Synthesising Salts
From OneSchoolWiki
Main Content
Malaysia SPM Form 4 Chemistry, Chapter 8: Salt
Contents |
[edit] What is Salt?
Salts: Replacing the Hydrogen of Acids
- A salt is the substance formed when the hydrogen of an acid is partly or completely replaced by a metal or ammonium.
- All salts are ionic compound.
Example
When the hydrogen ion in nitric acid is replaced by sodium ion (Na+), calcium ion (Ca2+), ammonium ion (NH4+) or aluminium ion (Al3+) it will form salts of sodium nitrate [NaNO3], calcium nitrate [Ca(NO32], ammonium nitrate [NH4NO3]and aluminium nitrate [Al(NO3)3]respectiely.
| Example 1 |
State whether the following chemical are salt or not salt..
Answer
|
[edit] Solubility of Salt
Solubility is the ability of water to dissolve a substain in a solvent (normally water). Recognising the solubility of a salt is the very first step to determine which method is suitable to be used to prepare a salt. The solubility of a salt is also one of the most important property used to identify the ions present in a salt. Therefore it is very important for a student to memorise the solubility of certain salts, oxides and hydroxides listed in the table below.
| Salt | Solubility |
|---|---|
| Salt of potassium, sodium and ammonium | All soluble in water |
| Salt of nitrate | All soluble in water |
| Salt of sulphate | Mostly soluble in water except: (Pb) Lead sulphate |
| Salt of chloride | Mostly soluble in water except: (Pb) Lead chloride |
| Salt of carbonate | Mostly insoluble in water except: Potassium carbonate |
[edit] Solubility of Oxide and Hydroxide of Certain Matel
Oxides or hydroxides of metal are not salt. The are base instead. However, the solubility of oxides and hydroxides are equally important in identifying the ions present in a salt.
| Oxide and Hydroxide | Solubility |
|---|---|
| Oxide | Mostly insoluble in water except: K2O and Na2O. |
| Hydroxide | Mostly insoluble in water except: KOH and NaOH |
[edit] Preparation of Soluble Salts
If youn still remember, in previous chapter (Asid and Base), we have learned that there are 4 important reactions for all acids:
(1) Acid + Metal ---> Salt + Hydrogen gas
(2) Acid + Metal Carbonate ---> Salt + Water + Carbon Dioxide gas
(3) Acid + Metal Oxide ---> Salt + Water
(4) Acid + Metal Hydroxide ---> Salt + Water.
We can see that all the reactions of acid listed above produce salt. In other words, all of these reaction can be used to prepare salts. In fact, all of these reaction is used to prepare soluble salt in the laboratory.
[edit] Which Reaction to be Used
In general, when making salts, we must ask the question:
- If the soluble salt is the salt of sodium, potassium or ammonium, we will use method (4) because this method enable use to find the “end point” of the reaction.
- “End point” is the amount of acid or base that exactly neutralize each others.
- If the soluble salt is not the salt of sodium, potassium or ammonium, we will use method (1), (2) and (3). This is because the metal, metal oxide or metal hydroxide that we use is insoluble in water.
- An insoluble substance can easily been separated from soluble salt through filtration.
[edit] Metal and Acid
- This preparation is suitable for the more reactive metals like Mg, Zn, Fe, Al (but not Na, K and Ca).
- In general, the metal is added to the acid until there is no further reaction.
- This is when no more bubbles of hydrogen gas are produced.
- Excess metal is then filtered out, and the clear filtrate is then evaporated until crystals begin to form in the hot solution.
metal + acid ---> salt + hydrogen
zinc + sulphuric acid ---> zinc sulphate + hydrogen
Zn (s) + H2SO4 (aq) ---> ZnSO4 (aq) + H2 (g)
[edit] Metal Oxide and Acid
- Nearly all the metal oxides react with acids, but most require warming.
