# Equilibrium Constants

### Understanding Equilibrium Constants

• Equilibrium in a chemical reaction is when the forward and reverse reactions occur at the same rate, leading to a constant concentration of reactants and products.
• The equilibrium constant (Kc) is a value that quantifies the ratio of the concentrations of products to reactants when a reaction has reached equilibrium.
• The value of Kc does not change unless the temperature changes.
• It’s important to note that solid substances or pure liquids are not included in the expression for the equilibrium constant.

### Expressing Equilibrium Constants

• The expression for Kc is given by: Kc = [C]^c [D]^d / [A]^a [B]^b, where [A], [B], [C] and [D] denote the molar concentrations of the substances A, B, C and D; and a, b, c and d are their corresponding coefficients in the balanced chemical equation.
• The value of Kc is unitless in ideal conditions, but in some cases it may carry units to maintain dimensional consistency.

### Interpreting Values of Equilibrium Constants

• If Kc > 1, the equilibrium lies towards the products, meaning the concentrations of the reactants are smaller than those of the products.
• If Kc < 1, the equilibrium lies towards the reactants, indicating that at equilibrium, there is a larger quantity of reactants.
• If Kc = 1, the concentrations of reactants and products are equal at equilibrium.

### The Effect of Temperature on Equilibrium Constant

• The value of Kc is temperature- dependent. It remains constant only if the temperature is constant.
• If a reaction is exothermic (releases heat), an increase in temperature will reduce the value of Kc - the equilibrium shifts in the reverse direction.
• Conversely, for an endothermic reaction (absorbs heat), an increase in temperature will increase the value of Kc - the equilibrium shifts towards the products.

### The Relationship between Kc and Kp

• Kp is another form of the equilibrium constant, which deals with partial pressures instead of concentrations.
• To convert between Kp and Kc, the equation Kp = Kc (RT)^Δn is used, where R is the gas constant, T is the temperature in Kelvin, and Δn is the difference in the total number of moles of gaseous products and reactants.
• This equation is derived from the ideal gas law, and only applies to reactions involving gases.