PROPERTIES OF ACIDS

(A) PHYSICAL PROPERTIES OF ACIDS

1. Acids have a characteristic sour taste
2. Most acids are colourless liquids
3. Mineral acids are odourless. Organic acids have characteristic smell
4. All acids have pH less than 7
5. All acids turn blue litmus paper red, methyl orange red and phenolphthalein colourless.
6. All acids dissolve in water to form an acidic solution. Most do not dissolve in organic solvents like propanone, kerosene, tetrachloromethane, petrol.

 

(B) CHEMICAL PROPERTIES OF ACIDS.

1. Reaction with metals

All acids react with a reactive metal to form a salt and produce /evolve hydrogen gas.

Metal + Acid -> Salt + Hydrogen gas

Experiment: reaction of metals with mineral acids.

(a)Place 5cm₃ of dilute hydrochloric acid in a small test tube. Add 1cm length of polished magnesium ribbon.

Stopper the test tube using a thump. Light a wooden splint. Place the burning splint on top of the stoppered test tube. Release the thump stopper. Record the observations made.

(b)Repeat the procedure in (a) above using Zinc granules, iron filings, copper turnings, aluminium foil in place of Magnesium ribbon

(c)Repeat the procedure in (a) then (b) using dilute sulphuric(VI) acid in place of dilute hydrochloric acid.

 

Sample observations

(i) Effervescence/bubbles produced/fizzing in all cases except when using copper

(ii) Colourless gas produced in all cases except when using copper

(iii) Gas produced extinguishes a burning wooden splint with an explosion/pop sound.

 

Explanation

Some metals react with dilute acids, while others do not. Metals which react with acids produces bubbles of hydrogen gas.

Hydrogen gas is a colourless gas that extinguishes a burning splint with a pop sound.

This shows acids contain hydrogen gas.

This hydrogen is displaced/removed from the acids by some metals like Magnesium, Zinc, aluminium,iron and sodium.

Some other metals like copper, silver, gold, platinum and mercury are not reactive enough to displace/remove the hydrogen from dilute acids.

 

Chemical equations

1. Magnesium + Hydrochloric acid -> Magnesium chloride + Hydrogen

Mg(s) + 2HCl (aq) ->MgCl₂ (aq) + H₂(g)

2. Zinc + Hydrochloric acid ->Zinc chloride + Hydrogen

Zn(s) + 2HCl (aq) ->ZnCl₂ (aq) + H₂(g)

3. Iron + Hydrochloric acid -> Iron(II) chloride + Hydrogen

Fe(s) + 2HCl (aq) -> FeCl₂ (aq) + H₂(g)

4. Aluminium + Hydrochloric acid -> Aluminium chloride + Hydrogen 2Al(s) + 3HCl (aq) ->AlCl₃ (aq) + 3H₂(g)
5. Magnesium + Sulphuric(VI)acid -> Magnesium sulphate(VI) + Hydrogen

Mg(s) + H2SO₄ (aq) ->MgSO₄ (aq) + H₂(g)

6. Zinc + Sulphuric(VI)acid -> Zinc sulphate(VI) + Hydrogen

Zn(s) + H₂SO₄ (aq) ->ZnSO4 (aq) + H₂(g)

7. Iron + Sulphuric(VI)acid -> Iron(II) sulphate(VI) + Hydrogen

Fe(s) + H₂SO₄ (aq) ->FeSO₄ (aq) + H₂(g)

8. Aluminium + Sulphuric(VI)acid -> Aluminium sulphate(VI) + Hydrogen

2Al(s) + 3H2SO₄ (aq) ->Al₂(SO4)₃ (aq) + 3H₂(g)

2. Reaction of metal carbonates and hydrogen carbonates with mineral acids.

All acids react with carbonates and hydrogen carbonates to form a salt, water and produce /evolve carbon(IV)oxide gas.

Metal carbonate + Acid -> Salt + Water + Carbon(IV)oxide gas

Metal hydrogen carbonate + Acid -> Salt + Water + Carbon(IV)oxide gas

 

Experiment: reaction of metal carbonates and hydrogen carbonates with mineral acids.

(a)Place 5cm₃ of dilute hydrochloric acid in a small test tube. Add half spatula full of sodium carbonate. Stopper the test tube using a cork with delivery tube directed into lime water. Record the observations made. Test the gas also with burning splint.

(b)Repeat the procedure in (a)above using Zinc carbonate, Calcium carbonate, copper carbonate, sodium hydrogen carbonate, Potassium hydrogen carbonate in place of Sodium carbonate.

(c)Repeat the procedure in (a) then (b) using dilute sulphuric(VI) acid in place of dilute hydrochloric acid.

Set up of apparatus

Sample observations

(i)effervescence/bubbles produced/fizzing in all cases.

(ii)colourless gas produced in all cases.

(iii)gas produced forms a white precipitate with lime water.

 

Explanation

All metal carbonate/hydrogen carbonate reacts with dilute acids to produce bubbles of carbon (IV)oxide gas.Carbon(IV)oxide gas is a colourless gas that extinguishes a burning splint.

When carbon (IV) oxide gas is bubbled in lime water, a white precipitate is formed.

