Chemistry equilibrium

Equilibrium

Equilibrium Reactions in Chemistry

• Many chemical reactions are reversible and exist in state of equilibrium

• Dynamic Equilibrium: The forward and reverse reactions occur at equal rates

• Single arrows are for non-reversible reactions

• Double headed arrow are for reversible reactions which are in equilibrium

Chemical Systems

• The terms “Reactants” and “Products” implies a reaction goes to completion when in reality, many reactions are in equilibrium

• Relative rates of a reaction depend on:

  • Temperature and Pressure

  • Concentration of reactants and products

  • Presence of catalyst

• At equilibrium:

  • Forward and reverse reactions occur at equal rates

  • No change in concentration of reactants or products

Equilibrium Law

• Law: At a given temperature, the ratio of the concentration of products to the concentration of reactants (each raised to the power of their molar coefficients) is a constant

  

• Constant is called equilibrium constant denoted by K_c

• This constant changes at different temperatures

• Large K_c value (K_c > 1) means products are favored over reactants

• Small K_c value (K_c < 1) means reactants are favored over products

• Pure solids and liquids are not included in when calculating the K_c value

• Homogenous equilibrium: reactants and products are present in one phase. (Common one is reactions in gaseous phase)

• Heterogeneous equilibrium: reactants and products exist in more than one phase

• K_c of the reverse reaction is the same as 1/K_c of the forward reaction

  • Kc (reverse) = 1/K_c (forward)

Effect of conditions on equilibrium constant

Pressure:

• If there are 4 mols of gaseous reactants and 3 mols of gaseous products, an increase in pressure would result in forward reaction being favored to reduce number of mols overall

Temperature:

• In exothermic reaction, heat is a product. Increase in heat would cause equilibrium position to move left to counter the imbalance caused by heat. A summary of all possibilities is in table below

Catalyst:

• Catalyst reduces activation energy

• It increases rate of forward and reverse reaction by equal amount thus no change in K_c

Le Châtelier principle

• Le Châtelier principle: If a change is made to a system that is in equilibrium, the balance between the forward and reverse reactions will shift to offset this change and return system to equilibrium

  • If concentration of reactant is increased, forward reaction will be favored to counteract this. Vice Versa

  • Value of K_c does not change

Reaction Quotient

• If system has not reached equilibrium yet, then

• Q is for reaction quotient and helps determine the progress of reaction as it moves toward equilibrium