LAWS OF CHEMICAL COMBINATION
Laws of chemical combination were given by Antoine L. Lavoisier and Joseph L. Proust.
These are the two laws, that are the law of conservation of mass and constant proportions.
LAW OF CONSERVATION OF MASS
Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
For example, Let A, B, X, Y are the elements.
AB + XY ——–> AX + BY
Here, the mass will remain same before and after the reaction.
2. LAW OF CONSTANT PROPORTION
This law was stated by Proust.
It states that in a chemical substance the elements are always present in definite proportions by mass.
For example, in water molecule H and O always present in the ratio 1:8 by mass.
DALTON’S ATOMIC THEORY
John Dalton gave the theory based on laws of chemical combination which explained its laws.
Postulates of this theory states that:
- Matter is made up of tiny particles called atoms that participate in chemical reactions.
- Atoms are indivisible, they cannot be created nor destroyed. (law of conservation of mass)
- Atoms of a given element are identical in mass and chemical properties.
- Atoms combine in the ratio of small whole numbers to form compounds.
- The relative number and kinds of atoms are constant in a given compound. (law of constant proportion)
ATOMS
Particles are made up of smallest unit called atoms.
They do not exist freely. They join to form molecules and ions.
Atoms are very small in size. Radius of an atom of hydrogen is 10-10m.
Atomic radius is measured in nanometers. (1nm= 10-9 m)
SYMBOLS OF ATOMS
Symbols for atoms were first used by Dalton. That symbol denotes a definite quantity.
Some of the symbols proposed by him:
- Berzillus suggested to use one or two letters of the name of element as symbols.
- IUPAC (International Union of Pure and Applied Chemistry) approves the names, symbols and units.
Many symbols are the first one or two letters of name of element. The first letter of a symbol is always written in capital letter and the second letter written in small letter. Such as Barium Ba, aluminium is Al.
Some symbols have first letter from the first alphabet in name of element and second letter is the alphabet appearing later in the name. Such as Zinc is Zn, magnesium is Mg.
Some symbols are taken from the Latin, German or Greek names of the element. Such as sodium is Na that is taken from the name natrium, potassium is K from kalium.
Symbols of some elements:
ATOMIC MASS
Mass of an atom is called atomic mass. It is expressed as unified mass (u) [by IUPAC].
Earlier the unit of atomic mass is atomic mass unit (amu). 1 atomic mass unit is equal to 1/12th mass of carbon atom.
- Relative atomic mass: Average mass of the atom compared to 1/12th mass of carbon-12 atom.
Relative atomic masses (in u) of some elements
MOLECULE
Molecule is combination of two or more atoms bonded chemically by attractive forces. It is the smallest particle in element or compound.
MOLECULE OF ELEMEMT: Molecule of an element is made up of combination of same type of atoms. For example, molecules of Ne will consist of same type of atoms in their molecule.
Monoatomic molecule- Molecule which has one atom. For example, Helium (He), Argon (Ar).
Diatomic molecule- Molecule in which two same type of atoms combine with each other. For example, H2 in which 2 H atoms bonded to each other. Some more examples are: O2, Cl2, etc.
Tetra atomic molecule- consist of 4 same type of atoms such as phosphorus (P4).
Polyatomic molecule- consist of more than 4 same type of atoms such as sulphur (S8).
ATOMICITY–> It is the number of atoms present in molecule. For example, atomicity of Cl2 is 2 as it consists of 2 atoms.
MOLECULE OF COMPOUND: Molecule of compound is made up of different types of atoms combined together in different proportions. For example, CO2, CCl4, etc.
Ratio by mass= ratio of the atomic masses of the given atoms. For example,
Ratio by mass of CO2 = atomic mass of C atom/ 2 (atomic mass of O) = 12/2 (16) = 12/32 = 3:8
Ratio by number of atoms = ratio by mass of atom/ atomic mass of atom
For example, CO2
Ratio by mass atomic mass ratio by number of atoms
C 3 12 3/12
O 8 16 8/16= ½
Simplest ratio that is ratio of the ratio by number of atoms
Simplest ratio for CO2 = 3/12 = 1/2 = 1:2
ION
Ion is a charged atom or group of atoms containing a net charge present in ionic (metal & non -metal) compound.
