10 sci chp 3 rvsn notes

   10 SCIENCE REVISION NOTES   

CHAPTER 3 ACIDS, BASES AND SALTS     

Elements are classified based on their properties as

→ Metals

→ Non-metals

→ Metalloids

Physical properties of Metals:

→ Metals are lustrous

→ All metals except mercury are hard and exist as solids at room temperature.

→ Metals are good conductors of heat and electricity.

→ Metals are malleable and ductile

→ Metals have high melting and boiling point.

→ Metals are sonorous and have high density

Exceptions:

→ Mercury is a metal which is liquid at room temperature

→ Lead and mercury are comparatively poor conductors of heat

→ Gallium and Caesium have very low melting point.

→ Alkali metals such as Sodium, Lithium and Potassium are soft and can be cut with a knife

→ The best conductors of heat are Silver and Copper.

Physical properties of Non-metals:

→ Non-metals are non-lustrous except lodine

→ Non-metals exists in solid, liquid and gaseous state.

→ Non-metals are non-malleable and non-ductile.

→ Non-metals have low melting and boiling points

→ Non-metals are non-conductors of electricity except Graphite.

→ Non-metals are generally not sonorous

→ Non-retals exhibits allotropic property. Diamond and graphite are allotropic forms of carbon

Exceptions:

→ Graphite is a non-metal but is a good conductor of electricity.

→ lodine is a non-metal but is lustrous.

→ Diamond is a non-metal but is the hardest substance known.

C'hemical properties of metals:

→ Reaction with Oxygen: When metals react with oxygen, metallic oxides are formed.

2Mg + O₂ → 2MgO (Magnesium oxide)

2Cu + O₂ → 2CuO (Copper oxide)

4Al + 3O₂ → 2A1₂O₃ (Aluminium oxide)

→ Amphoteric oxides: Metallic oxides which have both acidic and basic behaviour are called amphoteric oxides.

Examples: A1₂O₃ , ZnO

Al₂O₃ + 6HCI  → 2AICI₃ + 3H₂O

Al₂O₃+ 2NaOH → 2NaAIO₂ + H₂O

→ Some metallic oxides dissolve in water to form alkali

Na₂O(s) + H₂O(l)  → 2NaOH(aq)

K₂O(s) +H₂O(l) → 2KOH(aq)

→ Reaction with water:

Potassium and sodium react with water to liberate hydrogen gas

2Na + 2H₂O  2NaOH + H₂

2K + 2H₂O  2KOH+ H₂

Iron and aluminium react with steam to liberate hydrogen

2A1 + 3H₂O (steam) → Al₂O₃ + 3H₂

3Fe + 4H₂O (steam) → Fe₂O₃ +4H₂

Note: Hydrogen gas is not evolved when a metal reacts with nitric acid, because HNO₃ is a strong oxidising agent. It oxidises the hydrogen liberated to form water.

Reactivity of metals decreases in the order Mg > Al> Zn > Fe. This is assessed by the rate of formation of hydrogen bubbles.

→  Displacement reaction: The reactivity of metals is determined by displacement reactions

Example: Fe + CuSO₄ → FeSO₄ + Cu

Iron displaces copper from copper sulphate solution. Therefore iron is more reactive than copper.

Reactivity series: The reactivity series is a list of metals arranged in the order of their decreasing activities. This is determined by displacement reactions.

Most reactive → K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg. Ag, Au → Least

reactive

→ Reactions of metals with non-metals: Metals react with non-metals to form ionic compounds.

Na →Na⁺ + te,   Cl⁺ + e-  → CI^-

Na⁺ + CI^-   → NaCl

Properties of ionic compounds:

→ They are hard due to strong attractive forces between ions.

→ They have high melting and boiling points.

→ Electrovalent compounds are soluble in water.

→ They conduct electricity in their molten state or in aqueous solution

Occurrence and Extraction of metals:

Metals occur in nature in two forms:

→ Native form - Free state - Examples: Gold, silver, platinum and copper.

→ Combined form - Compound state - Examples: Sodium, potassium, magnesium, calcium, aluminium. zinc, iron, lead.

