Making Salts (T4) Flashcards
Most chlorides and sulfates are soluble. What are the exceptions?
- Barium Sulfate
- Lead Sulfate
- Lead Chloride
- Silver Chloride
Are all nitrates soluble or insoluble?
Soluble
Most Carbonates are insoluble. What are the exceptions?
- Ammonium Carbonate
- Potassium Carbonate
- Sodium Carbonate
Making soluble salts (EXCEPT sodium, potassium and ammonium salts) involve reacting a solid with an acid. What three different types of mixtures can you use?
- acid + metal (but only for the moderately reactive metals from magnesium to iron in the reactivity series)
- acid + metal oxide or hydroxide
- acid + carbonate
How would you make magnesium sulfate crystals?
Mg(s) + H2SO4(aq) > MgSO4(aq) + H2(g)
Enough magnesium is added to some dilute sulfuric acid so there is some left when the reaction stops bubbling. This makes sure there is no acid left in the final mixture.
The unused magnesium is then filtered off, and the magnesium sulfate solution is concentrated by boiling it until crystals will form when it is cooled.
A small drop of the mixture on the end of a glass rod is cooled rapidly to see if crystals form.
Finally the solution is left to cool and colourless magnesium sulfate crystals form. Any uncrystallised solution can be poured off the crystals and the crystals can be blotted dry with paper tissue.
When making magnesium sulfate crystals why not just evaporate the solution to dryness?
Because evaporation won’t give you magnesium sulfate crystals - instead you would get a white powder of anhydrous magnesium sulfate.
How would you make copper (II) sulfate crystals from copper (II) oxide?
CuO(s) + H2SO4(aq) > CuSO4(aq) + H2O(l)
Enough black copper (II) oxide is added to some HOT dilute sulfuric acid so there is some left when the reaction stops bubbling. This makes sure there is no acid left in the final mixture.
The unused black copper (II) oxide is then filtered off, and the copper (II) sulfate solution is concentrated by boiling it until crystals will form when it is cooled.
A small drop of the mixture on the end of a glass rod is cooled rapidly to see if crystals form.
Finally the solution is left to cool and copper (II) sulfate crystals form. Any uncrystallised solution can be poured off the crystals and the crystals can be blotted dry with paper tissue.
How do you know if you need to heat the mixture when making a salt?
Carbonates react with dilute acids in the cold, and so does magnesium.
Most other things you are likely to come across need to be heated.
What are the problems posed with making sodium, potassium and ammonium salts?
All sodium, potassium and ammonium compounds are soluble in water.
The solid you added would not only react with the acid, but any excess would just dissolve in the water present.
You wouldn’t have any excess to filter off and there is no simple way of seeing when you have added just enough of the solid to neutralise the acid.
How would you solve the problem posed with making sodium, potassium and ammonium salts?
You normally make these salts from sodium or potassium hydroxide or ammonia solution but you can also use the carbonates.
As all these solutions are alkaline you can find out when you have a neutral solution by using an indicator.
This method is called a TITRATION and the point at which an indicator changes colour during the titration is called the END POINT of the titration.
How would you make sodium sulfate crystals?
2NaOH(aq) + H2SO4(aq) > Na2SO4(aq) + 2H2O (l)
25 cm3 of sodium hydroxide solution is transferred to a conical flask using a pipette, and a few drops of methyl orange are added as the indicator.
Dilute sulfuric acid is run in from the burette until the indicator turns from yellow to orange.
The volume of acid is noted, and the same volumes of acid and alkali are mixed together in a clean flask without any indicator.
The solution can be crystallised by evaporating it to the point that crystals will form on cooling, then leaving it for the crystals to form.
The crystals are finally separated from any remaining solution and allowed to dry.
How would you make sodium chloride crystals?
NaOH(aq) + HCl(aq) > NaCl(aq) + H2O(l)
The titration would need to be carried out using hydrochloric acid.
Once you have re-mixed the acid and the alkali without the indicator you can evaporate the sodium chloride solution to dryness.
This is because sodium chloride crystals do not contain any water of crystallisation, so you can save time by evaporating all the water off in one go.
The disadvantage is you end up with either a powder or very tiny crystals.
What is happening with regards to ions in a precipitation reaction?
The ions of an insoluble salt are clumping together to form the solid.
How do you write ionic equations for precipitation reactions?
Write down the formula for the precipitate on the right hand side.
Write down the formulae for the ions that have clumped together to form it on the left hand side.
Don’t forget the state symbols!
Eg:
Ag+(aq) + Cl-(aq) > AgCl(s)
In short, what are the steps in making an insoluble salt?
- heat a beaker containing dilute acid slowly, making sure the acid does not boil.
- add small amounts of the insoluble base to the warm acid, agitating the acid carefully until the creation of gas stops
- use filter paper to remove any excess of the insoluble base, collecting the rest in a shallow basin as a solution
- heat the shallow basin gently to force dehydration and crystallisation of the salt and use filter paper to blot off any remaining water
