CHAPTER 23 REACTION RATE AND CHEMICAL EQUILIBRIUM
23-1 Stability of compounds
a thermodynamically stable substance does not decompose
spontaneously: delta G for decomposition is positive
the free energy decreases, delta G is negative, in a
spontaneous reaction because the system is changing
to a more stable state
a process may be spontaneous(delta G<0) but proceed so
slowly that no change is apparent. The reactants are
said to be kinetically stable in this situation
23-2 Reversible Reactions and equilibrium
not all reactions go to completion
the combined equation(<--> represents a reversible reaction/
they eventually reach equilibrium
23-3 reaction rate
reaction rate is the rate of disappearance of reactant or rate
of appearance of product
23-4 nature of reactants
reaction rate depends upon the nature of the reactants
ionic reactions involve no electron transfer and are rapid
the energy required to form the activated complex(in a change
colliding molecules may form one) is known as the
activation energy
23-5 Concentration
reaction rate depends upon concentration(molarity) of the
reactants
[ ] indicates mol/cubic decimeters
the specific rate constant,k, has one value for a given T
when the pressure on a gas at constant T increases the V
decreases. When the V decreases, concentration increases,
and thus reaction rate increases
a heterogeneous reaction involves 2 or more phases
a homogeneous reaction involve reactants in same phase
increasing the available surface will increase the reaction rate
activation energy is the additional energy needed to form an
activated complex (start a reaction)
23-6 temperature
reaction rate depends upon the T of the reactants
when molecules have sufficient EK to approach closely enough to
react chemically, they form an activated complex
increased T increases the number of activated complexes formed
23-7 Catalysis
catalysts increase reaction rate, but appear to be unaffected in
the reaction
a heterogeneous or contact catalyst works by adsorbing one of the
reactants
a homogeneous catalyst exists in the same phase as reactants
adsorption is the adherence of one substance to the surface of
another
inhibitors stop reactions by tying up a reactant
a catalyst changes the reaction mechanism so that the activation
energy required is lower
23-8 reaction mechanism
most reactions occur in a series of steps known as the reaction
mechanism/slowest step determines the rate of the overall
reaction
the coefficients in the equation become the exponents in the
rate expression for a single step reaction
for a single step reaction, the exponents of concentration factors are the order of the rate law expression
reactants must collide with the proper activation energy and
collision geometry(orientation) in order to form products
23-9 Equilibrium Constant
at equilibrium opposing reaction rates are equal
an equilibrium constant is the ratio of the rate expression of
the forward reaction to the rate expression of the reverse
reaction. the expression is: Keq=kf/kr
the exponents in the equilibrium constant are the coefficients
from the balanced equation
if Keq is small(much less than 1) very little product is formed
if Keq is large(much greater than 1) the reaction will approach
completion
Keq value is unique for any reaction at a given temperature
23-10 Le Chatelier's principle
LCP: a system at equilibrium that undergoes a stress will shift
toward restoring the original conditions
energy can be treated as a reactant or product in a reaction
optimum conditions produce the highest yield
23-11 Gibbs free energy and equilibrium
delta G<0 for spontaneous reactions
when Keq is very large the reaction will go almost to completion
gibbs free energy change for a reaction is related to its
equilibrium constant by the expression
delta G = -2.30 RT(log Keq)