Chem ch 13

• the area of chemistry concerned with the speeds, or rates, at which a chemical reaction occurs
• the speed of a reaction is not neccessarily related to the energy change

• the change in the concentration of a reactant or a product with time
• units of M/s
• always highest at the beginning of a reaction
• 2A --> B
• rate = - 1/2 ^[A] / ^t = ^[B] / ^ t (rate expression)

• Ea - the minimum amount of energy required to initiate a chemical reaction
• -must have a collision in order to have a reaction
• -collisions must occur with enough kinetic energy to break the existing bonds
• if energy is too small the molecules will merely bounce off each other intact

• expresses the realtionship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers
• for rxn aA +bB --> cC
• the rate law is: rate = k[A]^x[B]^y
• x and y must be experimentally determined
• sum of the exponents gives "order"of reaction

• k rate =k[A]
• every time a reaction takes place, at a constant temperature, the same value for k is obtained

• rate = k[A] = - ^[A] / ^ t
• In [A]0 / [A]t = kt
• [A] at time = 0, [A] at time = t
• for gas-phase reactions we can replace the concentration terms with the pressures of the gaseous reactant

• t 1/2 - the time required for the concentration of a reactant to decrease to half of its initial concentration
• at t 1/2, [A] = 1/2[A]0
• t 1/2 = 0.693/k
• in 1st order reaction it is independant of the [A]

• A --> products
• rate = - ^[A] / ^ t = k[A]2 (squared)
• 1/[A]t = kt + 1/[A]0
• half life = 1 / k[A]0
• initial concentration does matter in 2nd order rxns

• rate = k[A]0
• rate = k
• half life = [A]0 / 2k

• molecules must collide in order to react
• rate increases with the # of collisions per second

• aka - transition state
• a temporary species formed by the reactant molecules as a result of the collision before they form the product

• more molecules have the kinetic energy necessary to overcome the acivation energy
• molecules move faster so they have more collisions
• collisions are more energetic

• k = Ae ^(-Ea / RT) OR In k = In A - Ea / RT
• Ea = activation energy in kJ/mol
• Ae = frequency factor (we usually don't know)

• In k1/k2 = Ea / R ( 1/ T2 - 1/ T1)
• for any order reaction

a series of simple reactions that represent the progress of the overall reaction on the molecular level

• the sequence of elementary steps that leads to product formation
• details how the reaction is thought to take place
• reactions are hypotheses (always subject to revision)

• appear in the mechanism of the reaction (elementary steps) but not in the overall balanced equation
• they are canceled as you add the elementary steps

the slowest step in the sequence of steps leading to product formation

an elementary step in which only one reacting molecule participates

an elementary step involving two molecules

• an elementary step involving 3 molecules
• they are very rare
• require 3 molecules to "slam" into each other at once

the number of molecules reacting in an elementary step

• a substance that increases the rate of a chemical reaction without itself being consumed
• reacts within an elementary step of the mechanism but is regenerated in a subsequent step
• it works by allowing an alternative route which lowers the activation energy

catalytic converter - gas vs solid

same phase

biological catalyst

• catalyst appears as a reactant and later gets spit back out
• intermediate appears as a product and gets sucked back in