Factors that Affect Reaction Rates

Transcription

1 Factors that Affect Reaction Rates Preface: There are 2 kinds of reactions: Homogeneous reactions - all reactants are in the same phase (don't consider products) eg.) 3H 2(g) + N 2(g) 2NH 3(g) Ag + (aq) + Cl - (aq) AgCl (s) (both gases) ( both (aq) ) Heterogeneous Reactions - more than one phase in reactants. eg.) Zn (s) + 2HCl (aq) H 2(g) + ZnCl 2(aq) C (s) + O 2(g) CO 2(g) (2 diff. phases) (2 diff. phases) A. Effect of temperature - when molecules move faster more collisions per unit time faster rate - also - when they move faster they collide with more kinetic energy. (hit harder) Kinetic energy distributions # of Molecules Activation Energy (E a ) This shaded region represents the molecules which have sufficient energy for a successful collision A few molecules are SLOW (low KE) A large number of molecules have medium KE. A few molecules are moving FAST (high KE) Kinetic Energy

2 If temperature is increased, the average KE increases so that there are less slow (low KE) molecules and more fast (high KE) molecules Note: The TOTAL AREA UNDER THE CURVE is the same for the high temperature as for the low temperature as the # of molecules remains unchanged. Number of Molecules Curve at Lower Temp. (T 1 ) Activation Energy (E a ) Curve at Higher Temp. (T 2 ) Kinetic Energy At Temperature T 1 (lower temp.), only these molecules have sufficient energy for a successful collision At Temperature T 2 (higher temp.), these molecules also have sufficient energy for a successful collision Generally, if the temperature is increased by 10 o C reaction rate will about double. (ie. about twice the number of molecules have sufficient KE for a successful collision.) Temperature s role - the temperature determines how many (or what fraction of the) molecules will have energy > Ea (to make it over the barrier & have a successful collision). At Higher Temperatures: Number of Molecules Activation Energy (E a ) Curve at Higher Temp. (T 2 ) Kinetic Energy At a higher Temperature (T 2 ), there are less molecules which don t have enough KE for a successful collision. At Temperature T 2 (higher temp.), there are more molecules which have sufficient energy for a successful collision

3 At the higher temperature, a greater fraction of the molecules have sufficient energy to make it over the Activation Energy barrier. (ie. a greater fraction of the molecules posses enough energy to form the Activated Complex): A few of us don t have the energy to make it over the barrier! Yaaay! A greater fraction of us made it this time! Potential Energy REACTANTS PRODUCTS Progress of Reaction Looking at the diagram above, you can see that at a higher temperature, a greater fraction of the molecules have sufficient energy to make it over the barrier. Therefore the reaction is faster. So if you study the graphs on the previous pages, you will see that: Increasing the temperature increases the fraction of molecules which have sufficient energy to form the Activated Complex (ie. sufficient energy to make it over the activation energy barrier.) NOTICE that a change in temperature does NOT change the Potential Energy diagram. Temperature does NOT affect the Activation energy or the ΔΗ!! B. Effect of concentration low conc. both high conc. blue high conc. both low conc. red low chance higher chance very high chance of collision of collision of collision (slow reaction) (faster reaction) (much faster reaction)

4 C. Effect of Catalysts catalyst- an introduced substance which produce an alternate mechanism with a lower activation energy. PE DIAGRAM SHOWING ONLY THE UNCATALYZED REACTION PE (kj) REACTANTS PRODUCTS Progress of Reaction PE DIAGRAM SHOWING THE UNCATALYZED AND THE CATALYZED REACTION 2 00 PE (kj) E a (uncatalyzed rx.) Uncatalyzed route 12 0 Catalyzed route REACTANTS E a (catalyzed rx.) ΔΗ PRODUCTS Progress of Reaction energy required (Ea) is less with the catalyst, so at the same temperature, more molecules can make it over the barrier so reaction rate speeds up catalyzed reactions usually involve more steps but it s highest Ea (highest bump) is never as high as the uncatalyzed reaction a catalyst NEVER changes the PE of reactants of products - only the route between them. (no change in ΔΗ! ) the catalyst is not a reactant, therefore, it is not consumed in the reaction

5 Heterogeneous Catalysts work by... providing a surface for the reactant molecule to adhere to and begin breaking bonds during the collision eg.) formation of water using a platinum electrode eg.) synthesis of ammonia for fertilizer from N 2 and H 2 using iron catalyst (*Haber process) eg.) enzymes (biological catalysts) for breaking down starch to glucose Homogeneous Catalysts work by... mixing throughout the reaction mixture eg.) 2H 2 O 2(l) 2H 2 O (l) + O 2(g) (very slow uncatalyzed) -add some KI (s) (I - is the catalyst) Catalyzed Mechanism: D. Effect of Surface area (affects heterogeneous reactions only!) -when 2 different phases react, reaction can only take place on surface. solid liquid solid liquid gas gas - increase surface area by cutting solid into smaller pieces (liquids in smaller droplets) These "inside" surfaces are added Chunk (small surface area) Sliced (larger surface area) Powder (huge surface area!)

6 E. Effect of Stirring (affects heterogeneous reactions only!) Ensures the reactant particles are in close proximity to each other for a collision to occur F. Effect of Light For some reactions, the intensity of visible or ultraviolet light upon the reactants can affect the rate of a reaction. Eg.) ozone, O 3, is decomposed by ultraviolet light: 2 O 3 (g) 3O 2 (g) Some points 1.) Temperature affects rate of all reactions 2.) Concentration only affects (aq) or (g) reactants 3.) Surface area - affects only heterogeneous reactions. 4.) Stirring - affects only heterogeneous reactions. Affecting the Rate of Reactions involving Fuels: - concentration of O 2 important - to increase combustion rate - increase [ O 2 ] - increase surface area - increase temperature - catalyst (wood stoves etc) - to decrease combustion rate - water on fire -smothers it (decreases O 2 ) - lowers the temperature - fire retardant - forest fires - children's clothing - airplane fuels - when spilled