Chemistry X- COMMON ORES OF ALUMINIUM, IRON AND ZINC

Ore	Chemical name	Formula
1. Bauxite	Hydrated aluminium oxide	A1203.2H20
2. Cryolite	Sodium aluminium fluoride	Na3AlF6
3. Corundum	Anhydrous aluminium oxide	A1203

Ore	Chemical name	Formula
Red haematite	Anhydrous ferric oxide	Fe203
Brown haematite	Hydrated ferric oxide	2Fe203.3H20
Magnetite	Triferric tetraoxide	Fe304
Iron pyrites	Iron disulphide	FeS2
Siderite	Ferrous carbonate	FeC03

Common ores	Chemical name	Formula
Zinc blende Zincite Calamine	Zinc sulphide Zinc oxide Zinc carbonate	ZnS ZnO ZnC03

10th class Chemistry chapter 1. Chemical change

Chemistry Adda

1. Chemical reactions- Chemical changes or chemical reactions are the changes in which one or more new substances are formed.

2. Chemical Equations – Representation of a chemical reaction in terms of symbols and formulae of the reactants and products is known as chemical equation.

3. Balanced Chemical equation – The chemical equation in which the no. of atoms of different elements is same on both sides of the arrow is called balanced chemical equation.

4. The chemical reactions can be classified into different types such as

(a) Combination reactions- The reactions in which two or more substances combine to form a new substance are called combination reactions. For example,

2Mg(s) + O2 (g) → 2 MgO (s)

(b) Decomposition reactions – The reactions in which a single compound breaks up into two or more simpler substances are called decomposition reactions. For example,

2Pb (NO3)2 (s) → 2PbO(s) + 4NO2 (g) + O2 (g)

The decomposition of a substance on heating is known as thermal decomposition. The decomposition of a substance by passing electric current through it is known as electrolysis.

For example, when electric current is passed through acidified water, it decomposes into hydrogen and oxygen. Electric current

2H2O (l) → 2H2 (g) + O2 (g)

Certain compounds when exposed to light undergo decomposition. For example, Silver chloride when exposed to sunlight turns grey due to its decomposition into silver and chlorine. Sunlight 2AgCl(s) → 2Ag(s) + Cl2 (g)

The decomposition of a substance by absorbing light energy is called photolysis or photochemical decomposition.

(c) Displacement Reactions- The chemical reactions in which a more reactive element displaces a less reactive element from a compound are known as displacement reactions.

 For Example,

(i) Zn(s) + CuSO4 (aq) → ZnSO4 (aq) + Cu(s)

(ii) Cu(s) + 2AgNO3 (aq) → Cu (NO3)2 (aq) +2Ag(s).

(d) Double Displacement Reactions- The chemical reactions in which compounds react to form two different compounds by mutual exchange of ions are called double displacement reactions. These reactions take place in solution. Two common types of double displacement reactions are precipitation reactions and neutralization reactions.

(i) Precipitation reaction : In precipitation reactions, aqueous solution of two salts are mixed whereby some salts precipitate due to mutual exchange of ions between the two salts. For example

AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3

(ii) Neutralization reaction: In this type of reaction an acid reacts with a base to form salt and water by exchange of ions. NaOH (aq) + HCl (aq) → NaCl (aq) + H2O

(e) Redox reaction : If a substance gains oxygen or looses hydrogen during a reaction it is oxidized. On the other hand if a substance loses oxygen or gains hydrogen during a reaction, it is reduced. Both oxidation and reduction take place simultaneously and hence these are called redox reaction. Oxidation

ZnO               +            C       →    Zn + CO

Reduction           Oxidation

Cl2                    +        H2S          → 2HCl + S

Reduction

(f) Exothermic reaction and endothermic reaction : On the basis of energy changes during chemical reaction, they can be classified as

(i) Exothermic reaction- A chemical reaction in which heat energy is produced is called exothermic reaction .For e.g. (i) C (s) + O2 → CO2 (g) + Heat