For example, Cl–, OH–, Na+, Al3+, etc.
ANION: It is negatively charged ion.
CATION: It is positively charged ion.
For example,
In HCl, cation is hydrogen ion (H+), and anion is chloride ion (Cl–).
WRITING A CHEMICAL FORMULA
A chemical formula is a symbolic representation of composition of the compound.
For writing a chemical formula, we should know about the symbols and the valencies of the atoms or ions present in that compound.
Valency represent combining capacity of element. In charged ions, valency of that ion is the charge present on it.
RULES
Balance the valencies or charges on the atoms or ions respectively.
In a compound, first symbol will be of metal which is written. For example, ZnO (Zn is metal and O is a non-metal).
If there is polyatomic ion more than one in number then its number is written as subscript outside the enclosing bracket of the ion. For example, Ca (OH)2. OH is 2 in number so it is written like (OH)2.
Simple compounds that have two different elements in it are called binary compounds.
Some chemical formula of binary compounds:
Formula of water
H O
Valency: +1 -2
After balancing, formula becomes: H2O
Formula of calcium carbonate
Ca CO3
Valency: +2 -2
After balancing, formula would be: CaCO3
SYMBOLS AND VALENCIES OF SOME COMMON IONS
MOLECULAR MASS AND MOLE CONCEPT
MOLECULAR MASS
Molecular mass of a substance is the sum of atomic masses of all atoms present in that molecule of the substance.
Its unit is- atomic mass unit (u).
For example, molecular mass of CO2= atomic mass of C + 2*(atomic mass of O)
= 12 + 2*(16) = 12 + 32 = 44u
FORMULA UNIT MASS
Formula unit mass is the sum of atomic masses of all atoms present in the formula of compound. Its unit is also atomic mass unit(u).
For example, formula unit mass of Na2O= 2*(atomic mass of Na) + atomic mass of O
= 2*(23) + 16 = 46 + 16 = 62u
MOLE CONCEPT
The word mole was introduced by Wilhelm Ostwald. It is also called chemist’s counting unit.
Mole indicates SI unit of quantity of a substance. It is expressed as mol. Number of moles is given by ‘n’.
1 mole (of anything) = 6.022 x 1023 number (of that thing)
For example, 1 mole of atom = 6.022 x 1023 number of atoms
1 mole of molecule = 6.022 x 1023 number of molecules
6.022 x 1023 is a constant called as Avogadro number (NA). It is given by unit mol-1.
Mass(u) of 1 mole of substance = gram atomic or molecular mass in grams (g) (also called Molar mass)
For example, atomic mass of 1 mole of C = gram atomic mass of C
12u = 12g
Number of moles= given mass/ molar mass
Number of moles= given number of particles/ Avogadro number
MIXTURES
A mixture is a substance made up by physical combination of two or more substance. In mixture, the properties of individual materials are retained.
Types of mixtures:
Homogenous mixture: Homo means same. This kind of mixture contains only one phase. It has uniform composition throughout. These mixtures cannot be separated physically.
Example: sugar dissolved in water, salt in water, etc. This means you cannot distinguish between the constituents. These mixtures can have a variable composition.
Heterogenous mixture: Hetero means different. This kind of mixture contains at least two phases which cannot be mixed. They can be separated physically. These mixtures have non-uniform composition.
Example: oil in water (separate layers will be formed but they will not be mixed), sugar and sand, cement, and water, etc.
SOLUTIONS
Solutions are the homogenous mixtures in which solute is dissolved in solvent. There is a common misconception that solutions can only be of liquid and gas states. Solutions can be of all the three states (solid, liquid and gas).
Solvent: It is the component of solution which dissolves the other component. Generally, it is in higher amount.
Solute: It is the component of solution which is being dissolved in Generally, it is less in amount.
For example: in a solution of sugar and water, sugar will be the solute and water will be the solvent.
Properties of solution:
Solutions cannot be separated physically.
It is stable (means when they are left undisturbed, they are still mixed and do not settle down in at the bottom)
Particles of solution are very tiny, they have less than 10-9-meter diameter.
