Reaction Kinetics

Reaction Kinetics Factors affecting Rates Mr. Shields Regents Chemistry L13 L05

What affects Reaction Rates • Let’s look at the factors that can affect how fast • a reaction occurs. In other words, What affects reaction • Rate? • It turns out there are 5 things that are important: • Nature of the reactants • Temperature • Concentration • Surface area • Catalysts

Nature of the Reactants The basic question we must ask when we’re considering the Nature of Reactants is … “ how complex are the bonds to be broken and how complex are the bonds that need to be formed” And this in turn is affected by several other factors that affect the rate of reaction. Namely… 1) If there are a lot of bonds to be broken the reaction Will probably proceed more slowly than a rxn in which Only a few bonds need to be broken

Nature of the Reactants 2) HOMOGENEOUS RXNS are more likely to proceed more Quickly than HETEROGENEOUS RXNS For Example … Fe(s) + O2(g)  Fe2O3(s) heterogeneous reaction H2(g) + Cl2(g)  2HCl(g) homogeneous reaction (faster than heterogeneous rxn)

Nature of the Reactants • 3) If atoms or molecules can move freely about they are also • More likely to react more quickly • NOTE that in general reaction rates follow these trends … • Gas rxn rates > Liquid rxn rates >> Solid rxn rates b) Ionic rxns >> inorganic rxns > Organic rxns (ions in sol’n) Fe(s) + S(s) Let’s see why …

Nature of reactants case (a): Gas rxn rates > Liquid rxn rates >> Solid rxn rates - Higher molecular velocities (i.e. higher KE) - The higher the velocities the greater the number of collisions/unit time In case (b): Ionic rxns >> inorganic rxns > Organic rxns - ions have a charge and quickly attract ions of opposite charge (nearly instantaneous - happens in solution) - inorganic compounds form bonds by electron transfer. This is a faster process than bond formation by electron sharing (organic type compounds)

T1 T2 Energy exceeding EA # of Molecules Inc Temp (KE)  More molecules at T2 With KE > EA Temperature Collision theory says I need an effective collision for reaction To occur. What 2 factors determine whether a collision Will be effective? Recall that as KE (i.e. Temperature) increases so do the Number of molecules that have higher molecular velocities KE = ½ mv2

-Inc Velocity -Inc KE Temperature The general rule of Thumb is that for Every 10 deg increase In temperature Rxn rate increases 2x Slow Faster Fastest

Concentration Nature of reactants and temperature will affect rxn rate. Third on our list of factors affecting Reaction rate is CONCENTRATION Why would a change in concentration (up or down) affect Reaction rate? Sure… referring back to Collision Theory, we know we need an affective Collision for reaction to occur. AND … Increasing the number of molecules will have several positive Impacts.

Concentration • By Increasing the Concentration of reactants you … • Decreases time between collisions • And there are now Greater number of molecules in a given • volume with KE equal to or greater than the EA Slower Rxn Faster Rxn

Concentration Let’s consider what would happen if we put a match to Steel (Fe) wool in Air (O2 (18%), N2, CO2 etc)? Fe + O2 (air)  Fe2O3 + heat Now, what would happen if we put a match to steel wool in pure O2? The same thing but now the RATE is much greater Why???

Factors affecting Rates • So far we’ve discussed three factors affecting Reaction • Rates: • Nature of reactants • Temperature • Concentration We’re now going to discuss the last two factors affecting Reaction Rate… 4)Surface area 5) Catalysts

Surface Area Ever go camping? Ever sit around a campfire? Which burns faster on the fire, a large log or a log split into several smaller pieces? Obviously the log split into several pieces. Why? Of course … Because there’s more surface area to expose The wood to Oxygen needed for combustion. If we expose more surface area we increase the frequency Of collisions between the Oxygen and the wood - more collisions means more effective collisions and therefore a faster reaction rate

Surface Area Very fine powder or dust can sometimes explode Spontaneously when dispersed into the air (i.e. O2) Exploding Grain elevators and coal Mines are examples The very fine grain or coal dust suspended in the air reacts with O2 at extremely high rates due the extremely Large surface area producing rapidly expanding gases … In other words, An explosion! Iron powder

Catalysts • The thing to remember about catalysts is • They effect the rate of rxn EQUALLY • in the forward and reverse direction. • AND • 2) They do not affect the final equilibrium concentrations • - They ONLY affect how quickly a • reaction reaches Equilibrium • Concentrations

Catalyst Gathering the Catalyst & & Reactants Reactants Product Catalysts - Catalysts are also NOT CONSUMED in a chemical reaction - they are simply expeditors speeding up the reaction - after reaction they can be recovered unchanged In our bodies ENZYMES act as catalysts to speed up biochemical reactions.

No Catalyst Catalysts The way catalysts perform their trick is to simply ALTER THE REACTION PATHWAY So what do we mean by “the reaction pathway” Think of it this way … A reaction must climb an Energy hill (EA). The catalyst makes the hill the Reactants must climb A lot smaller Takes less E To get to the Other side

Catalysts This change in the reaction pathway is accomplished by… 1) Creating an INTERMEDIATE between the REACTANTS and the CATALYST 2) The INTERMEDIATE then becomes PRODUCT and REGENERATES the catalyst By creating this new Intermediate the EA Is decreased

Reaction Kinetics