(ii) Endothermic reaction- A chemical reaction in which heat energy is absorbed is called endothermic reaction. CaCO3 + Heat → CaO + CO2

5. Effects of some oxidation reaction in everyday life:

(a) Corrosion – The process of slow conversion of metals into their undesirable compounds due to their reaction with moisture and other gases present in the atmosphere is called corrosion of metals. Some common examples of corrosion are rusting of iron, formation of green layer on the surface of copper, tarnishing of silver etc. Formation of brown layer on the surface of iron is called rusting of iron. Formula of rust is Fe2O3.xH2o. Rusting of iron is a serious problem.

 (b) Rancidity- The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is known as rancidity. Rancidity is caused due to oxidation of fat and oil present in food materials. Rancidity can be prevented by using various methods such as by adding antioxidants to the food materials, storing food in air tight container and by flushing out air with some inert gases like nitrogen.

Chemical Reactions

Chemistry Adda  Introduction to Chemistry - Part III

Chemical Reactions
Transformation of a substance into a new substance or substances is called a chemical reaction.  For example water (which is the compound formed when Hydrogen and Oxygen bind together), the chemical reaction can be written as :  

                                    2H₂ + O₂    2 H₂O   

Hydrogen atoms exists as a gas with 2 Hydrogen atoms sticking together and is written as H₂  and the same is true with Oxygen atoms. This means that two elements, Hydrogen and Oxygen combine in a reaction to give a water molecule. The numbers of atoms of each element have to be balanced before and after the chemical reaction takes place. This is an important fact that atoms are neither created nor destroyed in a chemical reaction. The left hand side of the equation is called the reactants and the right hand side is called the products.

Chemical reactions are generally accompanied by heat changes. A reaction, in which heat is evolved, is called an exothermic chemical reaction. A reaction in which heat is absorbed is called an endothermic chemical reaction.Chemical reactions, whether exothermic or endothermic, are also categorized as reversible and irreversible reactions. 
If you burn sugar in air, you get carbon and water. This is an irreversible reaction, That is if you take carbon and water and mix together, you will never get the original sugar back!! A chemical reaction that proceeds in one direction only is known as an irreversible chemical reaction. 

A chemical reaction in which substances react together to produce resultants and the resultants in turn react with one another to produce the original substances is known as an reversible chemical reaction. For example, if calcium oxide is kept in close contact with carbon dioxide, the two substances slowly unite to make calcium carbonate.

  CaO  + CO₂  CaCO₃

If you heat CaCO₃  you will get back CaO and CO₂  .

The two equations can be combined as 

CaCO₃      CaO  + CO₂   
                

The horizontal arrow shows that the reaction proceeds in both the directions. The vertical arrow shows that the carbon dioxide gas escapes.

Each chemical reaction is characterized by a reaction rate, which is the rate at which the reactants are combining to produce the final product. The rate depends on various factors such as temperature, nature of the reacting substances, etc.

In a reversible chemical reaction, if the rate of forward reaction and the rate of backward reaction are same, we say that the reaction has reached equilibrium.

Summary    
In this chapter we have seen what is an atom and how it can be represented as a symbol. When compounds are formed, we have seen how to write their formula. The formula tells us what is the exact quantity of element that is present in the compound. We have seen how elements display valency, which is a measure of how reactive they are. When chemical elements react, we have seen how to write down the reactions with the help of symbols. The exact balanced number of atoms of each element before and after a chemical reaction has to be strictly followed. More discussions about the types of reaction, etc. will be discussed in later chapters

Chemical Symbols, Chemical Formulae, Valency

Chemistry Adda  Introduction to Chemistry - Part II

2. Chemical Symbols            All Elements have been named by the scientists who had discovered them and do not have any particular nomenclature. Quite a few have Latin names. Some have English names also. Although the names are long, generally abbreviation of either first or first two letters represents the element. This is done for convenience only.  Some elements are known commonly by their English names but the abbreviations followed are of their Latin names!! The abbreviations are called symbols and all the elements are represented by different symbols. 

