· The change in Gibbs free energy (ΔG) determines whether or not a reaction is spontaneous.
· Chemical mechanisms propose a series of steps that make up the overall reaction.
o Intermediates are molecules that exist within the course of a reaction but are neither reactants nor products overall.
o The slowest step, also known as the rate-determining step, limits the maximum rate at which the reaction can proceed.
· The collision theory states that a reaction rate is proportional to the number of effective collisions between the reacting molecules.
o For a collision to be effective, molecules must be in the proper orientation and have sufficient kinetic energy to exceed the activation energy.
o The Arrhenius equation is a mathematical way of representing collision theory.
· The transition state theory states that molecules form a transition state or activated complex during a reaction in which the old bonds are partially dissociated and the new bonds are partially formed.
o From the transition state, the reaction can proceed toward products or revert back to reactants.
o The transition state is the highest point on a free energy reaction diagram.
· Reaction rates can be affected by a number of factors.
o Increasing the concentration of reactant will increase reaction rate (except for zero-order reactions) because there are more effective collisions per time.
o Increasing the temperature will increase reaction rate because the particles’ kinetic energy is increased.
o Changing the medium can increase or decrease reaction rate, depending on how the reactants interact with the medium.
o Adding a catalyst increases reaction rate because it lowers the activation energy. Homogeneous catalysts are the same phase as the reactants; heterogeneous catalysts are a different phase.
· Reaction rates are measured in terms of the rate of disappearance of a reactant or appearance of a product.
· Rate laws take the form of rate = k[A]x[B]y.
o The rate orders usually do not match the stoichiometric coefficients.
o Rate laws must be determined from experimental data.
· The rate order of a reaction is the sum of all individual rate orders in the rate law.
· Zero-order reactions have a constant rate that does not depend on the concentration of reactant.
o The rate of a zero-order reaction can only be affected by changing the temperature or adding a catalyst.
o A concentration vs. time curve of a zero-order reaction is a straight line; the slope of such a line is equal to —k.
· First-order reactions have a nonconstant rate that depends on the concentration of reactant.
o A concentration vs. time curve of a first-order reaction is nonlinear.
o The slope of a ln [A] vs. time plot is —k for a first-order reaction.
· Second-order reactions have a nonconstant rate that depends on the concentration of reactant.
o A concentration vs. time curve of a second-order reaction is nonlinear.
o The slope of a vs. time plot is k for a second-order reaction.
· Broken-order reactions are those with noninteger orders.
· Mixed-order reactions are those that have a rate order that changes over time.