# Chapter 18 ## Mass-Action

• Used to describe a system undergoing chemical change
mA + nB ---> pC + qD
Where m, n, p, and q represent the coefficients from the balanced equation

Mass-Action Expression for the equation is:
[C]p x [D]q
[A]m x [B]n

(the brackets represent the concentration of the substance in moles per cubic decimeter - mol/dm3 - raised to the power of the coefficient from the balanced equation)
```Mass-Action Expression  =  [Products]
[Reactants]``` ## Equilibrium Constant (Keq)

• The value of the equilibrium constant is not affected by the initial concentration of any of the substances taking part in the reaction
• It is not affected by the pressence of other substances either (provided none of these substances react with any of the reacts or products)
• Keq will change if the temperature is changed
490.C ---> 45.86
400.C ---> 54.5
• Small value of Keq favors the reacts (anything under 1) --- at equilibrium there is little of the products
• Large value of Keq favors the products (usually over 1) --- at equilibrium there is a large quantity of products
• Concentrations are never a negative, therefore, Keq will not be negative
• You can experimentally obtain Keq by measuring the concentration at equilibrium and then calculating using the Mass-Action Expression
• Always make sure you have a balanced equation 