Table below gives some of the names of the elements and their symbols. 

Common name of element	Latin Name	Symbol
Hydrogen	-	H
Carbon	-	Natrium	Na
Chlorine	-	Cl
Iron	Ferrum	Fe
Silver	Argentum	Ag
Gold	Ag
Gold	Aurum	Au

It has to be borne in mind that for an element represented by two lettered symbol, the first one is in capitals and second one is written as a lower case letter. This again is an adopted convention by scientists and is completely accepted by all.  

3. Chemical Formulae      
Substances (either compounds or mixtures) can be written in combination of the symbols, but have to be written in correct proportion of the elements that are the constituents. The written representation of a molecule of a substance using symbols of constituent element is called the molecular or chemical formula. For example, a molecule of potassium permanganate is written as KMnO₄; this means that one molecule of potassium permanganate contains one atom of potassium (K), one atom of manganese (Mn) and four atoms of Oxygen (O). A molecule of water is written as H₂O; which means that there are 2 atoms of Hydrogen (H) and one atom of Oxygen (O). In an ordinary table salt molecule, there is one atom of Sodium (Na) and one atom of Chlorine (Cl) and hence its chemical symbol is NaCl. To be able to write a chemical formula, we must first know the chemical reaction that precedes the formation of the compound. 

4. Valency  Our knowledge about the structure of atoms depends on the mathematical formulations predicted by Neils Bohr. He suggested that electrons are distributed in orbits and the number of electrons held in the orbit depends on the number of the orbit. The orbits are counted outwards from the nucleus. Higher the orbit number, farther are the electrons in that orbit from the nucleus. If the orbit number is “n”, then the maximum electrons held in the orbit is given as 2n². The first orbit has n=1, and will hold maximum of 2 electrons, the second orbit has n=2 and is capable of holding a total of 8 electrons; similarly the third orbit will be able to contain 18 electrons and so on. 

Electrons within an atom have definite energies. The electrons closest to the nucleus (n=1) are most tightly bound; the reason is because of stronger electrostatic attraction with the nucleus. Electrons in the highest orbit are least tightly bound. Electrons in the same orbit have same energies. The electron orbits are also called as electron energy levels or shells. Electronic shells are known as K shell, L shell, M shell, N shell corresponding to orbit number n=1,2,3 and 4 respectively. Higher number orbits are assigned shell names in alphabetical order after NThe distribution of electrons in various orbits in an atom is known as the electronic configuration of that atom. The electronic configuration gives each atom, and hence each element in nature, its unique physical and chemical characteristics.

Table below gives electronic configuration of some elements. 

Name of the element (symbol)	Atomic number (Z)	Total number of electrons	Electronic Configuration   K         L         M         N
Hydrogen (H)	1	1	1
Carbon (C)	6	6	2	4
Oxygen (O)	8	8	2	6
Chlorine (Cl)	17	17	2	8	7

The number of electrons in the outermost orbit decides the chemical nature of the atom. An atom tries to have its outermost orbit complete with generally 8 electrons. Helium is the only stable element whose (first and the last) outermost orbit has 2 electrons. If this situation is not met, then the atom is unstable and reacts with other elements to either give or borrow electrons. The tendency of atoms to donate or accept electrons to stabilize their outermost orbits is known as the valency of the element. Valency is the measure of reactivity of the element. For example, Sodium (Na) has Z=11, no of electrons =11. Its electronic configuration will be K=2, L=8, M=1. The last M shell has vacancy for accommodating 7 electrons. But instead it is far easier for an Na atom to give up 1 electron from the M shell so that its outermost orbit has L=8. Chlorine (Cl) has Z=17, no. of electrons = 17. Its electronic configuration will be K=2, L=8, M=7. The last M shell needs to borrow 1 electron or give up 7 electrons. The chemical compound Sodium Chloride (NaCl) is very easily formed as both Na and Cl complement each other’s need for stabilizing their outermost orbits. Both Na as well as Cl are said to have valency 1. (Na valency is said to be +1, Cl valency is said to be –1)

We will discuss more about valency of atoms when we discuss how compounds are formed. Also it has to be kept in mind that several elements show variable valency, that is their combining capacity with other elements change with the condition of reaction.  

