Chemical Kinetics An Introduction S 2 O I -  I S O C O ( g ) + N O 2 ( g )  C O 2 ( g ) + N O H 2 O 2 ( a q )  H 2 O ( l ) + O 2 ( g )

Questions about reactions 1.What’s happening? The chemical equation ? 2.How fast is it happening? 3.To what extent does it occur? Kinetics Equilibrium Chapters IO HSO 3 - → I SO H + + H 2 O H 2 O 2 (aq) → H 2 O(l) + O 2 (g) -- slow H 2 O 2 (aq) + catalyst → H 2 O(l) + O 2 (g) -- FAST HC 2 H 3 O 2 + H 2 O C 2 H 3 O H 3 O +

Chemical Kinetics The study of reaction rates and the sequence of steps by which a reaction occurs A Definition

Rates of Reaction Very fast –Explosions, neutralizations Almost instantaneous Slow –Formation of diamonds, decay of 235 U Up to millions of years Medium –Cooking, rusting minutes to years

Expressing Reaction Rates Speed of a car: mph = For a chemical reaction we want to track concentration of products or reactants over time: Concentration of reactants decreases Concentration of products increases

For the reaction A → B Rate of reaction = Or… Expressing Reaction Rate Cont’d Note the sign: A is disappearing

a A + b B → c C + d D Rate of reaction = rate of disappearance of reactants = Δ[C] ΔtΔt 1 c = Δ[D] ΔtΔt 1 d Δ[A] ΔtΔt 1 a = - Δ[B] ΔtΔt 1 b = - = rate of appearance of products General Rate of Reaction

Time(s) [O 3 ](mol/L) x x x x x x x10 -5 The reaction between ethylene and ozone: C 2 H 4 (g) + O 3 (g) C 2 H 4 O (g) + O 2 (g) Rate = - = -  [C 2 H 4 ]  t  [O 3 ]  t Rate = - = x mol/L·s (1.10x10 -5 mol/L) - (3.20x10 -5 mol/L) 60.0 s s  [O 3 ]  t Rate = - An Example

This rate is the average rate for a time period Does not show that rate is changing with time Does not show rate at a given instant

Plot of [O 3 ] vs. Time

But Wait! There’s More… Rate is dependent on concentration! Can see this experimentally

Plot of [C 2 H 4 ] and [O 2 ] vs. Time

Instantaneous rate Use smaller and smaller increments of time The slope of a tangent line to the curve at any point is the instantaneous rate Note that reaction rate usually refers to the instantaneous rate

Plot of [O 3 ] vs. Time

Reaction Rate Law For a chemical reaction: aA + bB +... → cC + dD +... The rate law for the forward reaction has the form: Rate = k [A] m [B] n...  k = the reaction rate constant  exponents m & n are the reaction orders  defines how rate is affected by concentration More on this next week For example, if the rate doubles when the concentration of A doubles, the rate depends on [A] 1, so m = 1; if the rate quadruples when the concentration of B doubles, the rate depends on [B] 2, so n = 2.

So, what affects reaction rate?  Concentration Molecules must collide in order to react. Reaction rate is proportional to the concentration of reactants. Rate = k (collision frequency) = k (concentration)

Factors Affecting Reaction Rate  Physical state Molecules must mix in order to collide. The physical state (solid, liquid, gas) will affect frequency of collisions, as well as the physical size of droplets (liquid) or particles in the case of solids.

Factors Affecting Reaction Rate  Temperature Molecules must collide with enough energy to react. Raising the temperature increases the reaction rate by increasing the number of collisions per time unit, and especially, the energy of the collisions.

Factors Affecting Reaction Rate  Nature of the reactants Some species are more reactive than others. You have seen this with the periodicity of reactivity. For example the reactivity of the group 1 metals.

Factors Affecting Reaction Rate  Presence of a catalyst Catalysts can provide alternate, lower energy, reaction pathways. Catalysts generally reroute the pathway of a chemical reaction so that this “alternate” path, although perhaps more circuitous, has a lower activation energy for reaction than the un-catalyzed reaction.

Lab This Week Explore several of the factors which affect reaction rate Perform in any order Perform either B or C CHECK REAGENT CONCENTRATIONS! Caution with strong acids Use waste beakers Part A.1 use 2M H 3 PO 4 Part D—Note H 2 O 2 decomposes slowly on its own.

Evidences of a Chemical Reaction Color change Precipitate formation Temperature change Gas evolution Formation of a weak electrolyte