CSEC Chemistry Paper 2

Acids, Bases & Salts-Salts Preparation (Are we there yet?)

Picking up from if the salt we are making is not an anhydrous chloride (a binary dry salt), then the next question to ask is,?Is the salt a soluble salt?? We are going the opposite of the order in which solubility was given, since there are less insoluble salts, than soluble ones.

Preparing an insoluble salt

If the salt is insoluble, then the precipitation method is employed. To precipitate means to cause a substance to be deposited in solid form, from a solution (any form of deposit, be it liquid, snow, hail is accepted in Geography. Thankfully this is Chemistry)

This method is quick, immediate and dramatic, especially if a brightly coloured salt like lead (II) chloride is being prepared. It literally is like magic.

The first step involves selecting two soluble salts that contain the ions of the insoluble salt to be prepared. So, in this case, a soluble salt that contain Pb2+(aq) is required and a soluble salt which contains Cl(aq) is needed.

Since all nitrates are soluble, one could use lead nitrate, Pb(NO3)for the lead 2+ ions and one could use sodium chloride or potassium chloride, NaCl or KCl to provide the chloride ions.

(One doesn’t need to state the concentration)

One just needs to add a solution of lead nitrate (give a volume) to a solution of sodium chloride and stir. After stirring, one filters, rinse the residue with small amount of distilled water, then pat dry in paper towel. 

On the molecular level, looking at the actual ions that cause the action, you will have:

Pb(NO3)2(aq) + NaCl(aq) PbCl2(S) + NaNO3(aq)  (unbalanced)

Pb(NO3)2(aq) + 2NaCl(aq)  — PbCl2(S) + 2NaNO3(aq)  (balanced)

Pb2+(aq) + 2NO3(aq) + 2Na+(aq)+ 2Cl(aq) PbCl2(S) + 2Na+(aq)+ 2NO3(aq) (broken down into ions)

Remove ions or entities with same charge and state symbols that are common to the LHS and the RHS of the equation. Those are called spectator ions. (They are not the ones in the game.)

What you are left with is the essence of what truly happens, soluble lead 2+ ions in the presence of water, mix with soluble chloride ions in the presence of water and form a solid compound or a precipitate, just like that, that does not dissolve in the water or is insoluble.

Pb2+(aq)  + 2Cl(aq) PbCl2(S) 

This part sounds like a strange declaration from a movie, ?If the solid is formed from two liquids mixed together, it shall not be conquered by water (immune to water.)? The precipitate formed from two solution is insoluble in water. 

N.B. The solid or precipitate formed would settle out of the solution to give a suspension. The liquid filtered off as the filtrate would be sodium nitrate (doubly soluble?get it??)

The principle outlined above can be applied to any insoluble salt. 

Check yuhself!!!

Outline the steps involved in preparing a dry sample of calcium carbonate.

Tomorrow, we will wrap up salts (in foil paper if light sensitive) with working through a salt preparation question.


Contributed by: Kemil Walford

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