Other than n=1 that is K shell, all other shells have sub-shells. The sub-shells very slightly differ in energy within each shell.

In reality, the outermost orbit of an atom is stable when the electrons are paired.

Variable Valency Quite a lot of transition metals and non-metals show variable valency. As mentioned before, valency is a measure of reactivity of an element. Physically, valency means how many electrons an element can donate or accept to make its outermost orbital stable.  A stable orbit may mean that there are no unpaired electrons left in the outermost orbit. Generally it is observed that an atom tries to acquire electronic configuration of its nearest noble gas atom. Amongst transition metals, Iron, Nickel show variable valency. Amongst non-metals, Oxygen and Nitrogen are good examples. These metals and non-metals combine to give various compounds having different properties. For example Fe2O3 is  different from FeO. In Fe2O3, Fe shows valency +3 and O shows valency –2. Fe2O3 is a magnetic compound. In FeO, Fe shows valency +2. FeO is non magnetic.   Water (H2O) and hydrogen peroxide (H2O2) are compounds of hydrogen and oxygen. In H2O, H shows a valence of +1 and O shows a valence of –1. In H2O2, H has a valency of +2 but O has a valency of –1. Water is a neutral compound but H2O2 is a highly acidic compound.

Atomic Structure

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Class IX Chemistry Test paper Chapter 4: Structure of the Atom by JSUNIL Tutorial
Dalton’s Atomic Theory
The important postulates of Dalton’s atomic theory are:
1. All elements are composed of atoms. Atom is too small so that it could not be divided into further simpler components.
2. Atom cannot be destroyed or produced.
3. Atoms of an element are similar in all respects. They have same mass and properties.
4. Atoms of different elements combine in a definite simple ratio to produce compounds.

