ch15

Chapter 15: Chemical Equilibrium

• Can be static (at rest) or can also be dynamic, whereby a forward process and the reverse process take place at the same rate so that no net change occurs.

• rate at which the products form from the reactants equals the rate at which the reactants form from the products.

15.1 The Concept of Equilibrium

15.2 The Equilibrium Constant

• It is common practice to write equilibrium constants without units for reasons that we address later in this section. The units of such ratios always cancel and, consequently, activities have no units.

• When the reactants and products in a chemical reaction are gases, we can formulate the equilibrium-constant expression in terms of partial pressures

• K_c is different than K_p so to indicate which constant is being used you can use the ideal gas equation

• General expression relating K_p to K_c

• Triangle n is moles of gaseous product minus moles of gaseous reactant

15.3 Understanding and Working with Equilibrium Constants

• a very small equilibrium constant indicates that the equilibrium mixture contains mostly reactants. 

• If k is greater than 1, equilibrium lies to the right and products dominate

• If k is less than 1, equilibrium lies to the left and reactants dominate

The rules:

• The equilibrium constant of a reaction in the reverse direction is the inverse (or reciprocal) of the equilibrium constant of the reaction in the forward direction:

• The equilibrium constant of a reaction in the reverse direction is the inverse (or reciprocal) of the equilibrium constant of the reaction in the forward direction:

• The equilibrium constant of a reaction that has been multiplied by a number is equal to the original equilibrium constant raised to a power equal to that number.

• whenever a pure solid or a pure liquid is involved in a heterogeneous equilibrium, its concentration is not included in the equilibrium-constant expression.

15.6 Applications of equilibrium constants

• The reaction quotient, Q, is a number obtained by substituting reactant and product concentrations or partial pressures at any point during a reaction into an equilibrium-constant expression.

• The equilibrium constant has only one value at each temperature. The reaction quotient, however, varies as the reaction proceeds.

• Of what use is Q? One practical thing we can do with Q is tell whether our reaction really is at equilibrium, which is an especially valuable option when a reaction is very slow. 

  • QくK: The concentration of products is too small and that of reactants too large. The reaction achieves equilibrium by forming more products; it proceeds from left to right.

  • Q=K:The reaction quotient equals the equilibrium constant only if the system is at equilibrium.

  • Q＞K: The concentration of products is too large and that of reactants too small. The reaction achieves equilibrium by forming more reactants; it proceeds from right to left.

15.7 Le Châtelier’s Principle

• If a system at equilibrium is disturbed by a change in temperature, pressure, or a component concentration, the system will shift its equilibrium position so as to counteract the effect of the disturbance.

• reducing the volume of a gaseous equilibrium mixture causes the system to shift in the direction that reduces the number of moles of gas.

• In an endothermic (heat-absorbing) reaction, we consider heat a reactant, and in an exothermic (heat- releasing) reaction, we consider heat a product: