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1 Aug ‘17Kinetics and Equilibrium
10.01 Kinetics
Dr. Fred Omega GarcesChemistry 100Miramar College
What determines the speed of a reaction?
2 Aug ‘17Kinetics and Equilibrium
Kinetics and EquilibriumKinetics is a concept that address, how fast will the reaction occur
Equilibrium address, to what extent will the reaction proceed.
• The rate of iron rusting depends on reaction conditions.4 Fe (s) + 3 O2(g) + xH2O(g) D 2Fe2O3•XH2O (s)
Whether it occurs overnight or over many years, the reaction conditions influence how fast it occurs.
• Conversion of graphite to diamonds is thermodynamically favorable (DG - ).C (graphite) D C (diamond)
Kinetics makes this reaction nearly impossible.
3 Aug ‘17Kinetics and Equilibrium
Reaction FavorableWhat influence the speed of a chem reaction ?
Chemical changes occurs when: Reactant bonds are brokenProduct bonds are formed
For a reaction or a chemical change to take place, reactant bonds must be broken (require energy, endothermic) and product bonds must form (produce energy exothermic). Any factors which favors bond breakage and bond formation, favors the rate (kinetics) of the reaction.
4 Aug ‘17Kinetics and Equilibrium
Collision TheoryTheory which explains how molecules come together before product is formed.
•Deciding Factoron the speed of a reaction-
•Frequency FactorHow “often” they collide
•Orientation (steric) alignmentHow “correct” they collide
•Impact of Collision How “hard” they collide
5 Aug ‘17Kinetics and Equilibrium
Collision FrequencyRate of a reaction depends on the number of encounter the reactants have to form product.
One particle system1 type collision
Two particle system4 type collision
The greater the amount of reactant, the greater the number of encounters, the more likely a reaction takes place.
A A
A A B AB B
6 Aug ‘17Kinetics and Equilibrium
Collision OrientationRates of reaction depends on proper alignment of chemical species upon collision for product formation.
With proper alignment Product formation
7 Aug ‘17Kinetics and Equilibrium
Collision ImpactFor reaction to take place, species must collide with enough energy so that reactant bonds breaks. The faster the reactant energy (kinetic energy) the harder the collision.
Two car collision: Minor damage @ 5 mph, major damage if speed > 50 mph.
Speed of reactants must be high enough so that bonds can be broken
correct orientation but not enough energy to break bonds
correct orientation and enough energy to break bonds
8 Aug ‘17Kinetics and Equilibrium
Energy of Reaction
(b) Heat reactant
(+)EndothermicReactant + E ® Product
The energy of a reaction depends on the energies of the reactant relative that of the product. If the reaction is downhill (a) then consider energy as a product (Exothermic Reaction). If the reaction is uphill (b), then consider energy as a reactant (Endothermic)
(a) Heat product
(-)ExothermicReactant ® Product + E
9 Aug ‘17Kinetics and Equilibrium
Activation barrierE act - Minimum energy a reactant must possess in order to
convert to products.The activation barrier (Eact) can determine how fast a reaction occurs. In general, the higher the activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction.
Consider the process of
someone trying to roll a
boulder over a hill. The
higher the hill, the slower
the task. The lower the
hill the faster the
process. The height of
the hill correspond to the
energy of activation (Eact).
E act (forward)
10 Aug ‘17Kinetics and Equilibrium
Reaction (Profile) Coordinate DiagramWhat are the events at the molecular level when a reaction
takes place ? And what are the energies involved ?
Conversion of methyl isonitrile, CH3NC to acetonitrile, CH3CN
11 Aug ‘17Kinetics and Equilibrium
Energy (Reaction) Coordinate DiagramAn Exothermic reaction between A & B to produce A-BA + B (Reactant) g A-B (Product)
In this reaction:Ereact > EproductExothermic reaction
E transition St. =E reactant =E product =E reaction = E prod- E react
=
Ef act =Er act =
A+B
A••••B
A––B
20kJ
50kJ
10 kJ
12 Aug ‘17Kinetics and Equilibrium
Energy (Reaction) Coordinate DiagramAn Exothermic reaction between A & B to produce A-BA + B (Reactant) g A-B (Product)
In this reaction:Ereactant > EproductExothermic reaction
E transition St. = 50 kJE reactant = 20 kJE product = 10 kJE reaction = E prod- E react
= -10 kJ
Ef act = 30 kJEr act = 40 kJ
A+B
A••••B
A––B
20kJ
50kJ
10 kJ
E transition St. =E reactant =E product =E reaction = E prod- E react
=
Ef act =Er act =
13 Aug ‘17Kinetics and Equilibrium
Features of Reaction Coordinate DiagramAn Endothermic reaction between A & B to produce A-BA + B (Reactant) g A-B (Product)
A+B
A••••B
A––B
In this reaction:Ereact < EproductEndothermic reaction
E transition St. =E reactant =E product =E reaction = E prod-E react
=
Ef act =Er act =
25kJ
20kJ
45kJ
14 Aug ‘17Kinetics and Equilibrium
Features of Reaction Coordinate DiagramAn Endothermic reaction between A & B to produce A-BA + B (Reactant) g A-B (Product)
A+B
A••••B
A––B25kJ
20kJ
45kJ In this reaction:Ereactant < EproductEndothermic reaction
E transition St. = 45 kJE reactant = 20 kJE product = 25 kJE reaction = E prod-E react
= +5 kJ
Ef act = 25 kJEr act = 20 kJ
E transition St. =E reactant =E product =E reaction = E prod-E react
=
Ef act =Er act =
15 Aug ‘17Kinetics and Equilibrium
Catalyst AffectCatalyst lowers the activation energy so that minimum energy needed for the reaction to take place is lowered.
Consider the task of moving coal over a barrier. A pathway with a lower barrier is analogous to a reaction affected by a catalyst. The task becomes easier for a pathway with a lower barrier .
16 Aug ‘17Kinetics and Equilibrium
Catalytic ConverterA catalytic converter works by taking exhaust gases from the engine, including CO and NO, passing them through the catalytic converter, where they are converted to harmless CO2 and N2 by catalyzed reactions.
17 Aug ‘17Kinetics and Equilibrium
Summary:Reaction Conditions Affecting Rates
Conditions to Increase Rate:Nature of Reactants - Different substances will have different
reaction rates.Concentration of Reactant- More reactants result in more
collision yielding more products.Surface area- More surface area, the greater the chance for reactants to encounter to form product.
Temperature- Higher temperature result in more energetic collisions.
Catalyst- Lowers the activation energy for the reaction.