9th chemistry chapter 7 solved exercise short questions
Q#1 : Define oxidation in terms of electrons . Give an example .
” Oxidation is the loss of electrons by an atom or an ion “
Q#2 : Define reduction in terms of loss or gain of oxygen or hydrogen . Give an example .
” The addition of hydrogen or removal of oxygen during a chemical reaction is called reduction “
Q#3 : What is difference between valency and oxidation state ?
Q#4 : Differentiate between oxidizing and reducing agents .
Q#5 : Differentiate between strong and weak electrolytes .
Q#6 : How is electroplating of tin on steel carried out ?
Electroplating of tin :
Tin is usually electroplated on steel by placing the steel into a container containing a solution of tin salt .
→ The steel is connected to an electrical circuit , acting as cathode .
→While the other electrode made of tin metal acts as anode .
→ When an electrical current passes through the circuit , tin metal ion present in the solution deposit on steel .
Q#7 : Why is steel plated with nickel before the electroplating of chromium .
Steel is steel plated with nickel before the electroplating of chromium .
Because →chromium do not adhere well on the steel surface . and
→ chromium allows moisture to pass through it and metal is stripped off .
That’s why before chromium plating , nickel is plated first on steel .
Q#8 : How can you explain that following reaction is oxidation in terms of increase of oxidation number ?
” The loss of electron is called oxidation .“
Al⁰ → Al³+ + 3e-
In above equation :
Al⁰ with zero oxidation state changes to Al³+ by loss of 3 electrons .
So, oxidation reaction takes place .
Q#9 : How can you prove with an example that conversion of an ion to an atom is an oxidation process ?
- In this equation , loss of electrons take place .
- Chlorine Ion (anion ) converts into chlorine molecule by losing electrons .
- So, oxidation takes place .
Q#10 : Why does the anode carry negative charge in galvanic cell but positive charge in electrolytic cell ? Justify with comments .
- In case of Galvanic cell , electrons are lost by the atoms at anode plate and make it electron rich .
So, anode carry negative charge .
- In case of Electrolytic cell , the anode is attracted to the positive terminal of the battery .
That is why , it carry positive charge .
Q#11 : Where do the electrons flow from Zn electrode in Daniel’s cell ?
In Daniel cell , the electrons flow from Zn electrode (anode) towards the cathode made up of copper .
Q#12 : Why do electrodes get their names ‘anode’ and cathode in galvanic cell ?
- In Galvanic cell;
→ Where oxidation takes place is referred as anode .
→ While where reduction takes place is referred as cathode .
Q#13 : What happens at the cathode in a galvanic cell ?
The galvanic cell , reduction of Cu+2 ions takes place at cathode .
The electrons are gained by “Cu” ions of solution and “Cu” atoms are deposited at cathode .
Cu+2 + 2e- → Cu0
Q#14 : Which solution is used as an electrolyte in Nelson’s cell ?
In Nelson’s cell, Brine (saturated solution of NaCl) is used as an electrolyte .
Q#15 :Name the by-products produced in Nelson’s cell ?
By-products produced in Nelson’s cell are :
- Hydrogen gas (H2 )
- Chlorine gas (Cl2 )
Q#16 : Why galvanizing is done ?
Galvanizing is done to protect the iron against corrosion . It protects iron even after damage to zinc layer .
” The process of coating a thin layer of zinc or iron is galvanizing .”
Q#17 : Why is an iron grill painted frequently ?
Iron grill is painted frequently .
Because , due to presence of oxygen and water in air , it may corrode .
Iron is painted , to prevent from rusting.
Q#18 : Why is O2 necessary for rusting ?
Oxygen acts as an oxidizing agent in rusting and oxidizes iron by gaining electrons from it in presence of H+ ions .
O2 + 4H+ + 4e– → 2H2O
Q#19 : In electroplating of Chromium , which salt is usd as an electrolyte ?
Chromium sulphate Cr2(SO4)3 with few drops of H2SO4 acts as an electrolyte in electroplating of chromium.
Q#20 : Write three redox reaction taking place during the electroplating of chromium ?
Redox reaction :
Cr2(SO4)3 → 2Cr+3 + 3SO-24
Oxidation at anode :
4OH– → 2H2O + O2 + 4e–
Reduction at cathode :
Cr+3 + 3e– → Cr
Q#21 : In electroplating of silver , from where do Ag+ ions come and where do they deposit ?
In electroplating of silver , Ag+ ions come from anode made up of pure silver and they deposit at cathode .
At anode :
Ag → Ag+ + e–
Ag+ + e– → Ag
Q#22 : What is the nature of electrode used in electroplating of chromium ?
- Anode is made up of antimonial lead while
- Cathode is made up of the object to be electroplated .
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