Making salts

Making soluble salts. You can use any of these mixtures but if you use alkali will require a diff method from using an insoluble (in water) base — aunque la vd se parecen bastante los methods

• Acid + metal (but only for the moderately reactive metals from magnesium to iron in reactivity series)

• Acid + metal oxide or hydroxide

• Acid + carbonate

Preparing a soluble salt from the reaction of an acid + insoluble base

Method? This method will always be the same

• Insoluble base is added in excess to ensure all the acid has reacted

TRIPLE CONTENT

Practical: making copper(II) sulfate crystals

• Add acid to a conical flask. Heat it in a water bath to this speed the reaction between acid and insoluble base

• Add copper(II) oxide to acid in excess so all the acid reacts (it has been neutralised) and stir with a stirring rod

• Filter off the excess copper(II) oxide and transfer the filtrate which is blue, to an evaporating basin

• Place evaporating basin in top of a beaker half full of water and heat it

• Keep heating until a saturated solution is formed, when we start seeing formation of crystals in the solution

• Stop heating and let it cool slowly (evaporate) at room temp so larger crystals can form

• Remove blue crystals by filtration

• Dry crystals with paper towel or leaving them to dry in a dry, warm place.

Where should we be heating the acid? Why?

Why is the base added in excess?

In a fume cupboard to avoid releasing acid fumes to room

So all the acid is neutralised. If not completely neutralised, then any acid would become dangerously concentrated during evaporation and crystallisation

After filtering, what is the name of the filtrate? What 2 components are inform the filtrate?

When we keep heating to obtain a saturated solution, how can we test this?

Since all the acid (H2SO4) has reacted and the excess solid base (copper(II) oxide) has been removed, we are left with water + salt of copper sulfate: we have a solution (aq) of copper sulfate

Dipping a glass rod. If crystals form on the rod when we remove it we know the solution is very close to saturated and crystals will also begin to form in the solution (on the basin)

What is the name of the blue crystals formed?

Hydrated copper(II) sulfate

Why don’t we heat solution of copper sulfate to dryness?

So why when we crystallise we form hydrated copper(II) sulfate?

It would seem much easier to just evaporate of all the water by boiling, rather than crystallising the solution slowly, but evaporating to dryness wouldn’t give you blue, copper sulphate crystals. Instead, you would produce a white powder of anhydrous copper(II) sulphate.

When many salts form their crystals, water from the solution becomes chemically bound up with the salt. This is called water of crystallisation. A salt which contains water of crystallisation is said to be hydrated.

Safety precautions?

Wear eye protection

Avoid contact with the crystals as they are irritating to both skin and eyes

Do not evaporate solution to dryness

Heating must be gentle to avoid ‘spitting’ of hot particles

TRIPLE CONTENT: make soluble salts starting from and acid and alkali

What method? Why can’t we use the one before?

Titration.

We can’t tell whether the reaction has finished — there is no signal that’s all the acid has been neutralised. You also can’t just add an excess of alkali to the acid because the salt is soluble and would be contaminated with the excess alkali

What kind of indicator should we use? Why can’t we use universal indicator?

For titration, you should use an indicator with a single, clear colour change (like methyl orange or phenolphthalein)

Because it’s colour change is gradual so there is no clear endpoint

Method?

First perform the titration

Once we have the exact volumes so that neutralisation happens, add this same volume of alkali to that calculated volume of acid without the indicator

The solution that remains when the reaction is complete is water and salt (that salt solution, e.g sodium chloride solution if we had reacted sodium hydroxide with hydrochloric acid)

Slowly evaporate off some of the water with Bunsen burner and then leave the solution to crystallise (leave in a dry warm room)

Filter off the solid and dry it. You have now pure dry salt.