Chemical Kinetics (Reaction Rates) Chemical Kinetics: the study of how fast or slow a reaction takes place

Chemical Reactions occur at different rates! There are slow reactions like a copper roof oxidizing (turning green) over a period of several years. When you have a lot of bonds to break the reaction will be slow. Covalent bonds are hard to break.

There are fast reactions: which require the least number of bonds to be broken. Fast reaction involve ions. Precipitation reactions are also fast. Pb2+ (aq) + 2I1- (aq) --> PbI2 (s) Neutralization reactions are also fast. H2SO4(aq) + 2 KOH (aq) --> 2H2O (l) + K2SO4 (s)

Rate of Chemical Reactions The rate of a chemical reaction is a ratio of the amount of change in a substance to the amount of time required for that change to take place. a Reactant b Product rate = - 1 D [Reactant] = 1 D [Product] D means change a Dt b Dt Example: BaCl2 + Na2SO4 BaSO4 + 2NaCl The rate is - 1 D[BaCl2] = -1 D[Na2SO4] = 1 D[BaSO4 ] = 1D[NaCl] 1 Dt 1 Dt 1 Dt 2 Dt

Mathematical Expression of a Rate Law Suppose you are asked to find the mathematical expression of the rate for the following reaction: C3H8(g) + 5O2 (g) --> 3CO2 (g) + 4 H2O (g) rate law= k [C3H8] x [O2] 5 k is the rate constant *note that only the reactants are in the expression*

Reaction Process Most chemical reactions take place in a series of steps called reaction mechanisms. Example: Step 1: Cr(CO)6 --> Cr(CO) 5 +CO slow Step 2: Cr(CO)5 + NH3 --> Cr(CO)5NH3 fast RxN : Cr(CO)6 + NH3 --> Cr(CO)5NH3 +CO The slowest step determines the rate of the reaction. It is called the rate determining step.

Collision Theory and Activation Energy

The collision theory states that for a product to be made the reactants must collide with enough energy and with the right geometry. If the molecules collide with enough energy, then the atoms will rearrange and form a new product. This is called an effective collision.

REACTION PROFILE activated complex activation energy Ea reactants products energy course of reaction

ACTIVATED COMPLEX: is the temporary, unstable arrangement of particles present at the highest point in a chemical reaction profile. ACTIVATION ENERGY (Ea): is the minimum energy with which particles must collide in order for the collision to be effective (resulting in the formation of a NEW substance)

Combustion Reactions A combustion reaction is exothermic. A combustion reaction is a reaction between a fuel and oxygen. If there is hydrogen within the molecules of the fuel, then water will be produced C8H18 + O2 what are the products? 2 C8H18 + 25 O2 16 CO2 + 18 H2O

Fire Triangle (review of sec 4) You need 3 things for combustion: a fuel (wood or propane) an oxidizing agent (oxygen- 21% in air) a certain temperature If you are missing any of these components, the rate will be zero, the fire will stop.