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Rates of ReactionsRates of Reactions

Chemical kinetics is the Chemical kinetics is the study of the speed with study of the speed with which a chemical which a chemical reaction occurs and the reaction occurs and the factors that affect this factors that affect this speed. speed.

The rate of a reaction is The rate of a reaction is the speed at which a the speed at which a reaction happens. reaction happens.

All the molecular level All the molecular level of life some reactions of life some reactions go very rapidly and go very rapidly and other take forever.other take forever.

The rate of reaction, The rate of reaction, rr, , is defined to be the is defined to be the slope of the slope of the concentration-time concentration-time plot for a species plot for a species divided by the divided by the stoichiometric stoichiometric coefficient of that coefficient of that species. species.

A + 2 B A + 2 B == 3 C 3 C r = -dr = -d[[AA]]/dt = -1/2 /dt = -1/2

dd[[BB]]/dt= 1/3 d/dt= 1/3 d[[CC]]/dt/dt

The reactants have a The reactants have a negative slope, because negative slope, because they are being they are being consumed in the consumed in the reaction. Products have reaction. Products have a positive slope, a positive slope, because they are being because they are being formed in the reaction. formed in the reaction.

What factors affect the rate of a reaction?What factors affect the rate of a reaction?

The concentration of The concentration of the reactants. The the reactants. The more concentrated more concentrated the faster the rate. the faster the rate. Remember for Remember for gasses, increasing gasses, increasing the pressure simply the pressure simply increases the increases the concentration so concentration so that's the same thing. that's the same thing.

What factors affect the rate of a reaction?What factors affect the rate of a reaction? TemperatureTemperature. Usually reactions . Usually reactions

speed up with increasing speed up with increasing temperature.temperature.

Physical state of reactantsPhysical state of reactants. . Powders react faster than blocks - Powders react faster than blocks - greater surface area and since the greater surface area and since the reaction occurs at the surface we reaction occurs at the surface we get a faster rate. get a faster rate.

The presenceThe presence of a catalystof a catalyst (or (or inhibitor). A catalyst speeds up a inhibitor). A catalyst speeds up a reaction, an inhibitor slows it reaction, an inhibitor slows it down. down.

Reaction with High Rates: Reaction with High Rates:

1. FeCl1. FeCl33 + KSCN → Fe( SCN) + KSCN → Fe( SCN)33 + 3 KCl + 3 KCl

2. Faro’s snake2. Faro’s snake

Reaction with Middle Rates:Reaction with Middle Rates:

FeClFeCl3 3 + Na+ Na22SS22OO3 3 → [ Fe( S → [ Fe( S22OO33))22 ] ]--

Halloween reaction / Halloween reaction / Old Old NassauNassau ReactionReaction

The reaction in this The reaction in this experiment takes place inexperiment takes place inseveral steps [5].  several steps [5]. 

First, sodiumFirst, sodiummetabisulphite metabisulphite reacts with water to formreacts with water to formsodium hydrogen sulphite: sodium hydrogen sulphite: NaNa22SS22 O O55 + H + H22O ==>O ==>2NaHSO2NaHSO33                                                                        

           Hydrogen sulphite ionsHydrogen sulphite ionsreduce iodate(V) ions toreduce iodate(V) ions toiodide ions: iodide ions: IOIO33

-- + 3 HSO + 3 HSO33--==>==>

II-- + 3 SO + 3 SO442+2+3H3H++

                                                                         

Once the concentration of Once the concentration of iodide ions is large iodide ions is large enough that the solubility enough that the solubility product of HgIproduct of HgI22 is is exceeded, orange exceeded, orange mercury(II) iodide solid is mercury(II) iodide solid is precipitated until all of the precipitated until all of the HgHg2+2+ ions are ions are used up (provided that used up (provided that there is an excess of Ithere is an excess of I-- ions). ions).

  HgHg2+2+ + 2 I + 2 I-- ==> HgI ==> HgI22 (orange or yellow)(orange or yellow)

                                                                 

If there are still I- and IO3 - ions in the mixture, the iodide-If there are still I- and IO3 - ions in the mixture, the iodide-iodate reactioniodate reaction

IOIO33--+5 I+5 I-- + 6 H + 6 H++ ==> 3 I ==> 3 I22 + 3 H + 3 H 22OO

                                                                        takes place and the blue starch-iodine complex is formed, takes place and the blue starch-iodine complex is formed, II22 +starch ==>complex (blue or black)   +starch ==>complex (blue or black)  

                                                  

        

Reactions with low rates:Reactions with low rates:

1. Pb( CH1. Pb( CH33COO)COO)22 + 2KI = PbI + 2KI = PbI22 +CH +CH33COOKCOOK

2.NiCO2.NiCO33+2 dmg= +2 dmg= [[Ni( dmg)Ni( dmg)22]]2+2+ + CO + CO332-2-

3.CoSO3.CoSO44+2NH+2NH44SCN=Co(SCN)SCN=Co(SCN)22 + (NH + (NH44))22 SO SO44

Thank you for your attentionThank you for your attention