 

Chemical equations

1. Sodium carbonate +Hydrochloric acid -> Sodium chloride + Carbon(IV)Oxide+ Water

Na2CO3(s) + 2HCl (aq) -> 2NaCl (aq) + H₂O(g) + CO₂ (g)

2. Calcium carbonate +Hydrochloric acid -> Calcium chloride + Carbon(IV)Oxide+ Water CaCO₃(s) + 2HCl (aq) -> CaCl₂(aq) + H₂O(g) + CO₂(g)
3. Magnesium carbonate +Hydrochloric acid -> Magnesium chloride + Carbon(IV)Oxide+ Water MgCO₃(s) + 2HCl (aq) -> MgCl₂(aq) + H₂O(g) + CO₂ (g)
4. Copper carbonate +Hydrochloric acid ->Copper(II) chloride + Carbon(IV)Oxide+ Water CuCO₃(s) + 2HCl (aq) -> CuCl₂(aq) + H₂O(g) + CO₂ (g)
5. Copper carbonate +Sulphuric(VI) acid ->Copper(II)sulphate(VI) + Carbon(IV)Oxide+ Water CuCO₃(s) + H₂SO₄(aq) -> CuSO₄(aq) + H₂O(g) + CO₂ (g)
6. Zinc carbonate +Sulphuric(VI) acid ->Zinc sulphate(VI) + Carbon(IV)Oxide+ Water

ZnCO₃(s) + H2SO₄ (aq) -> ZnSO₄ (aq) + H₂O(g) + CO₂ (g)

7. Sodium hydrogen carbonate +Sulphuric(VI) acid ->Sodium sulphate(VI) + Carbon(IV)Oxide+ Water

NaHCO₃(s) + H₂SO₄ (aq) -> Na2SO₄ (aq) + H₂O(g) + CO₂(g)

8. Potassium hydrogen carbonate +Sulphuric(VI) acid -> Potassium sulphate(VI) + Carbon(IV)Oxide+ Water

KHCO₃(s) + H₂SO₄ (aq) -> K₃SO₄ (aq) + H₂O(g) + CO₂ (g)

9. Potassium hydrogen carbonate +Hydrochloric acid -> Potassium chloride + Carbon(IV)Oxide+ Water KHCO₃(s) + HCl (aq) -> KCl (aq) + H₂O(g) + CO₂(g)
10. Sodium hydrogen carbonate +Hydrochloric acid -> Sodium chloride + Carbon(IV)Oxide+ Water NaHCO₃(s) + HCl (aq) -> NaCl (aq) + H₂O(g) + CO₂(g)

 

Neutralization by bases/alkalis

All acids react with bases to form a salt and water only. The reaction of an acid with metal oxides/hydroxides(bases) to salt and water only is called neutralization reaction. Since no effervescence/bubbling/fizzing take place during neutralization:

(i) The reaction with alkalis require a suitable indicator.

The colour of the indicator changes when all the acid has reacted with the soluble solution of the alkali (metal oxides/ hydroxides).

(ii) Excess of the base is added to ensure all the acid reacts. The excess acid is then filtered off.

Experiment 1: Reaction of alkali with mineral acids.

(i) Place about 5cm3 of dilute hydrochloric acid in a boiling tube. Add one drop of phenolphthalein indicator. Using a dropper/teat pipette, add dilute sodium hydroxide drop wise until there is a colour change.

(ii) Repeat the procedure with dilute sulphuric (VI)acid instead of hydrochloric acid.

(iii) Repeat the procedure with potassium hydroxide instead of sodium hydroxide.

 

Sample observation:

Colour of phenolphthalein change from colourless to pink in all cases.

Explanation

Bases/alkalis neutralize acids. Acids and bases/alkalis are colourless.

A suitable indicator like phenolphthalein change colour to pink, when all the acid has been neutralized by the bases/alkalis.

Phenolphthalein change colour from pink, to colourless when all the bases/alkalis has been neutralized by the acid.

 

Chemical equation

Sodium oxide + Hydrochloric acid ->Sodium chloride + Water

Na₂O(s) + HCl ->NaCl(aq) + H₂O(l)

Potassium oxide + Hydrochloric acid ->Potassium chloride + Water

K₂O(s) + HCl -> KCl(aq) + H₂O(l)

Sodium hydroxide + Hydrochloric acid ->Sodium chloride + Water

NaOH(s) + HCl -> NaCl(aq) + H₂O(l)

Ammonia solution+ Hydrochloric acid ->Ammonium chloride + Water

NH₄OH(s) + HCl -> NH₄Cl(aq) + H₂O(l)

Potassium hydroxide + Hydrochloric acid -> Potassium chloride + Water

KOH(s) + HCl -> KCl(aq) + H₂O(l) Sodium hydroxide + sulphuric(VI)acid ->Sodium sulphate(VI)+ Water

2NaOH(s) + H₂SO₄ -> Na₂SO₂ (aq) + 2H2O(l) Potassium hydroxide + sulphuric(VI)acid-> Potassium sulphate(VI)+ Water 2KOH(s) + H₂SO₄-> K₂SO₄ (aq) + 2H₂O(l)

Ammonia solution+ sulphuric(VI)acid ->Ammonium sulphate(VI)+ Water 2NH₄OH(s) + H₂SO₄ -> ( NH₄)2SO₄ (aq) + 2H₂O(l) Magnesium hydroxide + sulphuric(VI)acid ->Magnesium sulphate(VI) + Water Mg(OH)₂(s) + H₂SO₄ -> MgSO₄ (aq) + 2H₂O(l)

Magnesium hydroxide + Hydrochoric acid ->Magnesium chloride + Water Mg(OH)₂(s) + HCl(aq) -> MgCl₂ (aq) + 2H₂O(l)

 

See also:

INTRODUCTION TO ACIDS, BASES AND INDICATORS

PHYSICAL/TEMPORARY AND CHEMICAL CHANGES

METHODS OF SEPARATING MIXTURES

CLASSIFICATION OF SUBSTANCES

CHEMISTRY LABORATORY AND APPARATUS