CHEMICAL KINETICS

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CHEMICAL KINETICS

• Theories of Chemical Kinetics
• Reaction Rates Stoichiometry
• Reaction Mechanisms, Rate Law, Order of Reaction
• Factors Affecting the Rate of Reaction
• Zero, 1st 2nd Order Reactions

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Chemical Kinetics

• the area of chemistry concerned with the speeds
  or rates, at which reactions occur.

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Theories of Chemical Kinetics

• Collision Theory
• Transition State Theory

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Collision Theory

• the rate of reaction is directly proportional to
  the number of molecular collisions per unit time
  (collision frequency).
• notes
• 1. a chemical reaction can occur only when
  reacting molecules collide with one another.
• 2. when molecules collide, only a fraction of
  the collisions among molecules lead to chemical
  reaction.

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Types of Collisions

• Effective Collisions- will result to a
  chemical reaction
• Ineffective Collisions- no reaction will take
  place

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Collisions between reactant molecules must be
sufficiently energetic for reaction to occur, as
illustrated by a hypothetical reaction A2 B2
2AB. (a) If the reactants are moving too slowly,
they merely bounce off one another and no
reaction occurs. (b) If the reactants are moving
energetically enough and they are oriented
correctly, reaction can occur.
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Requirements for an Effective Collision

• Activation Energy (Ea)- minimum amount of energy
  that molecules must bring to their collisions for
  a chemical reaction to occur.
• Proper Orientation of Molecules

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Two possible ways that Cl atoms and NOCl
molecules can collide. (a) If molecules are
oriented properly, a sufficiently energetic
collision will lead to reaction. (b) If the
orientation of the colliding molecules is wrong,
no reaction occurs.
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Transition State Theory

• the reactant molecules as a result of the
  collisions form a temporary species (activated
  complex) before they form a product.

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Activated Complex

• a hypothetical species believed to exist in an
  intermediate state (transition state) that lies
  between the reactants the products. It is a
  temporary state where bonds are in the process of
  breaking forming
• it is a very unstable species with a high
  potential energy

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• in the hypothetical reaction,
• A2 B2 ? 2AB
• A B ? AB ? A ? B
• ? ? A ? B
• A B AB
• reactants activated products
• complex

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Reaction Profile for Exothermic Reaction
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Reaction Profile for Endothermic Reaction
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Reaction Rates

• the speed of a chemical reaction
• the decrease in the concentration of a reactant
  or the increase of the concentration of a product
  with time

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Expressing the Rate of Chemical Reaction

• Rate
• change in concentration ? concentration
• time ?
  time
• unit is M/s or moles/L-s

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• In the hypothetical reaction,
• A ? B
• Rate of disappearance of A
• R - ?A
• ?t
• Rate of appearance of B
• R ?B
• ?t

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Reaction Rates Stoichiometry

• Consider the reaction
• 4NH3(g) 5O2(g) ? 4NO(g) 6H2O(g)
• the rate of reaction expressed in terms of any of
  the reactants or products
• rate of rxn -1 ?NH3 - 1 ? O2
• 4 ?t 5 ?t
• 1 ?NO 1 ?H2O
• 4 ?t 6 ?t

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Exercises

• Suppose that at some point in the reaction,
• A 3B ? 2C 2D
• A 0.9986 M, and that 13.20 min later, A
  0.9746 M. What is the average rate of reaction
  of A during this time period, expressed in M/s?
• In the reaction 2A ? 3B, A drops from 0.5684 M
  to 0.5522 M in 2.50 min. What is the average
  rate of formation of B during this time interval,
  expressed in M/s?

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Reaction Mechanisms

• a detailed description of a chemical reaction or
  the manner in which reaction is taking place in a
  series of elementary steps.

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Classification of Reactions

1. Elementary Reaction- a single step reaction
   mechanism
2. Non-elementary Reaction- reaction mechanism of
   two or more steps

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• The reaction between chlorine nitric oxide
  actually occurs via two elementary steps
• 2NO(g) Cl2(g) ? 2NOCl(g)
• Elem step 1 NO(g) Cl2(g) ? NOCl2(g)
• Elem step 2 NOCl2(g) NO(g) ? 2NOCl(g)
• Overall Rxn 2NO(g) Cl2(g) ? 2NOCl(g)

• note
• chemical species such as NOCl2 are called
  intermediates and are eliminated from the overall
  reaction.

