5.3 b: Common Ion Effect

Common Ion Effect

Y12 Chem

​Syllabus dot points

• 5.3 b:

  perform calculations to find the value of Keq and concentrations of substances within an equilibrium system, and use these values to make predictions on the direction in which a reaction may proceed

​What we will cover...

• ​predicting the common ion effect

• ​calculating the effect of a common ion

​Le Chatelier's Principle

• when any system at equilibrium is subjected to a change ​the equilibrium shifts to partially counteract this change

• ​we can apply this to dissolving salts into solutions that already contain dissolved ions

Multiple Choice

For the reaction...
SO_2(g) + O_2(g) <−>  SO_3(g)
If the equilibrium shifts to the right, the concentration of O_2(g) will ___________.

• increase
• decrease
• remain the same

Multiple Choice

For the reaction...
N_{2 (g)} +  3 H_2 (g) <−> 2 NH_{3 (g)}

If the pressure in the system is increased, the reaction will __________________.

•  shift to the left
• shift to the right
• not shift

Multiple Choice

For the following equilibrium reaction:

N₂ (g) + 3 H₂ (g) <==> 2 NH₃ (g) ΔH=-386 kJ/mol

Predict the direction that the reaction will proceed, or shift, if...

The reaction is cooled.

• right

• left

• no effect

​What is a common ion?

• If you add a salt into a solution that already contains dissolved ions, and this salt has an ion in common with the ions in solution

  • ​it is called a common ion

• ​we have to think about whether a common ion would increase or decrease the solubility of the ionic compound?

Open Ended

Would having a common ion increase or decrease the solubility of the added ionic compound in solution? Explain.

​Effect of the common ion

• ​does a common ion increase or decrease the solubility of the added ionic compound in solution?

  • ​it greatly decreases the solubility

  • ​especially of the sparingly soluble/least soluble salt

• ​​if a sparingly soluble salt is added to a solution with a common ion, then it can be assumed that the concentration of the common ion comes almost entirely from the ions already in the solution

  • ​the reverse is also true, if a soluble salt is added to a solution with a sparingly soluble salt, it will favour the dissolution of the soluble salt

Open Ended

If you have a solution of NaCl, and solid AgCl is added to it, what direction will the equilibrium position shift in the dissolution equation of AgCl? Explain.

• ​It would shift left to favour the solid, because the solution already contains Cl ions from the NaCl

• ​Therefore, the solubility of AgCl is reduced by the presence of a common ion

Open Ended