CHEMICAL REACTIONS - CHEMICAL EQUATIONS
INTRODUCTION:
In our daily life we all know about physical changes and chemical changes, all these changes are observed in our day today life. We observe ice converts into water vapor and vice versa. In this example, the changes in the states of matter are observed and heat may be gained or released. Similarly, burning of wood also example.
CHEMICAL REACTION:
Chemical reactions represent the processes in which a single substance or substances undergo change to produce new substance or substances with different or new properties are called 'CHEMICAL REACTIONS'.
For example: When potassium nitrate gets heated potassium nitrite and oxygen are formed. This is a chemical reaction since potassium nitrate changes into potassium nitrite and oxygen, these are new substances.
A chemical reaction consists of REACTANTS and PRODUCTS.
From the above example, potassium nitrate is called Reactant and potassium nitrite and oxygen are the products, since the substance which takes part in chemical reaction is called Reactant and a new substance formed or substances formed in a chemical reaction, called products.
FIND OUT THE REACTANTS AND PRODUCTS from the following:
Calcium carbonate -----> Calcium oxide and carbon dioxide
Sodium combines with water to get sodium hydroxide and hydrogen
HOW THE REACTANTS ARE TRANSFORMED INTO PRODUCTS ??????
Initially, the compounds or molecules break down into atoms and these atoms take part in the reaction then products are formed by new bonds formed between the atoms.
CHARACTERISTICS OF CHEMICAL REACTION:
How can we understand that there is a existence of chemical reaction and it took place, by observing the characteristics we can understand that there is a chemical reaction observed ......
1) When metals react with acids, we can observe there is evolution of Hydrogen gas.
2) Formation of precipitate when two solutions are mixed, the precipitate is a solid substance separated from the solution. For example, lead iodide is a precipitate formed when lead acetate aq.solution combined with aq. sol. of potassium iodide.
3) Some times we observe the emission of heat such reactions are
exothermic reactions, we know that when Calcium oxide is added to water, and sometimes the equipment becomes cold when ammonium chloride mixed with barium hydroxide. These are called
endothermic reactions.
CHEMICAL EQUATIONS Vs CHEMICAL REACTIONS
We can understand with an example :
CaCO3 is the formula of Calcium carbonate and CaO is calcium oxide and CO2 is carbon dioxide.
The above is a chemical reaction, when it is represented with formula and symbols, that notation is known as chemical equation.
How can we write a chemical equation ??????
* the reactants and products in the reaction should be written with their formulae or symbols.
* when more than one reactant or product, then + symbol is used
* an arrow mark is used in between reactants and products, the arrow mark also tells us in which way the reaction is taking place.
What information is conveyed by a chemical equation ???
* we understand the reactants and products in the reaction, and their number of atoms
* we can calculate the masses of the compounds or molecules in the reactants and products
* we come to know the number of moles of the reactants and products.
EXAMPLE:
When we observe the formation of water from hydrogen and oxygen :
There are :
four hydrogen atoms, two oxygen atoms gives two water molecules
two moles of hydrogen and one mole of oxygen to give two mole of water
four grams of hydrogen and thirty two grams of oxygen to give 36 g of water
we can also calculate number of atoms and volumes also.
2 H2 + O2 ------> 2 H2O
HOW TO MAKE A CHEMICAL EQUATION MORE INFORMATIVE ???
Generally, a chemical equation do not represent any thing other than the reactants and products, but to make a chemical equation more informative, one has to represent :
* states of the reactants and products, s, l, g, aq.
* the evolution of gas with upward arrow line and precipitate with downward arrow line
* if heat is liberated, then in the products + heat should be written
* if heat is absorbed, then in the reactants + heat should be written.
TYPES OF CHEMICAL REACTIONS:
1) COMBINATION REACTIONS
2) DECOMPOSITION REACTIONS
3) SINGLE DISPLACEMENT REACTIONS OR SUBSTITUTION REACTIONS
4) DOUBLE DISPLACEMENT REACTIONS
WHAT IS THE DIFFERENCE BETWEEN BALANCED CHEMICAL EQUATION AND UNBALANCED CHEMICAL EQUATION ??
BALANCED CHEMICAL EQUATION : When the number of atoms of each element on both sides is equal then the equation is said to be balanced chemical equation.
UNBALANCED CHEMICAL EQUATION: When the number of atoms of each element on both sides is not equal, then the equation is said to be unbalanced chemical equation.
HOW TO MAKE AN EQUATION A BALANCED EQUATION - STEPS TO BE REMEMBERED:
** must know the formulae of compounds or molecules of reactants or products and the subscriptions in the formulas should not be altered at any cost.
for example: always the calcium carbonate is CaCO3 and Hydrogen molecule is H2 only.
TRIAL AND ERROR METHOD OF BALANCING A CHEMICAL EQUATION:
STEPS:
1) COUNT EACH TYPE OF ATOMS ON BOTH SIDES
2) IDENTIFY THE MOST COMPLICATED FORMULA AND COUNT THE NUMBER OF ATOMS OF EACH ELEMENT
3) BY TRIAL AND ERROR METHOD, CHANGE THE NUMBER OF ATOMS ON BOTH SIDES TILL THE NUMBER OF ATOMS ON BOTH SIDES EQUAL.
EXAMPLES OF UNBALANCED AND BALANCED EQUATIONS:
1) KClO3 ---> KCl + O2
After balancing,
2KClO3 ----> 2 KCl + 3 O2
2) CaCO3 ---> CaO + CO2
3) AlCl3 + Ca(OH)2 ----> Al(OH)2 + CaCl2
2 AlCl3 + 3 Ca(OH)2 ----> 2 Al(OH)2 + 3 CaCl2
4) Al + H2SO4 ----> Al2(SO4)3 + H2
After balancing:
2 Al + 3 H2SO4 ----> Al2(SO4)3 + 3 H2
How to balance a chemical equation? with algebraic expression method.
A chemical equation tells us the type of reaction between different reactants. Chemical equation gives the states of the reactants and products. One can calculate the amount of products required for required amount of products.
To balance a chemical equation, very interesting method is algebraic expression method.
for example: C3H8 + O2 -----> CO2 + H2O
The above reaction is to be balanced using algebraic expressions
step 1) the coefficients of Each reactants and products assumed to be a,b,c,d,...
aC3H8 + b O2 -----> c CO2 + d H2O
step 2) identify and count the number of elements on both sides of the reaction
carbon: 3a = c
hydrogen: 8a = 2d
oxygen: 2b = 2c + d
step 3) let us assume a=1, then simplify the equations for other variable values
c = 3, 2d = 8 then d = 4 if d=4 and c =3 then 2b=6 + 4 = 10, b=5
step 4) write the chemical equation using the variable values
C3H8 + 5O2 ---> 3CO2 + 4 H2O
step 5) count the number of atoms on both sides
carbon atoms = 3
hydrogen atoms = 8
oxygen atoms = 10
step 6) we conclude that the equation is said to be balanced.
ASSIGNMENT ON BALANCING CHEMICAL EQUATIONS:
1) Reaction between Iron oxide and carbon to get Iron and carbon dioxide
2) reaction between hydrogen and oxygen to get water
3) reaction between acetic acid and ethyl alcohol to get ethyl acetate and water
4) when sulphuric acid reacts with calcium carbonate to get calcium sulphate along with H2CO3
5) combustion of methane to get carbon dioxide and water
6) reaction between copper oxide and sulphuric acid to get copper sulphate and hydrogen
7) reaction between nitrogen and oxygen
8) reaction between ammonia and oxygen
9) reaction between nitrogen and hydrogen
10) when ammonia reacts with hydro chloric acid to get ammonium chloride
11) when sodium reacts with water to get sodium hydrogen and hydrogen
12) when calcium reacts with hydro chloric acid to get calcium chloride and hydrogen
GENERAL KNOWLEDGE - USEFUL for ALL COMPETITIVE EXAMS...........
