Chapter 13 Flashcards
chemical kinetics
is the study of reaction rates, including how reaction rates change with varying conditions and which molecular events occur during the overall reaction.
Factors Affecting Rate of Reaction
- Concentrations of reactants
- Concentration of catalyst
- Temperature at which reaction occurs
- Surface area of a solid reactant or catalyst
Reaction rate
is the increase in molar concentration of product of a reaction per unit time or the decrease in molar concentration of reactant per unit time. The unit is usually mol/(L • s) or M/s
reaction rate equations give
the average rate over the time interval t.
As t decreases and approaches zero,
the equations give the instantaneous rate.
Rates are determined experimentally in a variety of ways. For example
samples can be taken and analyzed from the reaction at several different intervals. Continuously following the reaction is more convenient. This can be done by measuring pressure, as shown on the next slide, or by measuring light absorbance when a color change is part of the reaction
The reaction rate usually depends on
the concentration of one or more reactant. This relationship must be determined by experiment
the rate law,
an equation that relates the rate of a reaction to the concentration of a reactant (and catalyst) raised to various powers. The proportionality constant, k, is the rate constant.
reaction order with respect to a specific reactant is the
exponent of that species in the experimentally determined rate law.
The overall order of a reaction is the
sum of the orders of the reactant species from the experimentally determined rate law.
The rate law tells us the relationship between
the rate and the concentrations of reactants and catalysts.
To find concentrations at various times, we need to use the
ntegrated rate law. The form used depends on the order of reaction in that reactant.
The half-life of a reaction, t½, is the
time it takes for the reactant concentration to decrease to one-half of its initial value.
the rate law is in the form of
y = mx + b, allowing us to use the slope and intercept to find the values
__________ can also be used to determine the order of reaction in that reactant.
Graphs of concentration and time
The rate constant depends strongly on
temperature
Collision theory
assumes that reactant molecules must collide with an energy greater than some minimum value and with the proper orientation
The minimum energy is called the
activation energy
Transition-state theory
explains the reaction resulting from the collision of two molecules in terms of an activated complex (transition state)
activated complex (transition state)
an unstable grouping of atoms that can break up to form products.
The potential energy diagram for a reaction visually illustrates the
changes in energy that occur
the reaction may take place in several steps. That set of steps is called the
reaction mechanism
Each step in the reaction mechanism is called an
elementary reaction
the rate of an elementary reaction is proportional to the
concentration of each reactant molecule.
The slowest step in the reaction mechanism is called the
rate-determining step (RDS). The rate law for the RDS is the rate law for the overall reaction.
The rate of a reaction is determined completely by
the slowest step, the rate-determining step.
reaction intermediate
a species produced during a reaction that does not appear in the net equation because it reacts in a subsequent step in the mechanism.
Catalysis is an
increase in the rate of reaction that results from the addition of a catalyst.
A catalyst is not
consumed in a reaction. Rather, it is present in the beginning, is used in one step, and is produced again in a subsequent step. As a result, the catalyst does not appear in the overall reaction equation.
A catalyst increases the reaction rate by
providing an alternative reaction path with a lower activation energy. When Ea is reduced, k increases exponentially.
