2. 1 Factors affecting Rates Mr. ShieldsRegents Chemistry L13 L05

3. 2 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: 1)Nature of the reactants 2)Temperature 3)Concentration 4)Surface area 5)Catalysts

4. 3 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

5. 4 Nature of the Reactants 2) HOMOGENEOUS RXNS are more likely to proceed more Quickly than HETEROGENEOUS RXNS For Example … Fe(s) + O 2 (g)  Fe 2 O 3 (s) heterogeneous reaction H 2 (g) + Cl 2 (g)  2HCl(g) homogeneous reaction (faster than heterogeneous rxn)

6. 5 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 … a)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 …

7. 6 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)

8. 7 Temperature Recall that as KE (i.e. Temperature) increases so do the Number of molecules that have higher molecular velocities KE = ½ mv 2 Inc Temp (KE)  # of Molecules T1 T2 Energy exceeding E A More molecules at T2 With KE > E A Collision theory says I need an effective collision for reaction To occur. What 2 factors determine whether a collision Will be effective?

9. 8 Temperature The general rule of Thumb is that for Every 10 deg increase In temperature Rxn rate increases 2x -Inc Velocity -Inc KE Slow Faster Fastest

10. 9 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.

11. 10 Concentration By Increasing the Concentration of reactants you … 1)Decreases time between collisions 2)And there are now Greater number of molecules in a given volume with KE equal to or greater than the E A Slower Rxn Faster Rxn

12. 11 Concentration Let’s consider what would happen if we put a match to Steel (Fe) wool in Air (O 2 ( 18% ), N 2, CO 2 etc)? Fe + O 2 (air)  Fe 2 O 3 + heat Now, what would happen if we put a match to steel wool in pure O 2 ? The same thing but now the RATE is much greater Why???

13. 12 Factors affecting Rates So far we’ve discussed three factors affecting Reaction Rates: 1)Nature of reactants 2)Temperature 3)Concentration We’re now going to discuss the last two factors affecting Reaction Rate… 4) Surface area 5) Catalysts

14. 13 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

15. 14 Surface Area Very fine powder or dust can sometimes explode Spontaneously when dispersed into the air (i.e. O 2 ) Exploding Grain elevators and coal Mines are examples The very fine grain or coal dust suspended in the air reacts with O 2 at extremely high rates due the extremely Large surface area producing rapidly expanding gases … Iron powder In other words, An explosion!

16. 15 Catalysts The thing to remember about catalysts is 1)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

17. 16 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. Catalyst Gathering the Catalyst & & Reactants Reactants Product

18. 17 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 (E A ). The catalyst makes the hill the Reactants must climb A lot smaller No Catalyst Takes less E To get to the Other side

19. 18 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 E A Is decreased