1501 Kinetics - faculty.sdmiramar.edu

1 05.2015 Kinetics and Equilibrium

1501 Kinetics

Dr. Fred Omega Garces Chemistry 152 Miramar College

What determines the speed of a reaction?


Kinetics and Equilibrium Kinetics 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 (ΔG - ). C (graphite) D C (diamond)

Kinetics makes this reaction nearly impossible.


Reaction Favorable What influence the speed of a chem reaction ? Chemical changes occurs when: Reactant bonds are broken Product 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.


Collision Theory Theory which explains how molecules come together before product is formed.

•Deciding Factor on the speed of a reaction-

•Frequency Factor How “often” they collide

•Orientation (steric) alignment How “correct” they collide

•Impact of Collision How “hard” they collide


Collision Frequency Rate of a reaction depends on the number of encounter the reactants have to form product. One particle system 1 type collision Two particle system 4 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 A B B


Collision Orientation Rates of reaction depends on proper alignment of chemical species upon collision for product formation.

With proper alignment Product formation


Collision Impact For 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


Energy of Reaction

(b) Heat reactant

(+)Endothermic Reactant + 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

(-)Exothermic Reactant → Product + E


Activation barrier E 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)


Reaction (Profile) Coordinate Diagram What 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


Energy (Reaction) Coordinate Diagram An Exothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product)

In this reaction: Ereact > Eproduct Exothermic 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


Energy (Reaction) Coordinate Diagram An Exothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product)

In this reaction: Ereactant > Eproduct Exothermic reaction E transition St. = 50 kJ E reactant = 20 kJ E product = 10 kJ E reaction = E prod- E react

= -10 kJ Ef act = 30 kJ Er 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 =


Features of Reaction Coordinate Diagram An Endothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product)

A+B

A••••B

A––B

In this reaction: Ereact < Eproduct Endothermic reaction E transition St. = E reactant = E product = E reaction = E prod-E react

= Ef act = Er act =

25kJ

20kJ

45kJ


Features of Reaction Coordinate Diagram An Endothermic reaction between A & B to produce A-B A + B (Reactant) g A-B (Product)

A+B

A••••B

A––B 25kJ

20kJ

45kJ In this reaction: Ereactant < Eproduct Endothermic reaction E transition St. = 45 kJ E reactant = 20 kJ E product = 25 kJ E reaction = E prod-E react

= +5 kJ Ef act = 25 kJ Er act = 20 kJ

E transition St. = E reactant = E product = E reaction = E prod-E react

= Ef act = Er act =


Catalyst Affect Catalyst 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 .


Catalytic Converter A 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.


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.

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1501 Kinetics - faculty.sdmiramar.edu