Kinetics and Equilibria

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Rate Equations

Rate= [A]m[B]n    -m&n are orders or reaction (1,2 or 0)   -A & B are Reactants   -K is Rate Constant

- If [A] changes and the rate stays the same ---> 0 order                                                                      -If the rate is proportional to [A] ---> 1st order                                                                                        -If the rate is proportional to [A]2 (if [A] doubles, the rate will double) ----> 2nd order

Overall order is m + n

Rate Equations                                                                                                                                     e.g. CH3COCH3 (first) + I2 (zero)     -- H+ (first)----> CH3COCH2I  + H+  + I-                                Rate = K[CH3COCH3}1[H+]1[I2]0  ---->  Rate = K[CH3COCH3][H+]

Rate Constant                                                                                                                                -Write out equations       -Insert the numbers that you have     -Rearrange the quation to calculate what you want     -work out the units.

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Rates Continued

Initial Rate                                                                                                                                       -Find using a concentration-time graph, calculate the curve of the graph by using a tangent.      -Then divide the reactant part of the tangent by the time part.

Rate Determining step                                                                                                        -Mechanisms have one or a series of steps.      -In a series of steps the overall rate is determined by the step with the slowest rate ----> Rate determining step.

Orders of reaction and rate determining step:                                                                               -Order of reaction with respect to a reactant shows the number of moleccules that reactant are involved in the rate determining step.                                                                                                -So if a reaction is second order with respect to X, there wil be 2 molecules of X in the rate-determining step.

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Equilibrium Constant

Reversable Reaction Symbol:  

As reactants are used up the forward (-->) reaction slows down.    When more products are formed the reverse (<--) reaction speeds up.                                                                                        Dynamic equilibrium is when the forward and backwards reaction are the same rate. (this only happens in a closed system and at a constant temperature)

Equilibrium constant = Kc

aA + bB       dD  + eE

Kc=        [D]d [E]e         

               [A]a [B]b    

Calculating Kc                                                                                                                               -Put numbers in the equation.   -Work out units, they may vary.        -Might have to work out equilibrium concentrations:   1. Work out moles at equilibrium  2. divide moles by volume of container  3. Put numbers in Kc.                                                                                                      

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Changing the Equilibrium

Changing the temperature, Concentration and pressure alters the position of the quilibrium.              <---more reactants  or ---> more products

Le Chateliers Principle: The position of equilibrium will shift to conteract a change in temperature, concentration or pressure.

TEMPERATURE: -If it increases, equilibrium will shift to the endothermic (+) direction.                     -If it decreases equilibrium will shift exothermic (-) direction.      -Affects the value of Kc.

CONCENTRATION: -Kc is fixed at a given temperature.   -If concentration of one thing changes, the concentration of others must change to keep the  value of Kc the same.

CATALYST: -No effects on the position of equilibrium or Kc.   -It only increases the forward and backwards rate by the same amount.    -Equilibrium is reached faster.

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