As their size is very small, scattering of light is not possible. Beam of light is clearly seen from a solution.
ALLOYS: An alloy is a mixture of – two or more metals
– Metal or non-metal
Which cannot be separated with help of physical methods.
Alloys can have variable composition. Examples of alloys are brass (copper and zinc), bronze (copper and tin), etc.
CONCENTRATION OF SOLUTION
We can classify solution on the basis of amount of solute present in the solution.
Dilute: If solute amount is less than that of solvent, it is called dilute solution.
Concentrated: If solute amount is more than that of solvent, it is called concentrated solution.
Saturated: A solution is called saturated when a solution has dissolved an amount of solute and no more solute can be dissolved in it at a given temperature.
Unsaturated: If the solution has dissolved solute less than its saturation level, it is called unsaturated solution.
Some other definitions:
Solubility: When the saturation level is attained, at that time and temperature amount of solute present in the solution is called its solubility.
*Different solutes in a same solvent can have different solubility.
CONCENTRATION: It is the amount of solute (by mass or volume) present in a given amount of solvent (by mass or volume).
We can calculate concentration by three methods:
Mass by Mass percentage: (Mass of solute/Mass of solution) *100
Volume by volume: (Volume of solute/ Volume of solution) *100
Mass by volume percentage: (Mass of solute/Volume of solution) *100
Example: A solution have 100 grams of solute present in it. Mass of solvent is 350 grams. Calculate mass by mass percentage of the solution.
Answer: Given that,
Mass of solute = 100 g, Mass of solvent = 350 g
So, total mass of solution = mass of solute + mass of solvent
= 100 + 350 = 450g
Mass by mass percentage is = (Mass of solute/Mass of solution) *100
= (100/450) *100
= 22.22%
SUSPENSION:
If in a heterogenous mixture, solute particles are not dissolved but they remain suspended in it, that is called suspension. Even the solute particles are visible by naked eye.
Properties:
It is unstable. It means if it is left undisturbed then the solute particles will settle down.
Before the particles settle down, they will scatter light. But after the settling solution will not scatter as it will become a clean solution then.
Colloidal solution: If in a heterogenous mixture, solute particles are not dissolved. they are very tiny in size so that they cannot be seen with naked eye. But they can scatter beam of light. They show Tyndall effect.
Tyndall effect: Scattering of beam of light through tiny particles is called Tyndall effect. It was named after scientist who discovered it.
Properties:
Colloids have enough size that they can scatter a beam of light and hence the path is visible.
They are somewhat stable as they do not settle down when left undisturbed.
Their physical separation is not possible, but they can be separated by centrifugation technique.
CENTRIFUGATION – It is a technique which is used to separate particles from solution according to their size, shape, density, etc. It includes use of rotor and then spinning them in definite speed.
Colloidal solution has –
Dispersed phase – It can be considered as solute. These are the particles which are dispersed.
Dispersing medium – It can be considered as solvent, in this the phase is dispersed.
Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.
Examples:
METHODS TO SEPARATE COMPONENTS OF MIXTURE
EVAPORATION: To separate volatile solvent from the non-volatile solute. For example- separating copper sulphate from water, separating dye from ink.
CENTRIFUGATION: This is used to separate solid particles (not filtered by filter paper) from liquid. In this, when the mixture sin at high speed, denser particles sit at the bottom and lighter particles come at the top. For example- separation of butter from cream, blood tests, etc.
SEPARATING FUNNEL: Two immiscible liquids can be separated by using separating funnel on basis of their densities. For example- separating oil from water, extraction of iron from its ore.
SUBLIMATION: This is used for separating mixture of sublimable (directly converts from solid to gas) component and non- sublimable component. For example- separation of camphor and salt, etc.
CHROMATOGRAPHY: It is a technique by which components are separated on the basis of their color (Kroma- color). For example- separation of different colors in the dye, drugs in blood, pigments, etc.
DISTILLATION: It is the method used to separate two miscible components in a mixture on the basis of their boiling point. For example, separation of fuel from the crude oil, acetone from water, etc.
FRACTIONAL DISTILLATION: This is a type of distillation, but this is used when the difference in boiling point of two components is less than 25K. For example- separating gases from air mixture (separate at different heights), refining of petroleum products.