- This method is especially suitable for those metals which do not react with dilute acids. For example, copper metal has no reaction with dilute acids but copper(II) oxide, if warmed with dilute acids, forms salts.
metal oxide + acid ---> salt + water
copper(II) oxide + sulphuric acid ---> copper(II) sulphate + water
CuO (s) + H2SO4 (aq) ---> CuSO4 (aq) + H2O (1)
- We must add excess copper(II) oxide to the warm sulphuric acid so that all the acid is neutralised. *The unreacted oxide is then removed by filtering.
- The filtrate is a blue solution of copper(II) sulphate.
- The crystals are obtained by concentrating the solution by evaporation, and then leaving it to cool. *The crystals formed can be removed by filtration.
- As copper(II) sulphate crystals contain water of crystallisation, it is important not to evaporate the solution to dryness.
[edit] Metal Hydroxide and Acid
- This method is especially suitable for soluble metal hydroxides called alkalis.
- The process is called titration. This involves adding an acid from a burette into a conical flask, which contains the alkali coloured by 2–3 drops of a suitable indicator.
- The alkali is accurately measured out using a pipette and its filler.
- The end of the titration is called the end point, and is reached when the colour of the indicator in the alkali changes. At this point, neutralisation has occurred and a salt and water are formed:
metal hydroxide + acid ---> salt + water
potassium hydroxide + hydrochloric acid ---> potassium chloride + water
KOH (aq) + HCl(aq) ---> KCl (aq) + H2O (l)
- To obtain the salt, the titration is repeated, but this time without an indicator being present as we know the volume of the acid to add.
- Alternatively, at the end point, we could add some animal charcoal and boil the solution. This absorbs the colour of the indicator.
- Finally, to get crystals of the salt, we can evaporate the neutral solution. As potassium chloride contains no water of crystallisation and is stable to heat, we can evaporate to dryness using a gentle Bunsen flame.
[edit] Metal Carbonate and Acid
- This method is very similar to that involving a metal oxide and an acid, but this time, no heat is required. The carbonate fizzes and gives off carbon dioxide gas.
- Again, excess carbonate must be added, to ensure that all the acid is neutralised.
- The solution is then filtered (to remove unreacted carbonate), and evaporated to concentrate the solution for crystallisation.
Metal carbonate + acid ---> salt + water + carbon dioxide
Zinc carbonate + nitric acid ---> zinc nitrate + water + carbon dioxide
ZnCO3 (s) + 2HNO3 (aq) ---> Zn(NO3)2 (aq) + H2O (l) + CO2 (g)
Example
Write down all the equation of the reaction that we can use to preprare potassium sulphate, ammonium chloride, magnesium nitrate and copper sulphate.
potassium sulphate (1 reaction)
magnesium nitrate (3 reaction)
ammonium chloride (1 reaction)
copper sulphat(2 reaction)
[edit] Preparation of Insoluble Salts:
Ionic Precipitation
- Insoluble salts can be made by precipitation. This involves mixing a solution that contains its positive ions with another solution that contains its negative ions.
- For example, insoluble silver chloride is precipitated when solutions of silver nitrate and sodium chloride solution are mixed together. e.g.
silver nitrate (solution) + sodium chloride (solution) ---> silver chloride + sodium nitrate (solution)
AgNO3 (aq) + NaCl (aq) ---> AgCl (s) + NaNO3 (aq)
Ag+ (aq) + C1- (aq) ---> AgCl (s) (ionic equation)
- With precipitation reactions it is simpler to use the ionic equation which only shows the ions in solution which join together to form the insoluble precipitate.
| Examples of insoluble salt | Formula | Examples of insoluble salt | Formula |
| Lead (II) iodide | PbI2 | Lead (II) sulphate | PbSO4 |
| Lead (II) bromide | PbBr2 | Calcium sulphate | CaSO4 |
| Lead (II) chromate (VI) | PbCrO4 | Lead (II) chloride | PbCl2 |
Example Write down 2 equation of the reaction that been used to prepare Calcium sulphate
Example Write down 2 equation of the reaction that been used to prepare Lead(II) Chloride.