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Law of Mass Action

• the rate of a chemical reaction is directly
  proportional to the reactant concentrations
  raised to a certain power called the reaction
  order.

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• stated mathematically,
• rate ? reactantsorder
• rate kreactantsorder
• k Ae-Ea/RT (Arrhenius Equation)
• where
• k - rate constant
• A - collision frequency or frequency factor
  (s-1)
• Ea - activation energy (J)
• R - gas constant (8.314 J/mol-K)
• T - absolute temperature (K)

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• Consider the hypothetical reaction
• aA bB ? cC dD
•  
• rate kAxBy
• for elementary reactions
• x a, y b (the molecularity is equal to the
  order)
•  
• for non-elementary reactions
• x ? a, y ? b (the molecularity is NOT equal to
  the order thus should be determined)

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Notes

• the overall reaction order is equal to the sum
  of the orders.
• for non-elementary reactions, the slowest step is
  the rate-determining step.

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The flow of traffic on a toll road is limited by
the flow of traffic through the slowest toll
plaza. As cars pass from point 1 to point 3, they
pass through plazas A and B. In (a), the rate at
which cars can reach point 3 is limited by how
quickly they can get through plaza A getting
from point 1 to point 2 is the rate-determining
step. In (b), getting from point 2 to point 3 is
the rate-determining step.
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Exercise

• Given the following two-step reaction
• Step 1 A2B ? A2 B
• Step 2 A2B B ? A2 B2
• The overall reaction order was found to be
  r kA2B
• a. write the overall reaction
• b. which of the species are intermediate
• c. which is the rate determining step? (i.e.,
  the slowest step)

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Method of Initial Rates

• 1. Kinetic data for the reaction
• 2HgCl2 C2O42- ? 2Cl- 2CO2 Hg2Cl2
• Determine
• a. the order of reaction with respect to HgCl2
  
  C2O42-
• b.  the overall order of reaction
• c. the rate constant, k.

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• 2. Consider a hypothetical reaction
• A B ? C
• Determine
• a. the order of reaction with respect to A B
• b. the overall order of reaction
• c. the rate constant, k.
• d. what is the rate of formation of C when
  A is 1.0 M B is 1.5 M

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Factors Affecting the Rate of Chemical Reaction

• Reactant Concentration (or Pressure for Gases)
• most chemical reactions proceed faster if the
  concentration (or pressure) of one or more of the
  reactants is increased.
• example steel wool burns with difficulty in air
  (20 O2), but bursts into a brilliant whit flame
  in pure oxygen.

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• Temperature
• the rates of chemical reactions increase as
  temperature is increased.
• example food spoilage is prevented when
  refrigerated since the rate of bacterial growth
  is lowered at low temperature.

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• Nature of Reactants
• reactants differ in composition, size/surface
  area, reactivity, etc and some reacts faster or
  slower relative to others.
• example medicine in the form of tablet dissolve
  in the stomach enter the bloodstream more
  slowly than the same medicine in the form of a
  fine powder.

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• Catalysts
• rate of reactions can be increased by adding a
  substance known as catalyst.
• Fcatalyst is a substance which increases the
  rate of reaction by lowering the activation
  energy without being used up.
• example the digestion of food we eat is
  catalyzed by biological catalysts called enzymes.

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Exercises

• Among the factors that influence reaction rates,
  which is involved in the following
• Sodium reacts more rapidly than iron
• A bush fire reacts more rapidly on a windy day
  than on a still day
• In the reaction, C2H4(g) H2 ? C2H6, the
  reaction is carried out faster in the presence of
  Pt.
• N2O5 decomposes rapidly at a temperature above
  room temperature

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Zero, 1st 2nd Order Rxns

• Consider the hypothetical reaction,
• A ? products

Order Rate Graph Unit of k
Zero R kA0 k A vs time is straight line with negative slope A/time (ex M/s)
 
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• A ? products

Order Rate Graph Unit of k
1st R kA1 kA ln A vs time is straight line with negative slope time-1 (ex s-1)
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• A ? products

Order Rate Graph Unit of k
2nd R kA2 1/A vs time is a straight line with positive slope A-1time-1 (ex M-1s-1)
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• END
• 1st Long Exam
• MAY 10, 2003
• Coverage Thermodynamics
• Chemical Kinetics