In this a fractional column is inserted in the apparatus.
Liquid air- The air is compresses and cooled by increasing pressure and decreasing temperature.
Separation of Oxygen:
8. CRYSTALLISATION: This is the method used for separating pure solid from the solution. Pure solid is in form of crystals. For example, separating salt from sea, etc.
• This method is better than evaporation because in this solid can’t decompose or get charred.
PHYSICAL AND CHEMICAL CHANGES
PHYSICAL CHANGE
The change in which the chemical composition of substance remains same is called a physical change. In this the physical form of substance is changed.
Physical changes are reversible in nature.
For example, change of water into ice.
CHEMICAL CHANGE
The change in which the chemical composition of substance is changed and converted to a new substance is called a chemical change. This type of change occurs in a chemical reaction.
Chemical changes are irreversible in nature.
For example, conversion of milk into curd, burning of wood, etc.
TYPES OF PURE SUBSTANCES
Pure substances are divided into two types on the basis of chemical composition.
ELEMENTS
Robert Boyle first used the term element.
Antoine Laurent Lavoisier defined element as a basic form of matter that cannot be further broken down into simpler form by chemical reaction.
Elements are divided into-
Metals– They are lustrous, malleable (made into sheets), ductile (made into wires), sonorous etc. Example- copper, gold etc.
Non- Metals- They are non-conductors and are not lustrous, malleable or ductile. Example- Hydrogen, chlorine etc.
Metalloids- They possess properties of both metals and non- metals. Example- Boron, silicon etc.
93 elements are naturally occurring.
11 elements are in gas form at room temperature.
Gallium and Cesium become liquid above a level of room temperature.
COMPOUNDS
A compound is a combination of two or more elements. For example, H2O, it is a compound which contain elements H and O.
H2S gas is colorless and smells like rotten eggs.
DIFFERENCE BETWEEN MIXTURE AND COMPOUND
MIXTURE COMPOUND
Elements just mix and any new compound Elements combine with each other and new
is not formed. compound is formed.
Composition is not fixed. Composition is fixed.
Have same properties as of elements mix. Have different properties from
elements that combine.
They can change again into elements. They cannot be change back to
individual elements.
PHYSICAL NATURE OF MATTER
What is matter? Matter can be defined as something which occupies space and have some mass.
- Matter is having particle nature. It is made up of particles.
- Particles are having tiny size. They are so tiny that in one crystal of a substance, there are millions of particles.
CHARACTERISTICS OF PARTICLES OF MATTER
- Particles have spaces in between them – There is enough space present in between the particles of a matter. For example: if we add a spoon of sugar in glass of water and stir it. then it gets dissolved in water. Those sugar particles get place in spaces present between the particles of water.
- Particles of matter are continuously moving: Matter’s particles keep on moving. They possess energy called kinetic energy (which is the energy a substance attains when it is moving). The process of movement of particles from higher concentration to lower concentration is called diffusion. Diffusion can also be explained as self-intermixing of particles of two different types. For example: if in a beaker we put a single drop of ink or say water colour then within a very small period of time, it starts expanding.
*Rate of diffusion increases with temperature. With increase in temperature, the energy of particles also increases by which movement of particles also increases. hence, increase on diffusion rate. For example: we can smell hot food from far but not cold food. This is because from hot food, particle is coming in atmosphere with more kinetic energy. But in case of cold food they don’t have enough kinetic energy.
- Attraction between particles: Particles of a matter have force of attraction among them. This force is varying with respect to all matters. For example: great force is required to break iron nail with help of hammer because they are attached with greater force of attraction among iron nail’s particles.
STATES OF MATTER
SOLID STATE: It is the state of matter in which atoms attain fixed position with respect to each other. Molecules are tightly packed and have strong intermolecular force of attraction.
Properties:
Definite shape
Fixed volume
Incompressible (because the intermolecular space is very less)
Rigid
Rate of diffusion is very less
Layers of molecules are arranged in a highly ordered fashion
For example: furniture (of wood), sugar, etc.
LIQUID STATE: It is the state of matter which have somewhat spacing in between the molecules, definite volume with NOT fixed shape. Here the intermolecular forces of attraction are weaker than solids but higher than gas.
