7) Acids, Bases and Salts

Define ‘Acids’

Acids are proton donors, H+ ions

Define ‘bases’

Bases are proton acceptors

Define ‘amphoteric oxide’

Oxides that react with acids and bases to produce a salt and water.

Define ‘hydrated substance’

A substance that is chemically combined with water.

Define ‘anhydrous substance’

A substance containing no water.

Define ‘water of crystallisation’

The water present in hydrated crystals.

properties of acids

Acids are substances that can neutralise a base, forming salt and water.

pH values - below 7

Sour taste when edible. They are corrosive

When acids are added to water, they form positive hydrogen ions, which is what makes a solution acidic. Aqueous acid solutions contain H+ ions.

Reaction of acids and metals

Only metals above hydrogen in the reactivity series will react with dilute acids.

Acid + Metal --> Salt + Hydrogen

Reactions of acids and bases

Metal oxides and metal hydroxides (alkalis) can act as bases.

A neutralisation reaction occurs when bases react with acids.

Acid + Base --> Salt + Water

Reaction of acids and carbonates

Acids react with metal carbonates to form the corresponding metal salt, carbon dioxide and water.

Acid + Metal Carbonate --> Salt + Carbon dioxide + water

Properties of bases

Bases are substances that can neutralise an acid.

pH values - Above 7

They are oxides and hydroxides of metals.

Alkalis are soluble bases.

When alkalis are added to water, they form negative hydroxide ions, which is what makes an aqueous solution alkaline.

Aqueous alkali solutions contain OH- ions.

reaction of Bases and acids

A neutralisation reaction occurs when acids react with bases.

Acid + Base --> Salt + Water

reaction of bases and ammonium salts

When warmed with an alkali, ammonium salts undergo decomposition.

Ammonia is a weak and volatile base that can be easily displaced by another alkali.

Alkali + Ammonium salt --> Salt + Water + Ammonia

(Example no memorise:
NaOH + NH4Cl --> NaCl + H2O + NH3)

What are indicators used for

Indicators are used to distinguish between acids and bases

Litmus in acid and alkali

Red in acid

Blue in alkali

Thymolphthalein

Colourless in acid

Blue in alkali

Methyl orange

Red in acid

Yellow in alkali

Explain neutralisation reactions

Acids contain H+ ions and Bases (alkalis) contain OH- ions.

When they react, the H+ and OH- ions react to produce water, which has a neutral pH of 7, and is what makes a solution neutral.

H+(aq) + OH-(aq) --> H2O(l)

pH scale properties

pH is a measure of the concentration of hydrogen ions.

The scale goes 1-14.

The lower the pH the more acidic, and the more hydrogen ion concentration.

The higher the pH, the more alkaline, and the more hydroxide ions.

Label sections of the pH scale

pH 0-2 = strong acid

pH 3-6 = weak acid

pH 7 = neutral

pH 8-11 = weak alkali

pH 12-14 = strong alkali

What is universal indicator and what are the observations

Universal indicator is used to measure the pH of any solution. It changes colour (of the solution if liquid and the paper itself if paper) and is matched with a colour chart which indicates the pH of specific colours.

Yellow to Red towards acidic

Green when neutral.

Blue to purple when alkaline.

Describe proton transfer

H+ Ions are known as protons, as H+ ions have no electrons.

Acids are proton donors. They donate hydrogen ions to the system/ionise and produce protons.

Bases are proton acceptors. They accept the protons donated by the acid. Water can act as a base.

What is a strong acid and example

Strong acids completely dissociate (ionise) in aqueous solution and water, producing very low pH solutions.

Hydrochloric acid is a strong acid:

HCl (aq) --> H+(aq) + Cl-(aq)

What is a weak acid and example

Weak acids partially dissociate in aqueous solution, producing middle-low pH solutions.

Ethanoic acid is a weak acid:

CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)

What are oxides

Oxides are compounds made from one or more atoms of oxygen.

What are acidic oxides

Acidic oxides are formed when a non-metal element combines with oxygen, such as SO2 and CO2.

They react with bases to form salt and water, and form acidic solutions in water with low pH

What are basic oxides

Basic oxides are formed when a metal combines with oxygen, such as CuO and CaO.

They react with acids to form salt and water, and produce alkaline/basic solutions in water with high pH.

What are amphoteric oxides

Amphoteric oxides behave as both acidic and basic; common amphoteric oxides include:

Zinc oxide - ZnO

Aluminium oxide - Al2O3

The hydroxides of them are also amphoteric.

Solubility rules

1) All sodium, potassium and ammonium salts are soluble.

2) All nitrates are soluble.

3) All chlorides are soluble, EXCEPT lead and silver chloride (PbCl2 & AgCl).

4) All sulfates are soluble, EXCEPT barium, calcium and lead sulfate (BaSO4, CaSO4, & PbSO4)

5) All carbonates are INsoluble, EXCEPT sodium, potassium, and ammonium carbonate.

6) All hydroxides are INsoluble, EXCEPT sodium, potassium, ammonium and calcium hydroxide (calcium hydroxide is partially soluble).

Describe the preparation soluble salts by reacting an acid with an insoluble metal, base, or carbonate in excess

1) Add dilute acid into a beaker and heat using bunsen burner flame.

2) Add the insoluble metal, base, or carbonate, a little at a time, to the warm dilute acid and stir till the substance is in excess and stops disappearing from soluble salt formation. All the acid is used up.

3) Filter the mixture into an evaporating basin to remove the excess solid.

4) Heat solution to evaporate the water and to make the solution saturated (no more solute can dissolve in solvent). This can be checked by dipping a cold, glass rod into the solution and seeing if crystals form.

5) Leave the filtrate in a warm place to dry and crystallise.

Describe the preparation of a soluble salt by reacting an acid with an alkali by titration

1) Use a pipette to measure the alkali into a conical flask. Add a few drops of indicator (thymolphthalein or methyl orange)

2) Add acid into a burette.

3) Record the starting volume of acid in the burette.

4) Add the acid slowly into the conical flask using the burette until the indicator changes colour.

5) Record the final volume of acid in the burette and find the volume of acid added (difference). Final volume - initial volume.

6) Add this same volume of acid to the same volume of alkali except without the indicator.

7) Heat the resulting solution in an evaporating basin using a bunsen burner flame till water is evaporated and solution is saturated.

8) Leave the filtrate in a warm place to dry and crystallise.

Describe the preparation of insoluble salts by precipitation

1) Dissolve two soluble salts in water and mix together using a stirring rod to form precipitate.

2) Filter to remove the precipitate from the mixture.

3) Wash the residue on filter with distilled water to remove traces of other solutions.

4) Leave in an oven to dry.

Hydrated and anhydrous copper(II) sulfate formula

Hydrated copper(II) sulfate -
CuSO4 ∙ 5H2O (BLUE)
Anhydrous copper(II) sulfate -
CuSO4

Hydrated and anhydrous cobalt(II) chloride formula

Hydrated cobalt(II) chloride -
CoCl2 ∙ 6H2O (WHITE)

Anhydrous cobalt(II) chloride - CoCl2

How to convert between hydrated and anhydrous compounds

The conversion between anhydrous to hydrated compounds is reversible by heating.