Discovery of Electron
A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump.
(Diagram)
Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and momentum.
Properties of Cathode Rays
The properties of these particles are given below:
1. These particles are emitted from cathode surface and move in straight line.
2. The temperature of the object rises on which they fall.
3. They produce shadow of opaque object placed in their path.
4. These particles are deflected in electric and magnetic fields.
5. These particles are deflected towards positive plate of electric field.
Discovery of Proton
Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are also composed of metered particles. The positive rays are not emitted from anode. They are produced by the ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons are removed and positive particles are produced.
Properties of Positive Rays
1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge.
2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube.
3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are called Protons.
4. The charge on proton is equal to +1.602×10^-19 Coulomb. (-19 is power of 10)
Natural Radioactivity
The phenomenon in which certain elements emit radiation which can cause fogging of photographic plate is called natural radioactivity. The elements which omit these rays are called radioactive elements like Uranium, Thorium, Radium etc. There are about 40 radioactive elements. Henri Bequrel (1896) discovered radioactivity.Madam Curei also has valuable contribution in this field.
In natural radioactivity nuclei of elements are broken and element converted to other elements. Natural radioactivity is nuclear property of the elements.
Alpha Rays
1. They are helium nuclei. They are doubly positively charged, He2+.
2. They move with speed equal to the 1/10th of the velocity of the light.
3. They cannot pass through thick-metal foil.
4. They are very good ionizer of a gas.
5. They affect the photographic plate.
Beta Rays
1. They are negatively charged.
2. They move with the speed equal to the velocity of light.
3. They can pass through a few millimeter thick metal sheets.
4. They are good ionizer of a gas.
5. They can affect the photographic plate.
Gamma Rays
1. They are electromagnetic radiations.
2. They travel with speed equal to velocity of light.
3. They carry no charge.
4. They have high penetration power than alpha and beta rays.
5. They are weak ionizer of gas.
Rutherford Experiment and Discovery of Nucleus
Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected at an angle greater than 150.
Following conclusions were drawn from the Rutherford’s Alpha Particles scattering experiment.
1. The fact that majority of the particles went through the foil undeflected shows that most of the space occupied by an atom is empty.
2. The deflection of a few particles over a wide angle of 150 degrees shows that these particles strike with heavy body having positive charge.
3. The heavy positively charged central part of the atom is called nucleus.
4. Nearly all of the mass of atom is concentrated in the nucleus.
5. The size of the nucleus is very small as compared with the size of atom.
Defects of Rutherford Model
Rutherford model of an atom resembles our solar system. It has following defects:
1. According to classical electromagnetic theory, electron being charged body will emit energy continuously. Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus and atomic structure would collapse.
2. If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum is obtained.
Bohr’s Atomic Model
Neil Bohr (1913) presented a model of atom which has removed the defects of Rutherford Model. This model was developed for hydrogen atom which has only proton in the nucleus and one electron is revolving around it.
Postulates of Bohr’s Atomic Model
The main postulates of Bohr’s Model are given below:
1. Electrons revolve around the nucleus in a fixed orbit.
2. As long as electron revolves in a fixed orbit it does not emit and absorb energy. Hence energy of electron remains constant.
3. The orbit nearest to the nucleus is the first orbit and has lowest energy. When an electron absorbs energy it jumps from lower energy orbit to higher energy orbit. Energy is emitted in the form of radiations, when an electron jumps from higher energy orbit to lower energy orbit. The unit of energy emitted in the form of radiations is called quantum. It explains the formation of atomic spectrum.
4. The change in energy is related with the quantum of radiation by the equation :
E2 – E1 = hv where,
E1 = Energy of first orbit      E2 = Energy of the second orbit       h = Planck’s constant
v = Frequency of radiation
Atomic Number
The number of protons present in the nucleus of an atom is called atomic number or proton number. 

It is denoted by z. The proton in the nucleus of an atom is equal to number of electrons revolving around its nucleus.
Mass Number
The total number of the protons and neutrons present in the nucleus of an atom is called mass number. The protons and neutrons together are called nucleon. Hence it is also known as nucleon number. It is denoted by A. the number of neutrons present in the nucleus of an atom is rperesented by N.
Mass Number  =       No of Protons  + No of neutrons 
                    A =               Z              +        N
Isotopes
The atoms of same elements which have same atomic number but different mas number are called Isotopes. 

The number of protons present in the nucleus of an atom remains the same but number of neutrons may differ.
Isotopes of Different Elements
Isotopes of Hydrogen
Hydrogen has three isotopes:
1. Ordinary Hydrogen or Protium, H.
2. Heavy Hydrogen or Deutrium, D.
3. Radioactive Hydrogen or Tritium, T.
Protium
Ordinary naturally occurring hydrogen contains the largest percentage of protium. It is denoted by symbol H. It has one proton in its nucleus and one electron revolve around the nucleus.
Number of Protons = 1
Number of Electrons = 1
Number of Neutrons = 0
Atomic Number = 1
Mass Number = 1
Deutrium
Deutrium is called heavy hydrogen. The percentage of deutrium in naturally occuring hydrogen is about 0.0015%. It has one proton and one neutron in its nucleus. It has one electron revolving around its nucleus. It is denoted by symbol D.
Number of Proton = 1
Number of Electron = 1
Number of Neutrons = 1
Atomic Number = 1
Mass Number = 2
Tritium
Radioactive hydrogen is called tritium. It is denoted by symbol T. The number of tritium isotope is one in ten millions. It has one proton and 2 neutrons in its nucleus. It has one electron revolving around its nucleus.
Number of Proton = 1
Number of Electron = 1
Number of Neutron = 2
Atomic Number = 1
Mass Number = 3