Most pupil are puzzled regarding the use of various salt-making methods.
(1) why can't titration be used for all the salts?
(2) Since the first reaction for the topic 'Acids-Bases' introduced 'acids react with REACTIVE metals to give salt and hydrogen gas, can we use this method instead?
Discussion:
There are many methods to make a salt, HOWEVER, we need to see if the method is feasible ( workable).
For the case of 'reactive metals and acids'
Of course, you can use reactive metals and acids to make a salt.
BUT, it will be too dangerous if you are using a REACTIVE metal.
Eg: To make Potassium chloride (KCl), it will be DANGEROUS if you are going to react Potassium with Hydrochloric acid ! Potassium explodes with cold water. [Can You Imagine the reaction of potassium with acids?]
Hence, this method ( acids with reactive metals) can only be used for metals that are moderately reactive! example: magnesium, zinc, etc.
If you want to form Copper(II) sulphate, you cannot use this method because copper is unreactive to acids!
Hence reacting copper(II) oxide/carbonate with acids is the ONLY method!
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For the case of 'insoluble oxides/insoluble carbonates with acids'
Ever wonder why it has to be insoluble reagents in the first place?
Back to basics: 'what are acids made of' ?
'An acid is a substance that produces H+ ions when dissolve in water.'
In short, there is water in an acid.
If the oxides and carbonates are soluble in water, then how do you ensure that your acids will be FULLY reacted?
remember in our procedure for heating insoluble oxides/carbonate with acids, step #2
(1) Warm the acid.
(2) add EXCESS insoluble oxides and carbonate and stir to dissolve.
(3) FILTER the mixture ........
If your acids did not REACT with the insoluble oxides/carbonate FULLY, then the acid will contaminate the salt that is formed in the process!
In short, for the usage of heating acids with reactive metals/ insoluble oxide/ insoluble carbonates, the REASONS behind it include:
(1) the oxides /carbonates will be insoluble in water, hence will definitely be able to react with the acids FULLY. Upon complete reaction with acids, all excess metal/insoluble oxides/carbonates will remained and this is an indication that all acids had fully reacted.
(2) the metals used are moderatly reactive and is suitable to react with warm acids.
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What about 'titration'?
There are a few reasons for the usuage of titration.
The first reason is because the metal is too reactive and using their hydroxides is more appropriate.
Eg: to make sodium nitrate. We cannot use sodium metal as it is far too reactive with acids.
So, we use sodium HYDROXIDE and nitric acid.
In this case, we cannot use heating method as both are soluble and we cannot tell when neutralization takes place.
Hence, for the second reason:
Second reason: If you use soluble oxides and carbonates for the method of heating, one problem you foresee would be unable to know when neutralization occurs. Reason: all the reagents are soluble, you can't tell. ( recall for the case of heating insoluble oxides/carbonates, the reagent is insoluble so you can see when the reagent no longer dissolves in the acid).
But titration is able to tell because of the use of INDICATOR.
Hence, we will be able to know the EXACT amount of alkali needed to neutralize the acid to form the salt when the indicator CHANGES COLOUR.
Please do not state this reason as your answer for 'why titration is used': 'because if we use insoluble oxides/carbonates, it will not be able to pass through the burette' !!!!
I mentioned this point to the class because i wanted you to remember insoluble hydroxides and oxides cannot be used for titration in the LABORATORY. How can an insoluble reagent flow through a burette?
The answer why 'titration is used instead of heating method' because: ** both the reagents are soluble and there is NO INDICATION when neutralization takes place. For titration, indicator can be used to identify the point of neutralization.
Now, is it much clearer?!
