Acid, Bases and Salts

Definitions (Memorise them!)
An acid is a substance that produces hydrogen ions (H+) when dissolved in water.
A strong acid is one that dissociates completely in water to give hydrogen ions
A weak acid is one that dissociates partially in water to give hydrogen ions.

An alkali is a substance that produces hydroxide ions when dissolved in water.
A strong alkali is one that dissociates completely in water to give hydroxide ions.
A weak alkali is one that dissociates partially in water to give hydroxide ions.

A salt is an ionic compound formed from the reaction between an acid and a base.

An acidic oxide is one that reacts with a base to give salt and water only.
An amphoteric oxide is one that reacts with both acid and base to give salt and water only
A basic oxide is one that reacts with an acid to give salt and water only.
A neutral oxide is on that does not react with both acid and bases to give salt and water only.

Acid
Reactions of acids:
  1. neutralisation reaction: acid + base --> salt + water
  2. acid + metal --> salt + hydrogen [ copper, silver and gold DO NOT react with acids!]
  3. acid + carbonate --> salt + water+ carbon dioxide
Bases
Soluble base(s) are called alkali(s).
Reaction of bases:
  1. neutralisation reaction: acid + base --> salt + water
  2. base + ammonium salts --> ammonia + water+ salt
  3. base + soluble metallic salt --> insoluble hydroxide + salt
Solubility table [ memorise it well!]
All chlorides are soluble except lead (II) chloride and silver chloride.
All sulphates are soluble except lead (II) sulphate and barium sulphate. Calcium sulphate is slightly soluble.
All nitrates are soluble.
All carbonates, hydroxides and oxides are insoluble except group (I) . Calcium hydroxide is slightly soluble while barium hydroxide is soluble in water.
All ammonium salts are soluble.


Want a flashcard where you can memorize the solubility table above on the GO? Download the solubility app here. 

Salt formation (3 types!)
1. Group 1 and ammonium salts are formed by titration. [ for O level syllabus, you can remember the SPA (sodium, potassium and ammonium salts) and group I are prepared by titration]
2. All other soluble salts are formed by reacting an excess of their oxides, hydroxides or carbonates with acid.
3. All insoluble salts are formed by precipitation.

Method 1: Titration
Pipette 25 cm3 of ____A_____ into the conical flask. Add a few drops of methyl orange to the solution. Methyl orange turns ________(colour).
Add ___B_____ dropwise to the conical flask using a burette until methyl orange turns orange. Note the volume, V, of _____B____ added.
Repeat the experiment again by adding 25 cm3 of ____A____ using a pipette to the same volume, V, of ____B____ (using a burette) but without methyl orange.

To obtain a pure, dry crystals of the salt:
If salt is SODIUM CHLORIDE or question says that salt will not decompose on heating:
Evaporate the solution to dryness to obtain the salt. Wash the crystals with cold distilled water and dry the crystals between filter paper.

For all other salts:
Heat the solution obtained to evaporate some of the water so as to obtain a hot saturated solution. Cool the hot saturated solution to allow crystals to form. Filter the crystals using filter paper. Wash the crystals with cold distilled water and dry the crystals between filter paper.

Method 2: Adding excess oxides, carbonates, metal to the acid
Add _____A______ (oxide, carbonate or metal) to _____B_____ acid until no more can dissolve. Stir the solution. Filter the undissolved _____A_____.
Heat the solution obtained to evaporate some of the water so as to obtain a hot saturated solution. Cool the hot saturated solution to allow crystals to form. Filter the crystals using filter paper. Wash the crystals with cold distilled water and dry the crystals between filter paper.
[** Note, very reactive metals i.e those from Group I and calcium should not be used. Use the oxide, hydroxide or carbonate instead.
Non- reactive metals i.e those below hydrogen in the reactivity series (e.g. Copper, silver and gold) should not be used. Use the oxide, hydroxide or carbonate instead.]

Method 3: Precipitation
Add ____A____ solution to ______B____ solution. Filter the mixture. The residue is the required salt. Wash the salt with cold distilled water and dry the salt between filter paper.

Indicators
Indicator Colour when acidic Colour when neutral Colour when basic
Methyl orange Red Orange yellow
Phenolphthalein colourless colourless pink
*For strong acid and strong base titration, all indicators can be used.
*For strong acid and weak base titration, use methyl orange.
*For weak acid and strong base titration, use phenophthalein.

No comments:

Post a Comment