3. METALS AND NON-METALS
CHEMICAL
PROPERTIES OF METALS
What happens when metals burn in air?
- Try to burn various metals (aluminium, copper, iron, lead, magnesium, zinc and sodium) over a flame.
-
Sodium burns
easily with yellow flame (Magnesium: white flame, Copper: Blue-green,
Aluminium: White).
-
The metal
surface appears silver white after burning.
-
Reactivity of
metals to oxygen in decreasing order is
Na > Mg > Al > Zn >
Fe > Pb > Cu
-
Of the products (metal
oxides), sodium oxide is soluble in water. Other metal oxides are insoluble.
-
Almost all
metals combine with O2 to form metal oxides.
Metal + Oxygen → Metal oxide
E.g., when copper (Cu) is heated in air, it forms copper(II) oxide (CuO), a black oxide.
2Cu + O2 →
2CuO
Similarly, aluminium forms aluminium oxide (Al2O3).
4Al + 3O2 → 2Al2O3
-
Metal oxides are basic in nature.
But some metal oxides, such as aluminium oxide, zinc oxide show both acidic and
basic behaviour. Such metal oxides which react with both acids & bases to
produce salts and water are called amphoteric oxides. E.g. reaction of Aluminium
oxide.
Al2O3 + 6HCl → 2AlCl3 + 3H2O
Al2O3 + 2NaOH → 2NaAlO2 +
H2O
(Sodium
aluminate)
-
Most metal oxides are water insoluble
but some (sodium oxide, potassium oxide etc.) are soluble to form alkalis.
Na2O(s) + H2O(l) → 2NaOH(aq)
K2O(s) + H2O(l) → 2KOH(aq)
-
All metals show different
reactivities towards oxygen.
-
Metals such as potassium &
sodium react so vigorously that they catch fire if kept in the open. Hence,
they are kept in kerosene oil.
-
At ordinary temperature, the
surfaces of metals such as Mg, Al, Zn, Pb etc., are covered with a thin protective
layer of oxide. It prevents metal from further oxidation.
-
Iron
does not burn on heating but iron filings burn vigorously when sprinkled
in the flame of the burner.
-
Copper
does not burn, but the hot metal is coated with a black coloured layer of copper(II)
oxide.
-
Silver & gold do not react
with oxygen even at high temperature.
Anodising:
A process of forming a thick oxide layer of aluminium. This improves resistance
against corrosion. During anodising, a
clean aluminium article is made the anode and is electrolysed with dilute H2SO4.
The oxygen gas evolved at the anode reacts with aluminium to form a thick oxide
layer. This oxide layer is dyed.
What happens when metals react with water?
-
Metals react with water and
produce a metal oxide and hydrogen gas. Metal oxides that are
soluble in water dissolve in it to form metal hydroxide. But all metals
do not react with water.
Metal + Water → Metal oxide + Hydrogen
Metal oxide + Water → Metal hydroxide
-
Metals like potassium &
sodium react violently with cold water. The reaction is exothermic so that the
evolved hydrogen catches fire.
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) + heat energy 2Na(s) + 2H2O(l)
→ 2NaOH(aq) + H2(g)
+ heat energy
-
Reaction of calcium with water is
less violent. The heat evolved is not sufficient for the hydrogen to catch
fire.
Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
-
Calcium starts floating because
the bubbles of hydrogen gas formed stick to the metal surface.
-
Magnesium does not react with
cold water. It reacts with hot water to form magnesium hydroxide and hydrogen.
It also starts floating because bubbles of hydrogen gas stick to its surface.
-
Metals like aluminium, iron &
zinc do not react with cold or hot water. But they react with steam.
2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g)
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
-
Metals such as lead, copper,
silver and gold do not react with water at all.
-
Reactivity of metals with water
in decreasing order:
Na > K > Ca> Mg > Al > Fe
What happens when Metals react with Acids?
-
Metals react with acids to give a
salt and hydrogen gas.
Metal + Dilute acid → Salt + Hydrogen
-
But all metals do not react in
the same manner. E.g.
-
Put Al, Cu, Fe, Pb, Mg and Zn separately
in test tubes containing dilute hydrochloric acid.
-
Suspend thermometers in the test
tubes, so that their bulbs are dipped in the acid.
-
The following reactions occur.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
-
Here, magnesium shows fastest
reaction and hydrogen bubble formation and records highest temperature (most
exothermic).
-
The reactivity decreases in the
order Mg > Al > Zn > Fe.
- Copper
does not react with dilute HCl. So bubbles are not formed and no change in temperature.
- Hydrogen
gas is not evolved when a metal reacts with nitric acid. It is because HNO3
is a strong oxidising agent. It oxidises the H2 to water and itself
gets reduced to nitrogen oxides (N2O, NO or NO2). But
magnesium and manganese react with very dilute HNO3 evolving H2.
Aqua regia, (Latin for ‘royal water’)
is a mixture of conc. HCl & conc. HNO3 in 3:1 ratio.
It is highly corrosive, fuming liquid. It is one of the few reagents that can
dissolve gold and platinum.
How do Metals react with Solutions of other Metal Salts?
-
Put a clean copper (Cu)
wire in iron sulphate (FeSO4) solution and an iron (Fe)
nail in copper sulphate (CuSO4) solution taken in test
tubes.
-
After 20 minutes, it is observed
that blue CuSO4 solution turns green.
-
Iron is more
reactive than copper. It displaces Cu from CuSO4 to form FeSO4. So CuSO4 solution turns green.
Fe(s) + CuSO4(aq) →
FeSO4(aq) + Cu(s)
- This
is a displacement reaction. i.e., reactive metals displace less reactive
metals from their compounds in solution or molten form.
-
Displacement reactions are better
evidence for the reactivity of metals. If metal A displaces metal B from its
solution, it is more reactive than B.
Metal A + Salt solution of B → Salt solution of A + Metal B
-
All metals do not react with
reagents like oxygen, water and acids. So we cannot put all the metal samples
in decreasing order of their reactivity.
The
Reactivity or Activity Series:
It is a list of metals arranged in the order of their
decreasing activities.
Select Your Next Topic 👇
👉 Part 1: Physical Properties
👉 Part 2: Chemical Properties
👉 Part 3: How do Metals and Non-metals React?
👉 Part 4: Occurrence of Metals
👉 Part 5: Corrosion
