Class 8th (Chemistry) Chp 6 Chemical Reaction (Part 4)

Reactivity series of metals:

Metals differ greatly in their chemical reactivity. Some metals are highly reactivity; some are less reactive while some other metals are unreactive. On the basis of chemical reactivity, some important metals are arranged in order of decreasing reactivity. The series of metal in order of decreasing reactivity is called reactivity series.

In reactivity series, the most reactive metal is placed at the top while the least reactive metal is placed at the bottom. From the reactivity series it is clear that potassium is the top while gold which is the least reactive metal is placed at bottom. Silver, platinum gold which are less reactive metals. Therefore, these metals are found in natural state (Free State) in nature.

Hydrogen is not a metal, but it is listed in the reactivity series for comparison. Generally, metals that are above hydrogen in the series can displace hydrogen ions from water, acids od alkalis. These below hydrogen cannot do so. Predicting the Reactivity of Metals:

More reactive metals such as zinc, iron, magnesium and aluminium can displace metals below them in the reactivity series from their salt solutions. For example, zinc, iron and magnesium, being more reactive than copper, will displace copper from copper sulphate:

CuSO4 + Zn ZnSO4 + Cu

CuSO4 + Fe FeSO4 + Cu

CuSo4 + Mg MgSO4 + Cu

Metals such as zinc, iron and nickel that are present above hydrogen can displace hydrogen can displace hydrogen from dilute acids. For example,

H2SO4 + Zn ZnSO4 + H2

Q. Write and explain the reactivity series.

Decomposition Reactions to form Oxides:

Oxide is a compound in which oxygen is present as anion. Oxygen is highly reactive due to its small size and 6 electrons in the outermost shell. It combines with metals and non- metals to form oxides. Oxides of metals can be produced by applying heat on their ores in two ways: Calcination and Roasting.

Calcination:

Calcination is a decomposition reaction in which a metal ore is heated strongly in the limited supply or complete absence of air to remove carbon dioxide and water. The temperature is so maintained that the ore does not burn or melt. Carbonates of magnesium, zinc, and copper on calcination give respective metaloxides.

MgCO3 MgO + CO2

ZnCO3 ZnO + CO2

Q. Write what is calcination.

Roasting:

Roasting is a prosses in which the ore is heated strongly in the presence of air at a very high temperature. Sulphide ores such as zinc blende(ZnS) and galena (PbS) are converted to oxides by this process. Roasting can be conducted in a reverberatory furnace. A reverberatory furnace is a type of furnace in which materials are heated indirectly by a deflected flame that is directed either download or sideways. Impurities are converted into volatile oxide and get automatically removed. The metal oxides remain.

2ZnS + 3O2 2ZnO + 2SO2

2PbS + 3O2 2PbO + 2SO2

2FeS + 3O2 2FeO + 2SO2

Q. What is roasting?

Classification of Oxide:

The oxides are classified into four categories depending upon the nature and properties of the compounds:

1. Acidic oxide 2. Basic oxide 3. Neutral oxide 4. Amphoteric oxide

Acidic Oxide:

When non-metals react with oxygen, they form oxide through covalent bond, i.e. by sharing of electrons. These oxides have low boiling and melting points. CO2, SO2, NO2, etc are acidic oxides. Acidic oxides form acid when treated with water:

SO3 + H2O H2SO4

When we dip blue litmus paper in the aqueous solution of acidic oxide, it turns red, confirming that acidic oxides form acids when dissolved in water.

Basic Oxides:

Oxides of metals are basic. These oxides are formed through ionic or electrovalent bond. Hence, they have high boiling and melting points. CaO, MgO, ZnO, Na2O, CuO, etc are basic oxides. Basic oxides combine with water to form base:

Na2O + H2O 2NaOH

When we dip a red litmus paper in the aquerous solution of a basic solution, it turns blue, confirming that basic oxides form bases when dissolved in water. Amphoteric oxides

These are the oxides which show properties of both acid as well as base. On treatment with acid, these oxides undergo neutralization and form salt and water. Al2O3, ZnO and PbO are amphoteric oxides. The amphoteric oxide Al2O3, acts as an acid when it reacts with base like sodium hydroxide (NaOH). This reaction produces the salt NaAlO2. When the same oxide Al2O3 reacts with an acid such as H2SO4 it acts as a base and produces salt Al2(SO4)3.

Neutral oxides

These are the oxides which do not show either acidic or basic properties. For example, CO and NO neutral oxides.

Q. Give 2 examples of each:

a. Acidic oxides b. Basic oxides c. Amphoteric oxides d. Neutral oxides