Chapter 8 Chemical Reactivity Chemistry 9th Class Notes

Short Questions Chemistry 9th Notes

Q.1) Identify at least two groups which contain only metallic elements.

In the periodic table, elements in the group I except hydrogen and all the elements in group II are metals. Elements of group III are also metals except boron. All the transition elements are metals. The lanthanide and actinide series are also all metals.

Q.2) Write down the reaction of group-I metals with oxygen, with balance equations. 

Group-I metals tarnish because of the action of oxygen which results in the formation of their oxides.
4Na + O2 → 2Na2O
Na2O + H2O → 2NaOH

Q.3) State the physical properties of metals.  

Physical properties of metals

  • Almost all metals are solids (except mercury).
  • They have high melting and boiling points (except alkali metals).
  • They possess metallic luster and can be polished.
  • They are malleable, ductile and give off a tone when hit.
  • They are good conductor of heat and electricity.
  • They have high densities.
  • They are hard (except sodium and potassium).

Q.4) How does sodium acts a reducing agent and write down its reactions also.  

Sodium is a powerful reducing agent. It reduces most of the oxides and halides.
2Na + MgO → Na2O + Mg
4Na + TiCl4 →4NaCl + Ti

Q.5) Ionization energy of alkaline earth metals is higher than alkali metals, why?    

 The ionization energy values increase as we move left to right in period because of atomic size decreases and nuclear charge increases. As the atomic radii decrease in group II due to increase of nuclear charge, relatively higher amount of energy is required to remove valence electron. Alkaline earth metals have 2 electrons in their valence shell. They lose their valence electron to form dipositive ions M++. While alkali metals have 1 electron in their valence shell. They lose their valence electron to form uni-positive ions M+. So more energy is required to generate dipositive M2+  ion as compare to uni-positive ion.

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Q.6) Pure gold is not used for ornaments, why?     

Pure gold is too soft to resist the stresses applied to make ornaments. Alloying gold with other metals such as copper, silver and platinum increase its durability.

Q.7) What are the uses of magnesium?

Uses of magnesium
(i) It is used in photographic flashbulbs and fireworks.
(ii) It is low-density metal, so it is used in the formation of light but tough alloys, such as Duralium (a mixture of Al, Cu, Mg and Mn) and magnalium (a mixture of Al, Mg). These alloys are used for the construction of aircrafts, cars and moving parts of machines.
(iii) It is used as a fuse for Thermite process to ignite aluminium powder.
(iv) It is used to form magnesia MgO. It is used to make refractory bricks for furnace lining by mixing it with clay, pressing and drying.
(v) It is used as a deoxidant in metallurgy and in the extraction of Titanium and Uranium.
(vi) Magnesium sulphate (MgSO4) is used in textile, paper industry, soap formation and pharmaceutical industries etc.

Q.8) Write down the reaction of chlorine with sodium hydroxide with balance equation.

Chlorine reacts with cold dilute NaOH to give sodium hypochlorite and sodium chloride.
2NaOH + Cl2 → NaCl + NaOCl + H2O
Chlorine reacts with concentrated NaOH to give sodium chloride and sodium chlorate(V).
6NaOH + 3Cl2 → 5NaCl + NaClO3 + 3H2O

Read more: Chapter 7 Electrochemistry Chemistry 9 class Notes short, long question, numerical

Q.9) How does ionization energies vary in group? 

The ionization energy decreases as we move from top to bottom in a group because atomic radius increases due to addition of more and more shells between the valence shell and the nucleus. The valence electrons are loosely held by the electrostatic force of nucleus because they are at larger distance from the nucleus. Therefore, value of ionization energy decreases from top to bottom, as less energy is required to remove the loosely held electrons.

Q.10) What happens during displacement reaction in halogens?

During displacement reaction, more reactive halogen will displace the less reactive halogen from its halide solution. The reactivity of halogens decrease down the group.
F > Cl > Br > I > As
For example:
2NaBr + Cl2 → 2NaCl + Br2

Long Questions 9th Chemistry Notes 2021

Q.1) Compare and contrast the properties of alkali and alkaline earth metals, with reactions.


Alkali metalsAlkaline earth metals
They have lower ionization energy values; therefore, they are highly electropositive.They have higher ionization energy values;     therefore, they are less electropositive.
They tarnish because of the action of oxygen which results in the formation of their oxides.
4Na + O2 → 2Na2O
They react with oxygen on heating and form oxides.
2Mg + O2 → 2MgO
They rapidly react with water at room temperature to form alkaline solution and hydrogen gas.
2Na + 2H2O → 2NaOH + H2
They react slowly with cold water and more rapidly with hot water to produce weak bases.
Mg + H2O → MgO + H2
MgO + H2O → Mg(OH)2
They react vigorously with all the halogens to form halides.
2Na + Cl2 → 2NaCl
They react slowly with the halogens to form halides.
Ca + Cl2 → CaCl2
They react with hydrogen at high temperature to form ionic hydrides.
2Na + H2 → 2NaH
They react with hydrogen under strong conditions of temperature and pressure to form ionic hydrides.
Ca + H2 → CaH2
They do not react with carbon.They react with carbon on heating to form stable carbides.
Ca + 2C → CaC2
They do not react with nitrogen.They react with nitrogen on heating to form stable nitrides.
3Mg + N2 → Mg3N2

Q.2 a) Differentiate between hard and soft metals.