Properties:
Definite volume
No fixed shape, it attains the shape of container in which it is poured.
More compressible than solids but less compressible than gases. They are compressible because they have intermolecular spaces.
They can flow hence called fluid. (fluid is name given to those which can flow. Example includes liquids and gases)
Solids, liquids and gases can diffuse in liquids. Example like gases dissolve in water bodies, which is essential for aquatic life)
Rate of diffusion is more in liquids as compared to solids but it is less than gases. This is because in solid there was negligible intermolecular spacing but in liquid there is feature of some intermolecular spacing.
- GASEOUS STATE
It is the state of matter in which the intermolecular spacing is very large. They don’t have any fixed shape and fixed volume. Intermolecular force is very weak.
Properties:
- Highly compressible
- Neither fixed shape nor volume
- Highly diffusible
- High density
- Exert pressure on walls of container in which gas is filled up. This pressure is because of force per unit area by gas particles on the walls of container.
CHANGE IN THE STATE OF MATTER
EFFECT OF CHANGE OF TEMPERATURE
When heat is supplied the matter can change from solid to liquid and liquid to gas.
This is due to the increase in the kinetic energy of the particles. Particles start vibrating and can overcome the forces of attraction between them.
The particles vibrate faster when liquid changes to gas as compared to the solids.
Melting point: It is the minimum temperature at atmospheric pressure at which solid converted to liquid state. This process is called as fusion.
Temperature does not change when fusion. The heat which is supplied is used by particles for changing the state. This heat is called latent heat.
Latent heat of fusion: It is the heat which is required to convert 1Kg of solid into liquid at an atmospheric pressure at its melting point.
Boiling point: It is the temperature at atmospheric pressure, at which liquid starts to change into vapours. Boiling temperature of water is 100oC (273 + 100= 373K)
Latent heat of vaporization: It is the heat which is required to convert 1Kg of liquid into vapour at an atmospheric pressure at its boiling point.
Latent heat of vaporization provides more energy to steam.
Sublimation: Solid directly changes to gas state.
Deposition: Gas directly changes to solid state.
EFFECT OF CHANGE OF PRESSURE
When pressure is applied the particles get compressed and come closer to each other that led to increase in their force of attraction.
Solidification: When liquid changes to solid state.
Condensation: When gas particles changes to liquid state.
Dry ice is solid carbon dioxide. This solid carbon dioxide stored under high pressure. When the pressure is decreased to 1 atmospheric pressure, it changes to gaseous form.
EVAPORATION
Evaporation is a phenomenon by which below the boiling point, liquid changes into vapors.
It occurs in liquid, forces of attraction break in the particles due to high kinetic energy and are capable of changing in vapors.
FACTORS AFFECTING EVAPORATION
Surface area: Rate of evaporation increases with increase in surface area. For example, tea can be cool faster in plate than cup.
Temperature: Rate of evaporation increases with temperature due to more kinetic energy will available to particles to evaporate.
Humidity: It is the amount of water vapors present in air. If humidity is more in the air, then rate of evaporation decreases because air can accommodate only a definite quantity of water vapor.
Wind speed: Rate of evaporation increases with increase in wind speed because wind can move away the vapors in the air so that more vapors can go in air.
HOW DOES EVAPORATION CAUSE COOLING?
The liquid particles loss energy when the convert to vapors. So, for regaining the energy, the particles absorb heat from surrounding that causes cooling effect. For example,
When acetone is applies on the skin of palm, it causes cooling sensation on the skin. This is due to evaporation of acetone particles that gain heat from surrounding.
Cotton clothes in summer: Cotton clothes are worn in summers because cotton is an absorber of water that absorbs the sweat. Sweat evaporates in the atmosphere. Due to evaporation, heat energy comes out of the body which led to the cooling effect on the body.
Note: Scientists are suggesting the five states of matter that include- solid, liquid, gas, plasma and Bose- Einstein condensate (BEC).
Plasma– In the form of ionized gases. They are super excited and energetic.
BEC– It is formed by cooling gas of extremely low density and super low temperature.
