Kinetics

Kinetics: study the rate at which a chemical process occurs. * Also studies the reaction mechanism * Reaction mechanism: how the reaction occurs

Physical state of the reactants * In order to react, molecules must come in contact with each other. * The more homogeneous the mixture of reactants, the faster the molecules can react.
Concentration of reactants * As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. * Since during a reaction, concentrations are constantly changing, the rate of a chemical reaction is SELDOM constant
Temperature * At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy.
Presence of a catalyst * Catalysts speed up reactions by changing the mechanism of the reaction. * Catalysts are not consumed during the course of the reaction.

Rate of a reaction: the change in the amount of a reactant or product per unit time. * It’s often convenient to express amount in terms of concentration.
Concentration is used because the effect of concentration on the rate is generally predictable. * As concentration decreases, rate decreases.
Rate expression: mathematical representation of rate of reaction.
Reaction rates are always positive quantities.
Concentration is most often expressed in Molarity.
Rate expressions can be written in terms of reactant or product concentration

Average reaction rate: the rate at which a reaction proceeds over a time period. * Calculated using concentrations at the beginning and end of a time period.
Instantaneous reaction rate: the rate at which a reaction is proceeding at a specific time and/or conc. * Calculated using a graph (Conc. vs. time) and a slope of a straight line tangent to the curve at that specific time. * Or by determining the avg. rate of reaction over a very short time period.
Initial reaction rate: the instantaneous reaction rate at “time zero”.

The rate of a reaction may be expressed in terms of the change in concentration of any reactant or product.
Stoichiometric factors derived from a balanced equation may be used to relate reaction rates.

Chemical reactions occur as the result of collisions between reactant molecules.
The higher the concentration of reactant molecules, the more likely molecules will collide and react, and therefore the faster the reaction rate.
As reactants get consumed, collisions happen less frequently and the reaction rate decreases.

The rate constant, k is specific for a particular reaction at a particular temperature.
k is independent of reactant concentration.
k must be determined experimentally.

Experiments must be done to determine the rate law for a reaction. * Can’t just use a balanced chemical equation.
Elementary step: a single event in the course of a reaction
The rate is directly proportional to the product
To get the rate law from an elementary step, we simply multiply k by the concentrations of all of the molecules……..IF they all have coefficients of 1.
The rate law for an elementary step always = k times the product of each component, raised to the power of its coefficient.
Rate order: the sum of the exponents in the rate law
For a reaction with a single reactant: * A → products * rate = k[A]^m * m is the order of the reaction * m must be determined experimentally (not from balanced chemical equations). * Reaction orders are usually positive integers (0, 1, 2…) but can also be fractions or negative numbers.
Most reactions involve more than one reactant.
A + B → products
rate = k[A^]m[B]^n
There are three orders: * m is the order with respect to A * n is the order with respect to B * The overall order of the reaction = m + n

The **order with respect to a reactant **indicates the dependence of the reaction rate on the concentration of that particular reactant.
The **overall order *gives an understanding of how all the reactants contribute to the rate of a reaction. Also, dictates the units of the rate constant for that reaction.

One way to determine the rate law of a reaction is to measure the initial rate. * Initial rate: the rate at time zero.
If the initial rate is measured with a number of different initial reactant concentrations then the rate law can be determined.