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# Lecture22222

## on Aug 12, 2008

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a supplemental resource for students

a supplemental resource for students

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## Lecture22222Presentation Transcript

• Kinetics: the Rate Law and Effect of Concentration Lecture 22
• The rate law for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters (normally rate coefficients and partial reaction orders).
• For aA+bB+…  cC+dD+… Rate = k[A] m [B] n …
• Rate = k[A] m [B] n …
• Herein k is the rate constant. It depends on temperature.
• m and n are reaction orders. They depend on the reaction mechanism.
• Rate = k[A] m [B] n …
• If the rate doubles when [A] doubles, m=1.
• If the rate quadruples when [A] doubles, m=2.
• If the rate does not change when [A] doubles, m=0.
• Coefficients a and b in aA+bB+…  cC+dD+… may or may not be related in any way to the reaction orders m and n : Rate = k[A] m [B] n …
• The reaction mechanism must conform to to the rate law. The rate law is based on experimental fact.
• Components of the rate law must be found experimentally:
• Measure concentrations to find the initial rate.
• Use initial rates from several experiments to find the reaction orders.
• Use reaction orders to find the rate constant.
• How to define the reaction orders:
• Rate=k[A] . First order overall.
• Rate=k[A] 2 . Second order overall.
• Rate=k[A] 0 =k(1)=k . Zero order overall.
• How to define the reaction orders:
• NO (g) +O 3(g)  NO 2(g) +O 2(g)
• Rate=k[NO][O 3 ]
• First order with respect to NO.
• First order with respect to O 3 .
• Second order overall.
• How to define the reaction orders:
• 2NO (g) +2H 2(g)  N 2(g) +2H 2 O (g)
• Rate=k[NO] 2 [H 2 ]
• Second order with respect to NO.
• First order with respect to H 2 .
• Third order overall.
• Reaction orders cannot be deduced from the balanced equation.
• Special cases:
• If a reaction order is fractional, the rate depends on the square (cubic) root of the concentration: rate=k[A][B] 1/2
• If a reaction order is negative, the rate decreases when the concentration of that component increases.
• A sample problem on determining rate order from rate laws.
• It we do not know the reaction law, we have to find it from a series of experiments, starting each one with a different set of reactant concentrations and obtaining an initial rate in each case.
• A sample problem on determining reaction order from initial rate data.
• THE END