Fundamental Equilibrium Concepts
Chapter 13
Equilibrium helps us answer questions such as: at what point does this reaction stop? At what ratio of products to reactants is the reaction most stable? How many products and reactants are present when the reaction stops?
How do we know that a reaction has reached equilibrium? Let's say we have the following chemical reaction:
The equilibrium constant for this reaction can be expressed in the following way, assuming that all species in the reaction are either gases or aqueous.
Kc is the equilibrium constant for the reaction at a single temperature and is a ratio of products to reactants when the reaction is at equilibrium. Square brackets around a chemical formula mean molarity of that substance. Further, any stoichiometric coefficients from the balanced chemical reaction become exponents in the equilibrium constant expression.
A few notes about equilibrium constant expressions:
Solids and liquid substances are not included
The value of Kc is temperature dependent
The equilibrium constant can also be expressed using partial pressures instead of molarities. This version of the equilibrium constant is usually denoted Kp
The reaction quotient, Qc, is expressed identically to Kc; however, the molarities used in the expression are from any point of the reaction.
The relationship between Q and K can be evaluated to determine which direction the reaction will proceed in order to reach equilibrium:
if Q > K, then the reaction has not yet reached equilibrium and will go forward to establish equilibrium. This means that
product concentrations should decrease
reactant concentrations should increase
if Q = K, then the reaction is already at equilibrium. This means that
product concentrations should not change
reactant concentrations should not change
if Q > K, then the reaction has passed equilibrium and will go in reverse to re-establish equilibrium. This means that
product concentrations should increase
reactant concentrations should decrease
Special Equilibrium Constants
Over time in your chemical journey, you will encounter many different named equilibrium constants. Here is a comprehensive list of the most common types of named equilibrium constants you might encounter.
Acid Dissociation Constant, Ka
This can be used to define the strength of an acid.
Higher Ka = more dissociation = stronger acid
A generic acid dissociation reaction and its corresponding acid dissociation constant expression are shown below
Base Dissociation Constant, Kb
This can be used to define the strength of a base.
Higher Kb = more dissociation = stronger base
A generic base dissociation reaction and its corresponding base dissociation constant expression are shown below
Solubility Product Constant, Ksp
This can be used to tell us the degree to which a compound will dissociate.
higher Ksp = more soluble
An example of a solubility reaction and its corresponding solubility product constant expression are shown below
Formation Constant, Kf
This can be used to tell us the strength of the interaction between complex ions.
higher Kf = more stable formation
An example of a formation reaction for a complex ion and its corresponding formation constant expression are shown below
Autoionization Constant of Water, Kw
This can be used to tell us information about the pH and pOH of a solution.
The reaction for the autoionization of water and its corresponding equilibrium constant expression are shown below
Manipulating Equilibrium Constants
When working with reactions you might need to manipulate your equation, but how does that affect the values for our equilibrium constants? Below is a short synopsis of how manipulations of our original equation change our 𝐾𝑐 value.
Original equation we will consider
Flipping the reaction results in inverting the equilibrium constant:
Doubling the reaction results in squaring the equilibrium constant
Halving the reaction results in taking the square root of the equilibrium constant
Adding two reactions together will result in multiplying the respective equilibrium constants