Monday, September 05, 2005
CHEMISTRY - Preparation of Salts
Preparation of salts posted by britt
Salts can be ‘prepared’ by adding an acid to an alkali. This is called titration. However, when we say preparation, it means that we would want a clean, pure sample of the salt at the end of the experiment, not just a flask of liquid that might contain the salt and dunno what else.
So, if you think about it, adding an acid and an alkali, without thinking about the amount to add, is actually a lousy way of preparing a salt.
WHY?
Because unless you add an indicator (methyl orange, Phenolphthalein etc), you would not know WHEN to stop adding the acid/alkali to the other chemical.
Think about it, let say you wish to make sodium chloride. You take some sodium hydroxide solution and add hydrochloric acid. Well and good. But how do you know how much acid to add?
If you add too little, then there will be leftover sodium hydroxide.
If you add too much acid, then of course there will be leftover acid.
(‘What’s the big deal?’ you may ask, I still get the salt anyway.)
The big deal is: It’s a real pain in the rear end to purify the salt you want when there is leftover acid/alkali mixed in.
(‘Then use an indicator!’ you may say, when it changes colour, then you stop adding the acid)
The second big deal is: It’s an even bigger pain in the rear end to remove the indicator dye from the salt solution.
You may insist: It CAN BE DONE, right?? It’s not impossible to separate leftover acid/alkali or indicator dye from the salt, is it??
Yes, but why bother? There are other EASIER methods. One of which is to just REPEAT the titration using the reading from the first expt but without the indicator.
Another issue is that there are MANY salts that can’t be prepared using an acid-alkali titration method because the starting materials are NOT SOLUBLE in watter.
(See your IVLE workbin for videos on titratiosn and how the indicator changes colour when the reaction is completed.)
Methods of preparing a salt that is soluble in water.
The principle of preparing a salt is to use 2 substances in a reaction where ONE of them is insoluble in water and use THIS substance in EXCESS. Because it is insoluble, the EXCESS can then be FILTERED off when the reaction is finished.
Another advantage of using an insoluble starting material is that when you can actually SEE the leftover solid particles in the solution when the reaction is FINISHED.
Methods of preparing a salt that is insoluble in water.
This is a lot easier as if the product is insoluble in water, then both your starting materials should soluble in water. It does not matter whether the reagent used is in excess because the product can be recovered by FILTRATION, while the soluble reactants will be removed.
For more info, read page 185-197 of your text book. In the exam, you will be asked how to describe the methods in detail as well as to provide chemical equations, and steps in the recovering and purifying the salt. Sometimes, labeled diagrams are also required.
FAQ
Question: What if an exam question comes out and they give a salt for preparation and I don’t know if it is soluble or not?
Answer: There is no sure way of calculating or predicting the solubility of any given salt. The ONLY way to know is to MEMORIZE the list below:
SOLUBLE
All sodium salts
All potassium salts
All ammonium salts
All nitrates
All chlorides(except)
Sodium carbonate
Potassium carbonate
Ammonium carbonate
Sodium oxide
Potassium oxide
All sulphates (except)
INSOLUBLE
Silver chloride
Lead (II) chloride
Barium sulphate
Lead (II) sulphate
Calcium sulphate
All carbonates(except)
All oxides(except)
Question: OK, I know that the salt is soluble or insoluble. How do I know which method is the correct one I should use.
Answer: A lot of it depends on whether the question wants a purified, dried sample in the form of crystals. Below is another chart that you can use to help you. (cannot get to display here, deepest apologies)
Question: Let’s say I wish to prepare salts using an acid and a metal. Are certain metals that are suitable and certain metals that are NOT suitable?
Answer: YES. Moderately reactive metals (such as magnesium, zinc, aluminium and iron) are suitable for reactions with acids. However, very reactive metals (such as sodium, potassium and even calcium) cannot be added to acids because there will be an EXPLOSION. Yes, you will get the salt, as well as a funeral.
Certain metals, such as copper and lead, are unreactive and will not react with acids. (The reaction between copper and concentrated nitric acid is an exception, but we don’t use this method unless we are committing suicide, see video in IVLE workbin for explanation)
Question: If copper will not react with acids, how am I supposed to prepare copper salts?
Answer: You can use a base such as copper oxide (a black powder) or copper carbonate (a green coloured solid) to react with acids.
Question: How exactly do we use the titration method for producing a salt from a soluble base (alkali) and an acid?
Do the titration experiment as described in your text (page 194). After you do the experiment, you will have a reading from the burette as to how much alkali to add to the acid without any excess acid/alkali. Unfortunately, you cannot use what has been formed in the conical flask as it has indicator mixed in. Unfortunately, you can’t leave the indicator out as you need it to tell you when to stop adding the alkali from the burette
So, you repeat the experiment using exactly the same volume of acid and alkali WITHOUT the indicator. The flask (if you did the expt properly) will now contain just the salt, and water, without any excess acid or alkali).
with lots of love from the britt
Salts can be ‘prepared’ by adding an acid to an alkali. This is called titration. However, when we say preparation, it means that we would want a clean, pure sample of the salt at the end of the experiment, not just a flask of liquid that might contain the salt and dunno what else.
