Form 1:
Introduction to Acids,Bases and Indicators

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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 metals to form a salt and produce /evolve hydrogen gas.

Metal + Acid -> Salt + Hydrogen gas

Experiment :

Reaction of metals with mineral acids.

Procedure

(a)Place 5 cm3 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) -> MgCl (aq) + H2(g)

2. Zinc + Hydrochloric acid -> Zinc chloride + Hydrogen
Zn(s) + 2HCl (aq) -> ZnCl (aq) + H2(g)

3. Iron + Hydrochloric acid -> Iron(II) chloride + Hydrogen
Fe(s) + 2HCl (aq) -> FeCl (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) -> Al4(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 5 cm3 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) -> K2SO4 (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)

3.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 5 cm3 of dilute hydrochloric acid in a boiling tube. Add one drop of phenolphthalein indicator. Using a dropper/teat pipette, add dilute sodium hydroxide dropwise 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 equations

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 -> Na2SO4 (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)

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