Equilibrium Terms and Definitions

What is chemical equilibrium?


The state where the concentrations of all reactants and products remain constant with time; all reactions carried out in a closed vessel will reach equilibrium.

Describe the condition of equilibrium condition, when it is static.

No reaction is taking place, all product molecules remain as product molecules, and all reactant molecules remain as unused reactant molecules.

Describe the condition of equilibrium condition, when it is dynamic.

The reactions takes place; reactant molecules are converted to product molecules, while the product molecules are converted to reactant molecules through reverse reactions.


The forward and reverse reactions havesame rate if the system is in equilibrium.

In the beginning of the reaction, the necessity to achieve equilibrium causes…


Describe the process of initial concentrations becoming equilibrium concentrations

In the beginning of an unequal reaction, only reactant molecules exist, so only reactant molecules may collide with each other. During the middle the product concentration increases, collisions between products usually take place that lead to the reverse reaction. Once equilibrium is reached, the rates of forward reactions and reverse reactions are equal to one another

How does the Haber Process describe chemical equilibrium?

Haber Process :  N2(g)  +  3H2(g) yields 2NH3(g)

It is noted that Hydrogen gas (H2) is consumed at 3 times the rate of Nitrogen gas (N2) and that ammonia is formed at 2x the rate at which nitrogen is consumed

What is the Law of Mass Action?

For the balanced equation: aA  +  bB  yields  cC  +  dD    

The lower case letters are coefficients for the upper case elements.

The law of mass action is represented by the equilibrium expression:



What is K?

What is the reverse of K?



K is the equilibrium constant. The equilibrium constant varies depending on temperature and upon the coefficients of the above equation.

For the reverse reaction: cC  +  dD   yields aA  +  bB

 If this reaction is reversed then the new equilibrium expression is: 1/K=[A]a[B]b/[C]c[D]d


Define equilibrium position.

How many equilibrium positions are in one equilibrium constant?

How many equilibrium constants are in one equilibrium position?

A set of equilibrium concentrations

There is only one value of K for a reaction at a given temperature, but an infinite number of possibilities for equilibrium positions.

Equilibrium Gas Laws

How does Ka relate to Kp?


PV = nRT    or   P= (n/V)RT = CRT

n/V is the molar concentration of the gas, represented by C; therefore, there is a relationship K and Kp.

Kp= Ka(RT)Δn


Δn = (c + d) – (a + b) is the difference in the sums of the coefficients for the gaseous products and reactants.

Heterogeneous Equilibria: The effects of pure solids and liquids on the solution

The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids that exist because the concentrations of Pure Solids and Pure Liquids cannot change.

Solids or liquids that are involved in a chemical reaction don’t have their concentrations included in the equilibrium expression for the reaction.

What is the extent of the reaction based on K?

Reactions with large equilibrium constants that are greater than one go essentially to completion. Equilibrium position is far to the right with a generally large, negative energy.

Reactions with small equilibrium constants that are less than one consist of mostly reactants. Equilibrium position is far to the left.

The time required to achieve equilibrium is related to reaction rate and activation energy; it is not related to the magnitude of K.

What;is the relation between the reaction quotient (Q) and the equilibrium constant (K)?

Applying the law of mass action to initial concentrations gives the value of the reaction quotient.


In order to determine what;direction the system must move in order to achieve equilibrium, compare Q to K.


To determine the direction a system will shift:

Q is equal to K then the system is at equilibrium and no shift occurs.

Q is greater than K then system shifts to the left. The reverse reaction will consume; ; products and for reactants to reach equilibrium

Q is less than K then the system shifts to the right. The forward reaction willconsume reactants and form products until equilibrium is reached


What is Le Chatelier’s Principle?

If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change

How does adding or removing reactants affect the equilibrium concentration according to Le Chatlier’s Principle?

If a reactant or product is added to a system at equilibrium, the system will shift away from the added component (it will attempt to “use up” the added component).

If a reactant or product is removed from a system at equilibrium, the system will shift toward the removed component (it will attempt to “replace” the removed component).

How does;a change in pressure affect the equilibrium concentration according to Le Chatlier’s Principle?

When the volume of the container holding a gaseous system is reduced, the systemresponds by reducing its own volume.


When the container volume is increased, the system will shift so as to increase itsvolume.

How does a change in temperature affect the equilibrium concentration according to Le Chatlier’s Principle?

An increase in temperature increases the energy of the system. Le Chatelier’s principle predicts that the system will shift in the direction that consumes the energy.

For an exothermic system, energy is a product. The system will shift to the left to use up the excess energy.

For an endothermic system, energy is a reactant. The system will shift to theright to use up the energy.

A decrease in temperature will cause a system shift in the direction that “replaces” the lost energy.

How does chemical equilibrium affect the direction of the reaction?

There is less product; equilibrium lies far to the left.

There is less reactant; equilibrium lies far to the right.