So, if you think about it, adding an acid and an alkali, without thinking about the amount to add, is actually a lousy way of preparing a salt.
WHY?
Because unless you add an indicator (methyl orange, Phenolphthalein etc), you would not know WHEN to stop adding the acid/alkali to the other chemical.
Think about it, let say you wish to make sodium chloride. You take some sodium hydroxide solution and add hydrochloric acid. Well and good. But how do you know how much acid to add?
If you add too little, then there will be leftover sodium hydroxide.
If you add too much acid, then of course there will be leftover acid.
(‘What’s the big deal?’ you may ask, I still get the salt anyway.)
The big deal is: It’s a real pain in the rear end to purify the salt you want when there is leftover acid/alkali mixed in.
(‘Then use an indicator!’ you may say, when it changes colour, then you stop adding the acid)
The second big deal is: It’s an even bigger pain in the rear end to remove the indicator dye from the salt solution.
You may insist: It CAN BE DONE, right?? It’s not impossible to separate leftover acid/alkali or indicator dye from the salt, is it??
Yes, but why bother? There are other EASIER methods. One of which is to just REPEAT the titration using the reading from the first expt but without the indicator.
Another issue is that there are MANY salts that can’t be prepared using an acid-alkali titration method because the starting materials are NOT SOLUBLE in watter.
(See your IVLE workbin for videos on titratiosn and how the indicator changes colour when the reaction is completed.)
Methods of preparing a salt that is soluble in water.
The principle of preparing a salt is to use 2 substances in a reaction where ONE of them is insoluble in water and use THIS substance in EXCESS. Because it is insoluble, the EXCESS can then be FILTERED off when the reaction is finished.
Another advantage of using an insoluble starting material is that when you can actually SEE the leftover solid particles in the solution when the reaction is FINISHED.
Methods of preparing a salt that is insoluble in water.
This is a lot easier as if the product is insoluble in water, then both your starting materials should soluble in water. It does not matter whether the reagent used is in excess because the product can be recovered by FILTRATION, while the soluble reactants will be removed.
For more info, read page 185-197 of your text book. In the exam, you will be asked how to describe the methods in detail as well as to provide chemical equations, and steps in the recovering and purifying the salt. Sometimes, labeled diagrams are also required.
FAQ
Question: What if an exam question comes out and they give a salt for preparation and I don’t know if it is soluble or not?
Answer: There is no sure way of calculating or predicting the solubility of any given salt. The ONLY way to know is to MEMORIZE the list below:
SOLUBLE
All sodium salts
All potassium salts
All ammonium salts
All nitrates
All chlorides(except)
Sodium carbonate
Potassium carbonate
Ammonium carbonate
Sodium oxide
Potassium oxide
All sulphates (except)
INSOLUBLE
Silver chloride
Lead (II) chloride
Barium sulphate
Lead (II) sulphate
Calcium sulphate
All carbonates(except)
All oxides(except)
Question: OK, I know that the salt is soluble or insoluble. How do I know which method is the correct one I should use.
Answer: A lot of it depends on whether the question wants a purified, dried sample in the form of crystals. Below is another chart that you can use to help you. (cannot get to display here, deepest apologies)
Question: Let’s say I wish to prepare salts using an acid and a metal. Are certain metals that are suitable and certain metals that are NOT suitable?
Answer: YES. Moderately reactive metals (such as magnesium, zinc, aluminium and iron) are suitable for reactions with acids. However, very reactive metals (such as sodium, potassium and even calcium) cannot be added to acids because there will be an EXPLOSION. Yes, you will get the salt, as well as a funeral.
Certain metals, such as copper and lead, are unreactive and will not react with acids. (The reaction between copper and concentrated nitric acid is an exception, but we don’t use this method unless we are committing suicide, see video in IVLE workbin for explanation)
Question: If copper will not react with acids, how am I supposed to prepare copper salts?
Answer: You can use a base such as copper oxide (a black powder) or copper carbonate (a green coloured solid) to react with acids.
Question: How exactly do we use the titration method for producing a salt from a soluble base (alkali) and an acid?
Do the titration experiment as described in your text (page 194). After you do the experiment, you will have a reading from the burette as to how much alkali to add to the acid without any excess acid/alkali. Unfortunately, you cannot use what has been formed in the conical flask as it has indicator mixed in. Unfortunately, you can’t leave the indicator out as you need it to tell you when to stop adding the alkali from the burette
So, you repeat the experiment using exactly the same volume of acid and alkali WITHOUT the indicator. The flask (if you did the expt properly) will now contain just the salt, and water, without any excess acid or alkali).
with lots of love from the britt
# we mugged! @ 6:09 AM 
