chemistry for kids

7/30/2019 chemistry for kids

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Written by: - SHAHZAD IFTIKHARContact # 0313-7891989, 0333-5319544e-mail: [email protected] website: www.eservicespakistan.com

CHEMISTRY FOR 9TH CLASS (UNIT # 8)

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Read Instructions before study: B u l le t m e a n s y o u h a v e t o w r i t e t h e a n s w e r i n p a r a g r a p h s h a p e a n d y o u c o u l d n o t c h a n g e

t h e s e q u e n c e . B u l l e t s a r e u s e d o n l y f o r y o u r e a s e .

i ) R om a n n u m b e r e d , q u e s t i o n m a y b e w r i t t e n a s i t i s. H ow e v e r se q u e n c e c a n b e ch a n g e d .

*** This item should not be written in answer. It is just a hint for you.

Pay more attention on the bold text in answers.=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=

THEORY

Question: Define Electrochemistry?The branch of chemistry in which chemical energy is converted into electrical energyor electrical energy is converted into chemical energy is called electrochemistry.

Question: Define conductor and insulator?i) The substances through which electric current can pass are called

conductors i.e. Copper, Aluminium, Iron etc.

ii) The substances through which electric current cannot pass are calledinsulators or non-conductors. i.e. rubber, plastic, wood etc.

Question: Define a Cell?A vessel in which oxidation-reduction reaction takes place at electrodes is called cell.

Question: Define Electrolysis?When electricity is passed through fused ionic compounds or their aqueous solutions,

they are decomposed into their constituents. Such compounds are calledElectrolytes and the process is called Electrolysis.

Question: Define Electrolyte and also write its types?The compounds in molten state or in aqueous solution through which electricity canpass are called electrolyte. And the compounds through which electricity cannotpass are called non-electrolyte.Strong Electrolyte: -

Ionization is complete in these electrolytes. For example, in a dilute solution of NaClthere will no formula unit present. Their examples of strong electrolytes are mineralacids, caustic alkalies and their salts.Weak Electrolyte: -Ionization is incomplete in these electrolytes. Their examples are ammoniumhydroxide, tartaric acid, citric acid and carbonic acid.

Question: Define Electrodes?Electrodes are the terminals of the battery at which ions are deposited during the

process of electrolysis. They are: -Anode: -It is a positive electrode through which electrons enter the external circuit.Cathode: -It is a negative electrode through which electrons leave the external circuit.


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Question: On which factors substances librated at electrodes?During electrolysis the quantities of substances librated at electrodes depend upon

the following factors: -i) The quantity of current passed.ii) Time duration of passing the current at a uniform rate.iii) Charge on the ions being deposited.

Question: Define oxidation and reduction?Oxidation: - A chemical reaction in which: -

i) Oxygen is addedii) Hydrogen is removediii) Electrons are lost.

Reduction: - A chemical reaction in which: -i) Oxygen is removed.ii) Hydrogen is added.iii) Electrons are absorbed.

Question: Define Ionization?An electrolyte splits up into charged particles upon heating or in its aqueous solution.This process is called ionization.

Question: Describe in detail the Ionic Theory to explain electrolysis? OR

Arrhenius theory of ionization?In 1887, Arrhenius, a Swedish chemist, put forward his ionic theory to explain theprocess of electrolysis. According to this theory: -

i) When electrolyte are fused or dissolved in water, they are split intopositive and negative ions. This process is called ionization.ii) The total positive charge is equal to the total negative charge and thesolution is electrically neutral.

iii) Metallic ions, ammonium ions and hydrogen ions carry positive chargewhile on metallic ions and hydroxide ions carry negative charge.

iv) The number of electrical charges present on any ion is equal to thevalency of the atom or the radical.

v) The properties of ions are different from the properties of the relatedatoms.

vi) Ions move in all possible directions in fused salts or in aqueous solutionsof salts.

vii)

When an electric current is passed through an electrolyte, the positiveions are attracted towards the cathode while the negative ions tend tomove towards the anode.

Question: Define Degree of ionization?The level to which an electrolyte can ionize in water is called its degree of ionization.

