Chemical Kinetics

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Chemical kinetics

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The branch of chemistry concerned with both the mechanisms and rates of chemical reactions.

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Reaction mechanisms

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Series of steps by which a reaction takes place

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Rate

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The change in a given quantity over a specific period of time. For chemical reactions, the quantity that changes is the amount or concentration of a reactant or product.

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Reaction rate

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Change in concentration of a reactant or product per unit time.

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Activation energy

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Minimum energy required to start a reaction. The amount of energy may be large or small.

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Activated complex

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Transitional structure where old bonds are breaking and new bonds are forming

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Concentration of reactants

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According to the collision theory, the rate of reaction is proportional to the frequency of collisions which depends on the concentration of reactants

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Law of mass action

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The rate of a chemical reaction is proportional to the product of the concentration of the reactants

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Mechanism

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The rate of a chemical reaction occurring in a sequence of steps is determined by the rate of the slowest step.

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Surface area

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More area to react

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Temperature

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A rough rule is that an increase in temperature of 10°C doubles the rate of reaction.

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Catalyst

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Substance that speeds up or slow s down a reaction without being used up itself.

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Enzymes

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Increase the rates of reactions; important in biological reactions

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Homogeneous catalyst

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One that is present in the same phase as the reacting molecules

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Heterogeneous catalyst

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Exists in a different phase, usually a solid

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Differential rate law

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Expresses how the rate depends on concentration (rate law)

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Double concentration rate doubles

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1st order

2^n =2 n=1

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Double concentration rate increases by factor of 4

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2nd order

2^n=4 N=2

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Double concentration rate increases by factor of 8

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3rd order

2n= 8 n=3

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Integrated rate law

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Concentration of the reactant as a function of time

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Foe first order plotting the natural log of concentration vs . Time always gives a straight line

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.

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Half -life of a first -order reaction

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Time required for a reactant to reach half it's original concentration

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Reaction mechanism

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Series of steps by which most chemical reactions occur

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Intermediate

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Species that is neither a reactant nor a product but that is formed and consumed during the reaction sequence.

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Elementary step

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Reaction whose rate law can be written from its molecularity

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Molecularity

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Number of species that must collide to produce the reaction indicated by that step

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Unimolecular step

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Reaction involving one molecule

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Bimolecular

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Reactions involving the collision of two species

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Termolecular

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Reactions involving the collision of three species

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Reaction mechanisms Myst satisfy two requirements

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1. The sum of the elementary steps must give the overall balanced equation for the reaction

2. The mechanism must agree with the experimentally determined rate law.

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Rate -determining step

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Slowest step. Once the slowest step is known in a series of steps, the rate law can be written from the molecularity.

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Arrhenius

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Postulated that the number of collisions having an energy greater than or equal to the activation energy is given by the expression.

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R=

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8.3145 J/k * mol