→ The Earth's crust is the chief source of metals. Sea water contains soluble salts of sodium and magnesium

→ Based on the reactivity series, metals are divided into

a] Metals of high reactivity. Examples: K, Na, Ca, Mg, Al,

b] Metals of medium reactivity. Examples: Zn, Fe, Pb.

c] Metals of low reactivity. Examples: Cu, Hg, Ag, Au.

Metallurgy: The process of extracting metals from their respective ores and purifying them is called metallurgy.

The steps involved in metallurgy are:

i] Concentration of ore.

ii] Reduction of the concentrated ore

il] Refining of the metals obtained

Minerals: The elements or compounds which occur naturally in the earth's crust are known as minerals.

Ores: Minerals which contain a very high percentage of a particular metal from which metal can be extracted profitably is called ore.

Examples

i] Ores of iron - Haematite, Magnetite

ii] Ores of copper - Chalcopyrites, Cuprite

iiil Ore of aluminium - Bauxite

Enrichment of ore: The ores of metals contain large amounts of impurities called gangue. The process of removing gangue from ore by different methods is called enrichment of ore.

Extraction of metals with low reactivity: These metals are extracted by converting them to their oxides and then reducing the oxides

Examples:

2HgS(s) + 3O₂ (g) -heat→  2HgO(s) + 2SO₂(g)

2HgO(s) -heat→  2Hg(s)+ O₂ (g)

2Cu₂S(s) + 3O₂(g) -heat→  2CuO(s) + 2SO₂(g)

2CuO (s) + Cu₂Ss -heat→  6Cu (s) + SO₂ (g)

Extraction of metals of medium reactivity

Roasting: The process of converting sulphide ores into is oxides called by heating  strongly in the presence of  excess of of air is called roasting.

Example: 2ZnS(s) + 3O₂(g)  -heat→ 2ZnO + 2SO₂.

Calcination: The process of converting carbonate ores into oxides by heating strongly in limited supply of air is known as calcination.

Example: ZnCO₃ (s) – heat→ ZnO (s) + CO(g)

The oxides of metal are reduced to metal by using a suitable reducing agent.

Example: ZnO(s) +C(s) -heat→ Zn (s) + CO(g)

Note: Obtaining metals from their oxides and compounds is a reduction process. Carbon is used as the reducing agent in extraction of metals. Some metals such as Na, Ca, and Al displace the metals of lower reactivity. They can also be used as reducing agents.

3MnO₂ (s) + 4A1 (s) → 3Mn (s) + 2A1₂O₃ (s) + Heat

Fe₂O₃(s) + 2Al(s)  → 2Fe (s) + Al₂O₃ (s) + Heat

The above reaction is used to join railway tracks or cracks of machine parts. This reaction is known as Thermit process.

Extraction of metals of high reactivity

Highly reactive metals are extracted by a method called electrolytic reduction of their oxides.

Example: Aluminium is obtained by electrolytic reduction of aluminium oxide.

Refining of metals: The process of purification of metals is called refining of metals. The best method of refining of metal is electrolytic refining Cu, Zn, Sn, Ni, Ag and Au are refined by this method.

Electrolytic refining of copper:

→ Acidified copper sulphate acts as electrolyte.

→ Impure copper plate acts as anode

→ Pure thin copper plate acts as cathode

→ Impurities that collect at the bottom of the tank are called anode mud.

Corrosion

The gradual process of eating away of metals by the reaction of atmospheric air and moisture with the metals is called corrosion.

Examples:

→ Rusting of iron - formation of hydrated ferric oxide.

→ Tarnishing of silver - formation of black coloured silver sulphide.

→ Green coating on copper due to formation of copper carbonate.

Prevention of corrosion

→ Galvanisation: Process of coating iron or steel objects with a thin layer of zinc is called galvanisation. This nis prevents rusting.

→ Alloying : Improving the properties of a metal by mixing with other metals.

→ Painting

→ Greasing

→ Tin plating

→ Chromium plating

Alloys: The homogeneous mixture of two or more metals or a metal with a non-metal is called an alloy.

Examples:

→ Brass - an alloy of copper and zinc (Cu, Zn).

→  Bronze - an alloy of copper and tin (Cu, Sn)

→  Solder - an alloy of lead and tin (Pb, Sn).

Note:

→  An alloy of metals in which mercury is one of the metals is called amalgam

→  Gold is very soft. Therefore some percentage of silver or copper is added to make it hard. This enables making of gold jewellery.