Kinetics

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Chapter 14

• Kinetics

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Kinetics

• Defined

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Factors that affect Rate

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Review

• In any equation
• Reactants ? products
• N2(g) 3H2(g) ? 2NH3(g)
• Equilibrium

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RATE

• Speed of the Reaction
• Measure either

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Rate Equation

Rate
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Rate Equation

Rate
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Rate Equation
Average Rate

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Instantaneous Rate
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Rate and Concentration

• Generally, rate decreases as time passes.
• Why?

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• Study rate by looking at
• NH4(aq) NO2-(aq) ? N2(g) 2H2O(l)
• See table 14.3 pg 516

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Rate Law

• Rate
• Must determine the value of k, the rate constant
• Choose one set of data
• Look at the units

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Practice

• Using previous rate law, what is the rate when
  the concentration of each reactant is 0.334 M?
  K 2.7 x 10-4 M-1s-1

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Reaction Order

• If rate law kreact 1mreact 2n
• The exponents are

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Units of rate constants

• Depend on the
• Rate units are
• Rate constant units must allow that

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For example

• Second order reaction
• Units of rate

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Determining Rate Law

• Must be determined experimentally
• Compare rates as concentration of reactants is
  changed

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Condition 1

• Change concentration

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Condition 2

• Change in concentration
• Double concentration
• Triple concentration

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Condition 3

• Double concentration
• Triple concentration

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Rate

• aA bB ? cC d D

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Dependence of Rate on Concentration
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Conditions for Rxn

• Orientation of molecules
• Energy, specifically, Kinetic Energy

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Reactive Collisions

• Both previous conditions met
• KE is easy to change

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Activation Energy, Ea

• Minimum energy needed for reactive collision
• Increase T means
• Ea is also energy required to make

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Activated complex

• When reacting molecules stick together before
  completing reaction
• They must have
• Intermediate species

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Reaction completes

• When activated complex breaks apart
• New molecules result from this process
• No activated complex left in system

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Exothermic Reactions

• Stored chemical energy
• Disorder
• Products at

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Endothermic Reactions

• Energy is
• Increase of
• Disorder
• Products at

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Direction of Reaction

• Forward
• Reverse
• One is favorable
• Tendency to increase
• Tendency to decrease

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Heat of reaction

• ?Hrxn
• Enthalpy of reaction
• Measure of the energy change
• Formula
• ?Hrxn Ef Er

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Exo or Endothermic?

• If ?H gt 0 kJ
• Reaction is
• sign tells you
• If ?H lt 0 kJ
• Reaction is
• - sign tells you

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Practice

• What is the ?Hrxn if the Energy of the forward
  reaction is 65 kJ and that of the reverse is 32?
• Is this reaction exo or endothermic?

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Catalysts

• Something that increases the rate of reaction
• Not consumed
• Provides a
• Helps in the formation of the activated complex

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Formation of Acid Rain

• Coal and sulfur
• Produce SO2 in cars and energy plants
• Reacts with water in air
• Needs NO as

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Catalysts

• Decrease Ea
• Do not change the
• Only change the
• Provides a

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Enzymes

• CO2 H2O ? H2CO3
• Carbonic anhydrase
• Enzyme (-ase is the key)
• Facilitates conversion
• Changes the rate by a factor of 3.5 x 106

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Enzyme/Substrate Complex

• Decreases
• Therefore increases
• High Ea
• Low Ea