chemistry chp 1 chemical reactions

physical change

the change in which only the change in colour, shape and state takes place and no new substance is formed. usually physical changes are reversible

chemical change

the change in which one or two compounds are combined chemically to form a new substance. most of the chemical changes are irreversible

reactant

the substances which are on the [LHS] and take part in the chemical reactions

product

the substances which are on the [RHS] and are formed during a chemical reaction

the law that governs chemical reaction

the law of conservation of mass-

mass of the reactant is always equal to the mass of the product.

during a chemical reaction mass can neither be created nor be destroyed

observations that determine a chemical reaction

evolution of gas

formation of precipitate [insoluble solid formed during a chemical reaction]

change in temp

change in colour

change in state

change in state

mg[s] + o2[g] = mgo2[s]

2H2 +O2 = 2H2O

change in colour

the reaction of heat on green viterol= feso4 . 7H2O [green] + heat = feso4 [colourless] + 7H2O

and when we heat it further

2feso4 + heat = fe2O3 [reddish brown]+ SO2 +SO3

the reaction of heat on blue viterol= cuso4 .5H2O [blue] + heat = cuso4 [colourless] + 5H2O

change in temperature

there are two types of reactions

Exothermic reaction is when heat is liberated during a chemical reaction

Endothermic reaction is when heat is absorbed during a chemical reaction

endothermic reaction

the heating of calcium carbonate

CaCO3 + heat = CaO + CO2

the photosynthesis process is also an example of endothermic reactions as it uses CO2, H2O and sunlight the plant absorbs heat from the sunlight therefore we can say that it is an endothermic reaction

CO2 + H2O + heat = C6H12O6 + O2

note - all thermal decomposition reactions are endothermic in nature as they absorb heat to break down and form the product

exothermic reaction

the reaction of water on calcium oxide

CaO [quick lime] + H2O = Ca[OH]2 [slaked lime] + heat

every combustion reaction is an exothermic reaction

dilution of acids is also an exothermic reaction as it releases heat

evolution of gas

when gas is evolved during a chemical reaction.

AgCl = 2Ag + Cl2

formation of precipitate

an insoluble substance formed during a chemical reaction is called precipitate [ppt].

BaCl2 + Na2SO4 = BaSO4 [white ppt formed] + 2NaCl

types of chemical reactions

combination reaction

decomposition reaction -

• thermal decomposition reaction

• electrolytic decomposition reaction

• photolytic decomposition reaction

• single displacement reaction

• double displacement reaction

• redox reaction

combination reaction

when two or more reactants come together to form one single product

CaO + H2O = Ca[OH]2

decomposition reaction

when a single reactant is broken down into two or more products

there are three types of decomposition reaction

thermal decomposition - where the reactant is broken down by the application of heat ex- CaCO3 = CaO + CO2

electrolytic decomposition - where the reactant is broken down by the application of electricity ex- 2H2O = 2H2 + O2

photolytic decomposition - where the reactant is broken down by the application of sunlight ex- 2BaCl = Ba2 + Cl2

single displacement reaction

the reaction where a high a reactive element displaces with a low reactive element ex- Fe + CuSO4 = FeSO4 + Cu

double displacement reaction

the reaction where exchange of ions occur between two ionic compounds

NaCl + AgNO3 = NaNO3 + AgCl

BaCl + NaSO4 = BaSO4 + NaCl

generally in double displacement ppt is formed

redox reaction

the reaction where the process of oxidation and reduction take place simultaneously

combination reaction and single displacement reaction are redox reactions

oxidation

there are 2 concepts of oxidation

classical concept - 1. addition of oxygen is called as oxidation

2. the process of removal of hydrogen is known as reduction

electronic concept - 1. loss of electron is known as oxidation

2. increase in +ve charge is known as oxidation

reduction

there are 2 concepts of reduction

classical concept - 1. removal of oxygen from a compound is known as reduction

2. the process of addition of hydrogen is known as reduction

   electronic concept - 1. gain of electron is known as reduction

   2. the increase in -ve charge is known as reduction

if oxidation and reduction are so opposite then how do they work together?

if we want to give electron so we should have someone to accept it and this is what happens between reduction and oxidation

oxidising agent

the compound which itself undergoes the process of reduction but oxidises the other compound is an oxidising agent

reducing agent

the compound which itself undergoes the process of oxidation but reduces the other compound is a reducing agent

some of the redox reactions

ZnO + Cu = Zn + CuO in this reaction ZnO is the oxidising agent and Cu is the reducing agent

CuO + H2 = Cu + H2O in this reaction CuO is the oxidising agent and H2 is the reducing agent

CuO + C = Cu + CO in this reaction CuO is the oxidising agent and C is the reducing agent

Corrosion

it is the process by which a metal surface is attacked by a substance such as air or water. In short it is the oxidation of metal

methods for preventing corrosion

1. painting - a temporary way of increasing the life of a metal is by coating it with oil paints

2. Galvanising - deposition of zinc metal on the surface of the metal is called galvanising and this method is better than painting. the zinc sacrifices itself to protect the inner metal.

3. Using magnesium metal - magnesium sacrifices itself and prevents the corrosion of iron. underground pipes and ships are coated with magnesium.

4. Alloy formation - homogeneous mixture that is obtained by mixing two metals or non-metals is called an alloy. Alloy formation prevents rusting and increases the life of metal

corrosion of different metals

corrosion of iron is called rusting Fe2O3.XH2O

corrosion of copper is CuCO3.Cu[OH]2

corrosion of silver is also called tarnishing of silver Ag + H2S = Ag2S + H2

Rancidity

the process in which food containing oil and fat gets detoriated/degraded/spoiled due to oxidation reaction.

it causes change in taste,smell and decreases nutritional value of food

prevention of rancidity

we can prevent rancidity by flushing nitrogen gas in the bag of chips

addition of chemical materials called antioxidants of food such as BHT - butyalated hydroxytoulene and BHA - butyalated hydroxy anisole