Collision Theory For a rx to occur reacting molecules must collide so 1 - The old bonds “loosen up”, or start to break apart 2 - The correct orientation to allow new bonds to form e- clouds of atoms trying to bond together must touch each other

Collision Theory NO + NO 3  2NO 2 2 requirements for a chem reaction to occur from a collision between molecules 2 – proper alignment of molecules so new bonds can form 1 – enough energy to break the old bonds

Reaction rates How fast reactants products Rates are determined experimentally Only by experiment R = -  [reactants] = +  [products]  t  t

3 Types of Reaction Rates Average rate Slope of line between 2 data points Instantaneous rate Slope of tangent line at that moment Initial rate Instantaneous rate at start of the reaction Fast rate Rates slow down with time Decreased conc of reactants

Rate Law aA dD r = k[A] m nature reactants fast rx = large k slow rx = small k effects of temp t k therefore direct relationship

Rate Law aA + bB cC + dD r = k[A] m x [B] n nature reactants fast rx = large k slow rx = small k effects of temp t k therefore direct relationship

Reaction order 0, 1 st, or 2 nd definition r = k[A] m x [B] n m is order for A Order is usually whole # 0, 1, or 2 n is order for B m + n is overall order

Rx Mechanisms Terms: Reaction mechanism series of steps (rxs) from original reactants to the final product Elementary step Each single step (reaction) in the pathway Rx intermediate Subst. formed during the rx mechanism that is used up in subsequent steps and doesn’t show up as a reactant or product in the overall rx Rate determining step Slowest step in the reaction mechanism

Rx mechanisms Reactions probably occur in a series of steps Add individual steps together = overall equation Rate determining step: The slowest step in the reaction mechanism A B C D E fast slow fast fast

Factors that Influence Reaction Rates Nature reactants Conc. Of reactants Surface area Catalyst/inhibitor Temp

Nature of Reactants and Products How extensive is the rearrangement of the e- clouds of reactants and products Precipitation reactions No rearrangement of e- clouds Iron rusting (oxidizing) A lot of rearranging of e- clouds

Energy Diagrams Activation energy Energy required to start a chem rx Activated complex High energy, short lived particle formed by the collision of reactant molecules Can from products or reform reactants Depends on whether the collision causes a rx or not

Effects of Catalyst Reduce the activation energy

Reaction Mechanism Each step in reaction pathway is called an elementary (intermediate) step Elem step can contain reaction intermediates (or intermediate products) Products of elementary step that is used up in a subsequent elementary step They don’t show up in the overall reaction A ---> B ---> C ---> D ---> E ---> F fast slow fast Overall reaction