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Kinetics and Equilibrium
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Kinetics• The branch of chemistry known as
chemical kinetics is concerned with the rates of chemical reactions and the mechanisms by which they occur.
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• The rate of a chemical reaction is measured in terms of the # of moles of a reactant consumed (or produced) in a unit of time.
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• The mechanism of a chem rxn attempts to describe the order of events in which the reaction takes place.
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Energy and Reaction Rates• Energy is required to initiate a rxn.
Activation energy is defined as the min energy required to make a rxn occur.
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• The heat of reaction illustrates the difference in heat content before and after the rxn.
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• It is also the difference in potential
energy between the prod and reactants.
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• If the products have less energy than the reactants, the reaction is considered exothermic.
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• Endothermic reactions occur when the products have more energy than the reactants. They are given by a positive heat of rxn.
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Potential Energy Diagram• We can show the relationship between
activation energy and heat of reaction for a given rxn on a graph.
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• We place PE on the vertical axis, and the “reaction coordinate” on the horizontal axis.
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• In order for any rxn to occur, an activated complex must be formed.
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• This is an intermediate product, and may be considered to be a temporary association of atoms or molecules which when finished, will give us our products.
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• The highest point on the curve of a PE diagram shows the PE of the intermediate product (activated complex).
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• Once the formation of an activated
complex has occurred, the rxn can continue to give the products.
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• The PE of the products will be lower than the peak energy of the activated complex.
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• If the PE is higher for the products, the rxn was endothermic, and if the PE is higher for the reactants, the rxn was exothermic.
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• The heat of reaction is strictly the difference between the PE’s of the products and reactants, and is the same regardless of the amounts of each involved.
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Factors that Affect Reaction Rates• Chem rxns depend on collisions
between reacting species - atoms, molecules, ions, etc.
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• A rxn only occurs when adequate energy has been given to form an activated complex.
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• Generally speaking, anything that causes more collisions to occur will increase the rate of reaction.
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Nature of the Reactants• The nature of the reactants, with respect
to the types of bonds involved, contributes to determining the rate of rxn.
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• If few bond rearrangements are necessary, the rxn will occur quickly, such as with ionic substances in water.
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• If bonds need to be broken or newly created, more time is necessary, such as oxygen and hydrogen bonding to form water.
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Concentration• If the concentration of one or more
reactants is increased, the rate of rxn increases. Compressing a gas has the same effect.
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Temperature• An increase in temperature will increase
the rate of reaction for two reasons - increased frequency of collisions and increased energy values in each collision.
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Reaction Mechanism• Most reactions do not occur in a single
step, but in a series of steps called the reaction mechanism.
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• Each individual step is generally a two-particle collision, and the number of steps and their individual complexity affects the rate of the overall rxn. The more steps, the slower the rxn.
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Catalysts• Fast reactions have low activation
energies, while slow reactions have high activation energies.
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• The addition of a catalyst to a system changes its activation energy, thereby changing its rate of rxn.
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• A catalyzed rxn requires less activation energy, without changing the overall rxn.
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• Even though the activated complexes in a catalyzed rxn have less PE than a non-catalyzed rxn, the heats of rxn are the same.