Solutions – Common Ion Effect – MCAT General Chemistry | MedSchoolCoach

Solutions - Common Ion Effect

MCAT General Chemistry Topic

Common Ion Effect

When an electrolyte dissolves in solution, its associated ion pair dissociates into a separate cation and anion. The equilibrium concentration of ions dissolved in solution is denoted by the solubility product constant (Ksp). However, what would happen if extra ions were added to a solution at equilibrium? This effect, the common ion effect, will be demonstrated through two examples.

 

Common Ion Effect – Example 1

 

A solution of dissolved CaCO3 is at equilibrium. What would happen to the amount of dissolved CaCO3 if CaCl2 is added to the solution?

 

CaCO3 sCa2+ aq+CO32- (aq)


Applying Le Chatelier’s principle, a secondary equilibrium will begin, where CaCl2 will dissolve and dissociate to form calcium ions and chloride ions that will freely interact with solution. Both equilibriums present in the solution share a common ion, calcium. The dissolution of CaCl2 increases the total amount of calcium ion present in solution. Remembering that the dissolution of CaCO3 is at equilibrium, an increase in calcium will increase the ion product (Q) of CaCO3 past its Ksp. When Q > Ksp, excess solute precipitates out of solution. In this case, excess Ca2+ and CO32- will precipitate, forming CaCO3 until Q = Ksp. The take home point here is that addition of a common ion will decrease solubility, potentially forcing some excess ion to form a precipitate.

Common Ion Effect – Example 2

A solution of dissolved CaCO3 is at equilibrium. What would happen to the amount of dissolved CaCO3 if HCl is added to the solution?

 

CaCO3 sCa2+ aq+CO32- (aq)


Recognizing that HCl is a strong acid and that CO3 is a weak base, we can predict that the two are going to undergo a neutralization reaction. As the hydrogen chloride protonates the bicarbonate, carbonate will be removed from solution. If ions are removed from equilibrium, we will reach a situation where Q < Ksp. When Q < Ksp, the reaction is unsaturated, and fewer ions are dissolved in solution than will be at equilibrium. To increase the ion product (Q), calcium carbonate will dissociate further into calcium and bicarbonate ions, until Q = Ksp. The take home point from this is example is that removal of a common ion will increase solubility.

Your Complete Solution

Cover All the Bases with the MedSchoolCoach MCAT Bundle

MCAT Prep App

eLearning-01

Our MCAT videos are always free, covering 200+ topics in a bite-sized, digestible format. Flashcards, questions, lecture notes and more are available to you to maximize your score. Now on iOS, Android, and the web!

MCAT Practice Exams

Chemistry-01

Practice makes perfect! Our representative exams coupled with thorough explanations and in-depth analytics help students understand exactly where they stand.

MCAT Go

Musical Note-01

An audio learning experience by MedSchoolCoach unlike anything else out there. Comprehensive science coverage, quizzes, playlists, and more—stay tuned!

One-on-One Tutoring

Student-01

Are you ready to take your MCAT performance to a whole new level? Work with our 99th-percentile MCAT tutors to boost your score by 12 points or more!

MedSchoolCoach logo

Enter your email address to add this customizable packing list to your Google Drive for FREE!