Recrystallization of Sodium Chloride with HCl

Author(s): Miha Lee

Discrepant Event - Teacher's Guide
SED 695B

   

Recrystallization of Sodium Chloride with HCl

Detailed Explanation of Discrepant Event

Principles illustrated

  • Solubility equilibrium of solids
  • Le Ch‚telierís Principle in Chemical Equilibrium
  • Common ion effect

  If you add concentrated HCl solution to a saturated NaCl solution, you will drive the equilibrium of equation 1) back to the left by increasing the concentration of Cl-(aq). As a result, NaCl will re-crystallize in the solution.This is a demonstration of Le Ch‚telierís principle that states: if a system in equilibrium is disturbed, it will shift the equilibrium to counteract the disturbance.

1) NaCl(s) Na+(aq) + Cl-(aq)

2) HCl(aq) H+(aq) + Cl-(aq)

 

K= Ksp = [Na+(aq)][Cl-(aq)]

 

Ka =

 

[Cl-(aq)] = Ka[HCl(aq)]/[H+(aq)]

 

Ksp / [Na+(aq)] = Ka[HCl(aq)]/[H+(aq)]

 

Standards: Chemistry 9-12

Solutions

  • 6a. Students know the definitions of solute and solvent.
  • 6b. Students know how to describe the dissolving process at the molecular level by using the concept of random molecular motion.

Chemical Equilibrium

  • 9a. Students know how to use Le Chatelier's principle to predict the effect of changes in concentration, temperature, and pressure.

Questioning Script

Prior knowledge & experience:

  • To make a solute come out of a solution, we need to evaporat the solvent or cool the solution down.
  • Acid molecules are ionized in water, so the ion concentration would increase with increasing the acid concentration.
  • Neutral salt are stable and does not react with strong acids.

Root question

  • Is a saturated solution in a state of equilibrium between the dissolved solutes and the solid solute?

Target response

  • Students will understand that the equilibrium state of a saturated solution is dynamic process.
  • Students can predict the direction of the equilibrium shift when it is disturbed.

Common Misconceptions:

  • Students usually believe that sodium chloride donít react with hydrochloric acid because it is a neutral salt and create no change in the solution.
  • If another solution that doesnít react with the orginal solution is added, the total volume of solvent is increased, and thus its concentration is decreased. Therefore, students suppose that the solute does not precipitate.

Procedure

  1. Make a saturated NaCl solution by mixing an excess of salt into distilled water. The solubility of NaCl in H2O is approximately 35g/100ml at 20, so 100g of NaCl into 200ml of H2O should be sufficient.
  2. Transfer enough of the saturated solution into a Petri dish to cover the surface of the dish.
  3. Place the Petri dish on the overhead projector and add a few drops of concentrated HCl to the solution and stir the solution vigorously until crystals come out.

Caution

  • Be careful when you choose a table salt because some table salts contain Iodine and usually a flowing agent such as dextrose. The crystals will not form as readily as with regular salt.
  • When you prepare the saturated NaCl solution, heat up the solution with an excessive amount of salt and stir thoroughly to ensure that you are starting with a saturated salt solution.
  • The more concentrated the HCl the quicker the result.
  • When working with strong acids, wear eye protection and a lab apron. Keep you hands away from your eyes and wash your hands when finished.
  • Do not add water or a solution to acid. Remember Always add an acid to water.
  • Dilute the strong acid before disposing it.

  Background Knowledge

A saturated solution at a constnat temperature is in a dynamic equilibrium, solubility equilibrium, between the solid and dissolved states of a compound. If we mix a soluble salt containing an ion common to a slightly soluble salt solution, we will affect the position of the equilibrium of the slightly soluble salt system. Adding the common ion to the salt solution will lead to the decrease of its solubility. This common ion effect is an application of LeChatelier's Principle. According to LeChatelier's Principle, the increased common ion concetration place a stress upon the slightly soluble salt equilibrium, so the equilibrium will respond so as to undo the stress of added common ion. This means that the equilibria will shift so that the common ion will be reduced which means a shift to reduce the solubility of the slightly soluble salt system.

Common ion effect: the shift in equilibrium caused by the addition of a substance with an ion in common to the substances in equilibrium.

I asked my boys to compete with each other to find out who can make saturated NaCl solution faster. Look at the photos above. Then you can notice the amount of salt used in this preparation of saturated salt solution. The last photo is the final state of disturbance. I used a lid of jelly bean bottle to make it easy to see the crystals.

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