Soft metals

i) Soft metals can be cut by knife.
ii) There are weak attractive forces between their atoms.
iii) They have low melting and boiling points.
iv) They have low density.
v) They have low ionization energy.
vi) They have low tensile strength and cannot be used where stress is required.
vii) They are very electropositive
viii) They are very reactive.

Hard metals

Hard metals cannot be cut by knife.
There are strong attractive forces between their atoms.
They have high melting and boiling points.
They have high density.
They have high ionization energy.
They have high tensile strength and are used in construction of buildings .
They are less electropositive
They are less reactive.

Q.2 b) Give the reaction of magnesium with,

i) H2
ii) HCl
iii) O2
iv) H2O
v) Cl2

i) Mg + H2 → MgH2
ii) Mg + 2HCl → MgCl2 + H2
iii) 2Mg + O2 → 2MgO
iv) Mg + H2O → Mg(OH)2 + H2
v) Mg + Cl2 → MgCl2

Q.3) Discuss the reasons why some elements exist as free elements in nature while other occurs in combined states as compounds. Give two examples of each type.   

There are elements which have low ionization energies, so they are very reactive. That is why they do not occur free in nature. These elements are found combined with other elements because they are more stable when they are sharing or transferring electrons to other elements. A huge amount of elements have an unstable electron arrangement, due to which they form bonds with other elements.
Group I and group II elements do not occur free in nature because they are highly reactive, therefore, they are unstable. Their high reactivity is due to larger atomic size and low ionization energy, so they are present in the compound state.
Some elements exist in free state. The elements are in the free state if they are found in the elementary form.
Usually, less active elements exist free in nature. Transition metals are relatively inert and found free in nature
Noble metals also found free in nature. These metals are chemically inert species as noble metals resist oxidation and corrosion in moist air. they show poor affinity towards other elements as they could  not be oxidized or reduced thus they do not react and produce compounds like other metals. Common examples of noble metals are gold, silver, copper, platinum, etc.  
Some non-metals which exist as free elements in nature are oxygen, nitrogen, and noble gases.

Q.4) Define metal and non-metal and compare the properties (both chemical and physical) of metals and non-metals.

Metals are those substances which are good conductor of heat and electricity. They are malleable and ductile. Their oxides and hydroxides are bases. All of the metals, except mercury, exist in solid state at room temperature.
Non-metals are those substances which are nonconductors of heat and electricity. They are neither malleable nor ductile. Their oxides and hydroxides are acids. Non-metals exist in all the three states of matter at room temperature.

i) They are electron donor during chemical reactions. They are reducing agents.They are electron acceptors during chemical reactions. They are oxidizing agents.
ii) They become positively charged ion in solutions.They become negatively charged ions in solutions.
iii) Some metals can replace hydrogen from acids to form salts.Non-metals cannot replace hydrogen from acids.
iv) They form basic oxides. The soluble basic oxides which form alkalis when they dissolve in water. For example:
4Na + O2 → 2Na2O
They form mainly acidic oxides which dissolve in water to form acids. For example:
C + O2 → CO2
v) They form electrovalent (ionic) chlorides. For example:
2Na + Cl2 → 2NaCl
They form covalent chlorides. For example:
H2 + Cl2 → 2HCl
vi) They do not combine easily with hydrogen. Few hydrides formed are electrovalent. For example:
Mg + H2 → MgH2
They combine with hydrogen to form many stable covalent hydrides. For example:
N2 + 3H2 → 2NH3
vii) They are usually solids at room temperature (except Hg).They are often gases (except Br, S, P, I, C, and B).
viii) They are good conductors of heat and electricity.They are poor conductors of heat and electricity (except graphite).

Q.5) Halogens are very reactive elements, write down halogen’s reaction with hydrogen, oxygen, metals, non-metals, and other compounds.

(i) Reaction with hydrogen
Halogens react with hydrogen to given hydrogen halides. The chemical affinity for hydrogen decreases down the group. Fluorine reacts with hydrogen violently; chlorine reacts only in the presence of sunlight, while bromine and iodine react only on heating. Iodine reacts reversibly.
H2 + F2 → 2HF
H2 + Cl2 → 2HCl
H2 + Br2 → 2HBr
H2 + I2 ⇔ 2HI
(ii) Reaction with oxygen
Fluorine reacts with oxygen to produce monoxide and dioxide.
O2 + 2F2 → 2OF2
O2 + F2 → O2F2
(iii) Reaction with metals
Halogens react with metals to form the corresponding ionic halides. The ionic character decreases down the group.
Cu + Br2 → CuBr2
2K + I2 → 2KI
(iv) Reaction with non metals
Halogens react with non metals to form tri-halides and penta-halides.
2P + 3Cl2 → 2PCl3
2P + 3Br2 → 2PBr3
2P + 5Cl2 → 2PCl5
2P + 5Br2 → 2PBr5
(v) Reaction with other compounds
All halogens are oxidizing agents. They oxidize certain compounds and get reduced.
H2S + Cl2 → 2HCl + S
2NH3 + 3Cl2 → 6HCl + N2
Reaction of chlorine and lime water produces bleaching powder.
Ca(OH)2 + Cl2 → CaOCl2 + H2O

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