Rate Law
We will begin this discussion with a general description of reactions, rates and rate laws. We will discuss a general reaction
(1)As the reaction occurs, concentrations of reactants and products will change. The rate of the reaction can be characterized by any of the following.
(2)The above equation defines the rate of reaction. The rate of reaction is defined in terms of the rate of change of concentration for any component of the reaction, divided by the stochiometric coefficient for that component. Stochiometric coefficients are positive for products and negative for reactants.
The rate of a reaction is dependent on the concentrations of chemical species involved in the reaction. In general the rate of a reaction depends on the concentrations of species as follows:
The rate of a reaction is dependent on the concentrations of chemical species involved in the reaction. In general the rate of a reaction depends on the concentrations of species as follows:
rate = k [A]p [B]q (3)
This equation is the rate law for the reaction. The proportionality constant, k, is known as the rate constant for the reaction. In general, the rate constant depends on temperature, commonly increasing with temperature. The exponents in the rate law are often small integers (positive or negative) or simple fractions. These exponents are determined experimentally.
This equation is the rate law for the reaction. The proportionality constant, k, is known as the rate constant for the reaction. In general, the rate constant depends on temperature, commonly increasing with temperature. The exponents in the rate law are often small integers (positive or negative) or simple fractions. These exponents are determined experimentally.
Reaction Order
The exponent on the concentration terms in equation (3) is the order of the reaction with respect to each component of the reaction. For example, p is the order of the reaction with respect to the concentration of component A. If p is zero, the rate of reaction is independent of the concentration of A. The sum of the exponents is the overall order of the reaction.
Initial Rate Method:
By measuring the change in concentration with time of one of the components of the reaction system, one can collect rate information. This data can be used to determine the initial rate of the reaction, the rate early in the reaction. By taking the ln of equation (3),
Initial Rate Method:
By measuring the change in concentration with time of one of the components of the reaction system, one can collect rate information. This data can be used to determine the initial rate of the reaction, the rate early in the reaction. By taking the ln of equation (3),
ln(rate) = ln k + p ln[A] +q ln[B] (4)
one gets an expression relating the ln of the rate to the sum of the ln of the rate constant and the ln of the concentrations of the reaction components. The ln of the concentrations of the individual reaction components are multiplied by the reaction order for that component. By measuring the initial rate of the reaction while changing the initial concentration of one of the reaction components, one can determine the order of the reaction with respect to that component. This is done by plotting the ln of the initial reaction rate versus the ln of the initial concentration of the varying component (all other initial concentrations are held constant). This should result in a straight line of slope equal to the order for that component. This process can be repeated, varying concentrations of other individual components in the reaction system.
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