BOHR’S MODEL OF ATOM
Few postulates were given by Neil Bohr. These are:
Inside the atom only discrete orbits of electrons are allowed.
In these discrete orbits, electrons revolve but they do not radiate energy while revolving.
These orbits are called energy levels (n). These are denoted as K, L, M—- so on or n=1, 2, — and so on.
NEUTRONS
Discovered by- J. Chadwick.
Neutrons are present in nucleus and had no charge, have mass equal to protons.
Absent in nucleus of H atom.
Mass of an atom= mass of protons + mass of neutrons
DISTRIBUTION OF ELECTRONS IN DIFFERENT ORBITS OR SHELLS
It was suggested by Bohr and Bury.
RULES:
The maximum number of electrons that can be present in a shell is given by 2n2.
For example, in K shell (n=1), 2x (1)2 = 2. So, 2 electrons can be there in K shell and so on for other shells.
In outermost shell only 8 electrons could be present.
Electrons will not fill in a given shell, unless the inner shells are filled.
For example, Lithium (number of electrons= 3), so orbit representation will be:
VALENCY
Electrons present in outermost shell are called valence electrons.
Valency is the number of electrons gained, lost or shared so as to make the octet of electrons (8 electrons) in the outermost shell. It defines the combining capacity of the element.
Valency of inert elements such as Him, Ne is 0 as they have complete octet.
Examples,
Lithium atom (2, 1) has one electron in outermost shell so it loses that one electron to get stable, so it will have valency 1.
Chlorine atom (2, 8, and 7) has seven electrons in outermost shell. It is easy to gain one electron rather than losing seven electrons. So, its valency will be 1 not 7.
Aluminum atom (2, 8, and 3) has three electrons in outermost shell. So, it loses three electrons to get stable octet configuration. Its valency will be 3.
Valency of some common elements:
Element (number of electrons)
Configuration
Valency
Hydrogen (1)
1
1
Helium (2)
2
0
Carbon (6)
2,4
4
Nitrogen (7)
2,5
3
Oxygen (8)
2,6
2
Sodium (11)
2,8,1
1
Magnesium (12)
2,8,2
2
ATOMIC NUMBER: This refers to the number of protons present in the atom or say in nucleus. It is denoted by Z.
For example: 10, it means it have 10 protons inside its nucleus. It is Neon.
MASS NUMBER: This refers to the sum of protons and neutrons inside nucleus. Protons and neutrons together are called nucleons. It is denoted by A. For example:
Mass of nitrogen is 14. Z (atomic number) of nitrogen is 7, which means number of protons are 7.
And we know that:
A = Z + neutrons’ number
14=7 +neutrons
So, number of neutrons in nitrogen = 14-7 = 7
If X be any element’s symbol then as subscript its atomic number is written and as superscript its mass number is written. Like:
XAZ
For example, oxygen will be written as O 816 (8 protons and 8 neutrons)
ISOTOPES: These are the atoms of same element having same atomic number but different mass number.
It means they have same number of protons but different number of neutrons. Every isotope of element is a pure substance. The isotopes have different physical properties but they attain same chemical properties.
For example: Hydrogen have three isotopes namely protium, deuterium and tritium.
Protium H11 Deuterium H12 (D) Tritium H13 (T)
(p = 1 n=0) (p=1 n= 1) (p=1 n=2)
Same element is occurring with different atomic mass in different ratio. So, for getting a common atomic mass the formula is:
((Atomic mass of isotope 1) *(ratio of occurring of 1)/100 +(Atomic mass of isotope 2) *(ratio of occurrence of 2/100) +……. similarly, up to the total number of isotopes)
For example: Chlorine is having two isotopes, one with atomic mass 35 and other with atomic mass 37. Ratio of occurrence is 3:1 respectively. So, the average atomic mass of chlorine will be:
((35) *3/100) + ((37) *1/100) = 35.5u
Applications of isotopes are:
In nuclear reactors isotope of uranium is used as fuel.
For treatment of cancer isotope of cobalt is used.
For treatment of goitre isotope of iodine is used.
ISOBARS: These are the atoms of different elements which have same number of nucleons but different number of protons. For example: potassium and calcium. Both have same mass number of 40 but different atomic number or number of protons.
