Kinetics/ collision theory

Kinetics

• Study of factors that affect the rates of reactions and the mechanisms (steps) by which reactions take place

Reaction rate

• number of particles ( atoms, ions,molecules) that react in a given time to form products.

  Reaction equation:change [R]/ change in time

Collision theory

• Particles must collide in order to react

• Collisions must be effective ( must have sufficient energy to break existing bonds and form new ones)

• must have correct orientation ( hit a specfic way)

Activation energy

The minimum energy colliding particles must have in order to react

Potential energy graph

• Potential energy of reactants or products is the difference from when they are a straight line until the ground

• Heat of reaction is the difference between the straight line of the products vs the reactants

• The reverse reaction is from the hump up until the straight line of the products

• the activation energy is from the hump up until the straight line of the reactions

• the activation complex or transition state is from the hump until the ground.

Activation complex:

• Bonds break and rearrange themselves

• Forward reaction

Factors that affect reaction rate

• Temperature

• Surface area ( particle size —> greater surface area so particles can hit easier)

• Catalyst ( substance that lowers activation energy without getting used up)

• Concentration ( increasing amount will increase collision with particles)

Calculating rate of reaction

• Look at coefficients of the balanced equation

• Multiply by the amount of moles for the substance you want ( ex oxygen) over the amount of moles you already have ex) The reaction rate of ozone is 8.76 × 10^-3 M/s over a certain interval of time. What is the rate of appearance of O2 during this interval?

  Using the balanced equation you can do 8.76 × 10³ x 3/2 since in the balanced equations its 2O₃ —> 3O₂

• Compare coefficients for each subtance

Reversible reaction

ex) A —>B

• Can become B and then through collisions also go back to A

• rate: change in [A]/ change in time

• will reach equilibrium

Equilibrium

• Rate of forward reaction is equal to the rate of the reverse reaction

• RATE OF PRODUCTION OF REACTANTS IS EQUAL TO THE RATE OF PRODUCTION OF PRODUCTS

• RATE IS THE SAME BUT YOU CAN HAVE MORE OF REACTANTS OR PRODUCTS

Equilibrium expression:

K_eq= [ products]/ [reactants]

• If k > 1 products are favored ( reaction goes to completion)

• if k < 1 reactants are favored ( mostly reactants in container)

• if k=1 neither reactants nor products are favored ( significant amount of both reactant and products in container)

• In equilibrium expressions you may only include substances ( reactants or products) that are gases or aqeuous. Liquids and solids have “ fixed” concentrations.

La Châtelier’s principle

• if a “ stress’ ( change) is applied to a reaction at equilibrium the reaction “ shifts” to relieve the stress

• Types of stress: change concentration of a reactant or product , change temp by adding or removing heat, change the pressure by increasing or decreasing the volume of container ( gases only)

• “ shifts” one way ( forward of reverse) reaction has a temporary increase in reaction rate in one direction

How these stresses apply:

• If concentration of reactants increases it shifts right

• if concentration of reactants decrease it shifts left to make more of what was removed

• If concentrations of products increase reaction shifts left

• If heat is added to the container reaction shifts left ( shifts in the endothermic reaction)

• If he is removed reaction shifts right ( in the exothermic direction).

• If pressure of container is increased reaction shifts towards the side with fewer moles of gase

• If pressure of container is decrease rection shifts where there are more moles.

• INCREASING VOLUME= REDUCING PRESSURE

• DECREASING VOLUME = INCREASING PRESSURE

Endothermic vs exothermic graphs

• Endothermic reactants are low and products are high

• Exothermic reactants are high and products are low.

Are reversible activation energy greater or less than forward reactions

Reversible activation energy is greater than forward activation energy because they need overcome the activation energy and reach the transition state.