Chemistry - A Molecular Science

b) adding solid KCN to increase the CN

1- ion concentration

The increased concentration of CN

1- ions would shift the equilibrium back toward AgCN by

reacting with some of the Ag

1+ ions in solution. The Ag

1+ ion concentration would

decrease. Note that the addit

ion of a solid can shift the equilibrium if it dissolves to

increase the concentrations of at l

east one ion involved in the equilibrium.

c) adding more solid AgCN

Adding more AgCN has no effect on the Ag

1+ concentration because it does not change

the concentration of AgCN. Recall that solids

and liquids enter the equilibrium expression

as one (1), so the equilibr

ium constant expression for this equilibrium is K = [Ag

1+][CN

1-].

Thus, only changing [Ag

1+] or [CN

1-] can shift the equilibrium.

9.13

CHAPTER SUMMARY AND OBJECTIVES This chapter explored the kinetic and thermodynamic factors that dictate the feasibility, rate, and equilibrium position of a r

eaction. The enthalpy of reaction,

H, is the heat Δ

absorbed at constant pressure. When

H > 0, heat is absorbed anΔ

d the reaction is said to be

endothermic, but when

H < 0, heat is given off and the reaction is said to be exothermic. Δ

H of a gas phase reaction can be approximated as the difference between the energy Δ required to break the bonds in the reactants and

to form them in the products. Entropy is a

thermodynamic quantity that measures the ability of a system to disburse its energy; systems with more ways to disburse their energy have higher entropies. The number of ways the energy of a system can be disbursed

decreases whenever constraints are placed

on the system. Thus, the entropy of a substance

in the gas is much greater than its entropy

in the much more constrained liquid or solid state. Constraints in nature impose order on systems, so systems that are ordered (solid) have lower entropies than systems that are disordered (gas). The second law

of thermodynamics states that

SΔ

univ

0 for spontaneous

processes. The Gibbs free energy is

G = -TΔ

SΔ

univ

at constant T and P, so

G < 0 for a Δ

spontaneous process at constant T and P.

G = Δ

H - TΔ

S, so both the enthalpy and Δ

entropy of reaction help establish spontaneity.

G indicates the spontaneous direction of a Δ

reaction;

G < 0 indicates that the reaction is Δ

spontaneous in the direction written,

G < 0 Δ

indicates that it is spontaneous in the reverse direction, and

G = 0 indicates that the Δ

reaction is at equilibrium.

GΔ

o indicates the extent of reaction;

GΔ

o >> 0 means the

reaction is extensive and essentially all of the limiting reactant is consumed.

Kinetics deals with the rates of reactions

. Rates are governed by the frequency of

collisions between the reactants and the frac

tion of those collisions that result in the

Chapter 9 Reaction Energetics

© by

North

Carolina

State

University