea

Rate equation

Depends on the mechanism of the reaction

Expression describing the proportionality of the rate to the reactant concentrations

Rate constant k is temperature dependent

Reaction order

zero = rate is independent of the reactant concentration

first = rate is proportional to reactant concentration

second = rate is proportional to [A]²

Arrhenius equation

Uses the temperature dependence of the rate constant to determine the activation energy

Arrhenius factor takes into account the frequency of collisions with proper orientation.