Degree of Ionization = Number of molecules which ionize

Total number of molecules


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Reaction at Cathode: -2Cl- ----------------> Cl2 + 2e

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Reaction at Anode: -2Na+ + 2e- ----------------> 2Na

Question: Describe the laws of Faraday regarding Electrolysis?Faradays First law of Electrolysis: -

i) The mass of an element, which is deposited, on an electrode duringelectrolysis, is directly proportional to the quantity of electricity Q, whichpasses through the electrolyte.

ii) The unit of the quantity of electricity is called coulomb. One coulomb isthe quantity of electricity passed when one ampere current passes for onesecond. One Faraday = 96,500 coulombs. AndQ = I x tI is the current, t is time in seconds and Q is the quantity of electricity

passing through electrolyte.iii) According to this law: -

m I x tm = Z x I x tZ is constant and m is mass of substance deposited. Here m is expressed

in grams.iv) The quantity of substance, which is deposited when one Faraday of

electricity is passed through an electrolyte, is called OneGramChemicalEquivalentofthatsubstance.

Faradays Second law of Electrolysis: -i) When the same quantity of electricity is passed, for the same time,

through different electrolytes connected in series, the substancesdeposited at the electrodes will be in the ratio of their chemicalequivalents.

ii) Faradays laws are very useful for determination of electrochemicalequivalents of different substances. The following equation is used for thispurpose.

Electrochemical Equivalent (Z) = Quantity of Substance Deposited (W)Quantity of electricity Passed (Q)

Z = W/Q.

Question: Explain Electrochemical Series?

i) When an aqueous solution containing Copper and Zinc ions iselectrolyzed, zinc ions will remain in the solution while copper ions arereduced and discharged.

ii) Cations (positive ions) can be arranged in the order of increasing ease ofdischarge at the cathode similarly anions (negative ions) can also bearranged in the order of increasing ease of discharge at the cathode.

iii) The list of ions arranged in the order of their ease of discharge at theelectrodes is called electrochemical series.


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iv) Electrochemical series also throw light on the reactivity of metals, whichform ions in aqueous solutions.

Question: Define Electroplating and describe its objectives?

It is the process in which a metal is deposited on another metal electrolytically. Thereare three main objectives of electroplating: -Decoration: -We deposit noble metals (like Gold & Silver) on inferior metals to enhance theirbeauty.Protection: -Electroplating protect the inferior metals from rusting and reaction of organic salts.

Repair: -To weld the broken parts of the machinery by electro-deposition of metals.

Question: Describe the Procedure of Electroplating?

i) The article to be electroplated is first cleaned with sand and then washed withcaustic soda solution and then with plenty of water.ii) This article is then made a cathode, while the metal to be deposited is made

anode.iii) The electrolyte is a salt of the metal being deposited and the electroplating is

carried out in a tank made of cement, glass or wood. It is called electrolytictank.

iv) The electrolyte chosen should have following properties: -a. Very soluble in water.b. Good conductorc. Not easily oxidized or reduced or hydrolyzed.d. Cheap.v) Electrodes are dipped in the electrolytic tank and current is passed through it.

vi) A thin layer of the noble metal is deposited on the article to be electroplated.vii) The following conditions should be fulfilled to get good deposits.

a. High current density.b. Low temperature.c. Higher concentration of the metal in its electrolyte.

Question: What is difference between Chemical Reaction and ElectrochemicalReaction?The chemical changes taking place in the substance is called chemicalreaction.

Electrochemical reactions are always oxidation-reduction reaction. Transfer ofelectrons takes place through the external circuit.

Question: Differentiate between the following?Gram equivalent and electrochemical equivalent:Equivalent weight of substance expressed in gram is called gram equivalent. WhileElectrochemical equivalent is the amount of a substance produced at the electrode

when one coulomb of electric current is passed through electrolyte for one second.Electrolytic dissociation and thermal dissociation:


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Decomposition of a substance into its ions by electric current is called electrolyticdissociation.

Decomposition of a substance into its ions by heating in the absence of air is calledthermal dissociation.

Question: Why Gold, Silver and platinum are used for making ornaments?They are used to make ornaments because

i) They do not rust.ii) They are lustrous.iii) They are not oxidized by environment.iv) They are hard and elastic and can be moulded to any shape.

Question: What happens when electricity is passed through a solution?The ions of electrolyte migrate towards respective electrode.

Question: Define Electrolytic Cell and Voltaic Cell?Electrolytic Cell: - In this cell a non-spontaneous oxidation-reduction reaction takesplace with the help of electrical energy.Voltaic Cell: - In this cell a spontaneous oxidation-reduction reaction is used toproduce electric current.

=*=*=*=*=More works/Tasks:Do Exercise from your Book.

Do Objectives from book & from any other objective book.Do Numerical with the help of your teacher.Do Interesting Information from book.Do Scientific Reasons with the help of teacher.

chemistry for kids

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