MCAT General Chemistry - Chemical Kinetics

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Gibbs free energy (ΔG)

determines whether or not a reaction will occur by itself without outside assistance/is spontaneous; does not necessarily mean that it will run quickly

Catalytic enzymes

selectively enhance the rate of certain reactions by a factor of 10² to 10¹² over other thermodynamically feasible reaction pathways

mechanism

the series of steps in which a reaction proceeds, the sum of which gives the overall reaction

intermediate

molecule which does not appear in the overall reaction; often difficult to detect because they may be consumed almost immediately after they are formed; can be supported through kinetic experiments

rate-determining step

slowest step in any proposed mechanism; acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step

collision theory of chemical kinetics

the rate of a reaction is proportional to the number of effective collisions per second between the reacting molecules; orientation and energy

activation energy (E_a)/energy barrier.

The minimum energy of collision necessary for a reaction to take place; Only a fraction of colliding particles have enough kinetic energy to exceed the activation energy

rate = Z × f
where Z is the total number of collisions occurring per second and f is the fraction of collisions that are effective.

Arrhenius equation

much more quantitatively rigorous analysis of the collision theory

where k is the rate constant of a reaction, A is the frequency factor, Ea is the activation energy of the reaction, R is the ideal gas constant, and T is the temperature in kelvin.

frequency factor/attempt frequency (A)

measure of how often molecules in a certain reaction collide, with the unit s^–1 ; proportional to rate; Increased by Increasing Concentration

reaction coordinate

traces the reaction from reactants to products

transition state/activated complex

the old bonds are weakened and the new bonds begin to form; greater energy than both the reactants and the products and is denoted by the symbol ‡; energy required to reach this transition state is the activation energy; can either dissociate into the products or revert to reactants without any additional energy input; distinguished from reaction intermediates in that transition states are theoretical constructs that exist at the point of maximum energy, rather than distinct identities with finite lifetimes.

free energy change of the reaction (ΔGrxn)

difference between the free energy of the products and the free energy of the reactants

exergonic reaction

negative free energy change; energy is given off

endergonic reaction

positive free energy change; energy is absorbed

free energy diagram

illustrates the relationship between the activation energy, the free energy of the reaction, and the free energy of the system

Factors Affecting Reaction Rate

• concentration - proportional

• temperature - proportional (10°C increase doubles speed in biological systems — but optimum temperature)

• medium - aqueous/organic, state of matter, polarity (polar preferred - speeds up rxns)

  • catalysts - proportional

Catalysts

substances that increase reaction rate without themselves being consumed in the reaction; return to their original chemical state upon formation of the products; ONLY decreases activation energy

may increase the frequency of collisions between the reactants; change the relative orientation of the reactants, making a higher percentage of the collisions effective; donate electron density to the reactants; or reduce intramolecular bonding within reactant molecules

homogeneous catalysis

catalyst is in the same phase (solid, liquid, gas) as the reactants

heterogeneous catalysis

the catalyst is in a distinct phase from the reactants

rate

concentrations of reactants and products and their change over time

moles per liter per second or molarity per second

rate law

rate is proportional (by proportionality constant, k) to the concentrations of the reactants, with each concentration raised to some experimentally determined exponent; the values of x and y are almost never the same as the stoichiometric coefficients eXCEPT on the rate-determining/only step

law of mass action

equilibrium constant expression

rate constant (k)

its particular value for any specific chemical reaction will depend on the activation energy for that reaction and the temperature at which the reaction takes place

K_eq= k₁/k_-1

Experimental Determination of Rate Law

Identify a pair of trials in which the concentration of one of the reactants is changed while the concentrations of all other reactants remain constant.

Under these conditions, any change in the rate of product formation from one trial to the other (if there is any change) is fully attributable to the change in concentration of that one reactant.

Repeat this process for other reactants.

zero-order reaction

the rate of formation of product C is independent of changes in concentrations of any of the reactants; dependent on temperature/catalyst

rate = k[A]⁰[B]⁰ = k

k in M/s

t vs. [A] - negative slope = k

first-order reaction

rate that is directly proportional to only one reactant

ex. process of radioactive decay

rate = k[A]¹

[A]_t = [A]₀e^–kt

k is s^-1

t vs. ln[A] - negative slope = k

second-order reaction

rate that is proportional to either the concentrations of two reactants or to the square of the concentration of a single reactant

rate = k[A]¹[B]¹ or rate = k[A]²

k is M^-1s^-1

t vs. 1/[A] - positive slope = k

Mixed-order reactions

rate orders that vary over the course of the reaction

broken-order reactions

non-integer orders (fractions)