It is possible from the equilibrium constants of the direct reaction to obtain the equilibrium constant of the inverse reaction. We can also multiply the constants.

Be the chemical reaction:

Example:

Your reverse reaction will be:

### Adding a reaction

If two reactions are added, the new KC will be the product of the original constants.

### Multiplication of a reaction

If two reactions are multiplied by a given number, your new KC will be raised to this value.

See the model:

### Division of a reaction

If two reactions are divided by a certain number, your new KC will be your root.

See the model:

### Reaction with pure liquid or solid substance

Pure liquid and solid substances have constant concentration, where the value is incorporated to the constant KC. For this reason, in equilibrium constant expressions, their concentrations are not placed in the calculation of KC.

Example:

### Equilibrium constant as a function of partial pressures (Kp)

If a given chemical reaction has gaseous substances, the equilibrium constant may be given as a function of its partial pressures. Example:

For the calculation of partial pressure, the concept of molar fraction (x) is used. The molar fraction and then the partial gas pressure are calculated. Example:

3 moles of PCl5 (g) are placed in a container, achieving the following balance:

At equilibrium, 60% of the reagent dissociates. Knowing that the total system pressure is 4.8atm, calculate the value of KP:

1 °) calculate the quantity of moles, being 60%:

2 °) assemble the chemical balance table:

PCl5 | PCl3 | Cl2 | |

start | 3 | - | - |

reacted / formed | 1,8 | 1,8 | 1,8 |

balance | 3 -1,8=1,2 | 1,8 | 1,8 |

3 °) calculate the molar fractions:

nT = total number of moles

4 °) calculate the partial pressure of the gases:

The sum of the partial pressures must be equal to the total pressure:

That is:

5 °) calculate the KP: