CHAPTER 2- CHEMICAL REACTION AND CHEMICAL EQUATION
Q1: State law of conservation of mass (matter)? Give examples.
Ans.: During any process, mass is neither created nor destroyed this
law was put forward by a French chemist, lavoiser in 1785.
This law states that:
Matter is neither created nor destroyed during a chemical reaction.
During a chemical reaction, the total mass of the product is equal to
the total mass of the reactants.
Example no 1:
When a piece of iron is left in moist air its surface gradually turns
brown, the object gets rusted and gains mass.
4Fe+3O22Fe2O3
The increase in mass is just the mass of oxygen.
Example no 2
When coal burns it leaves behind ash. The ash is lighter than coal.
But the mass of coal will be equal to that of ash and the liberated
carbon dioxide.
C+O2CO2
Q2: state exothermic and endothermic reaction with examples?
Ans:exothermic reaction:-Reaction in which heat energy is evolved is
known asexothermic reaction in such reaction the system become warmer
and neat potential energy of substances decreases. The enthalpy of
product is lessthan the enthalpy of
reactants.
Examples:-
When unslaked calcium oxide is added towater. the water becomes warm
with formation of
Slaked time . in this reaction heat energy is released.
Endothermic reaction:-Reaction in whichheat energy is absorbed is
known as endothermic reaction . in such reaction enthalpy of reactants
is lower than those of product. Since energy
Is absorbed during reaction the temperature of reaction decreases. (example are)
Heat is absorbed in the decomposition ofcalcium carbonate. In this
reaction heating must be continuous to decompose calcium carbonate.
Q3: What are different types of reaction? Give chemical reaction of each type?
Ans: Chemical reaction are of following types:
1:Simple decomposition :-In this reaction a single compound is
decomposed into two or more simple substance.
2:Synthesis:-In this reaction two or moresubstance combine to from a
single compound it is also known as addition reaction.
3:Displacement:-In this reaction an atomor radical displaced by
another in a compound. Such reaction depend upon the electropositive
or electronegative nature of an atom or radical to displaced another.
4:Double decomposition / displacement:-In this reaction the two
reactants decomposed to form two new substances by exchanging their
radicals.
5:Neutralization:-In this reaction , the hydrogen ion of an acid
neutralized by the hydroxide ion of a base to from salt and water.
6:Hydrolysis:-In this reaction the salt of either weak acid and base
dissolve in water to from acid and base.
Q4:Explain Laws of Chemical Combinations with example.
Ans: Laws of Chemical Combinations
There are four laws of chemical combinations these laws explained the
general feature of chemical change. These laws are:
1. Law of Conservation of Mass
2. Law of Definite Proportions
3. Law of Multiple Proportions
4. Law Reciprocal Proportions
Antoine Lavoiser has rejected the worn out ideas about the changes
that take place during a chemical reaction. He made careful
quantitative measurementsin chemical reactions and established that
mass is neither created nor nor destroyed in a chemical change.
Law of Conservation of Mass
Statement
It is presented by Lavoiser. It is defined as:
"Mass is neither created nor destroyed during a chemical reaction but
it only changes from one form to another form."
In a chemical reaction, reactants are converted to products. But the
total massof the reactants and products remains the same. The
following experiment easily proves law of conservation of mass.
Practical Verification (Landolt Experiment)
German chemist H. Landolt, studied about fifteen different chemical
reactions with a great skill, to test the validity of the law of
conservation of mass.
For this, he took H.shaped tube and filledthe two limbs A and B, with
silver nitrate (AgNO3) in limb A and Hydrochloric Acid (HCl) in limb
B. The tube was sealed so that material could not escape outside. The
tube was weighed initially in a vertical position so that the solution
should not intermix with each other. The reactant were mixed by
inverting and shaking the tube. The tube was weighedafter mixing (on
the formation of white precipitate of AgCl). He observed that weight
remains same.
HCl + AgNO3 ———-> AgCl + NaNO3
Law of Definite Proportions
Statement
It is presented by Proust. It is defined as:
"When different elements combine to give a pure compound, the ratio
between the masses of these elements will always remain the same."
Proust proved experimentally that compound obtained from difference
source will always contain same elementscombined together in fixed
proportions.
Example
Water can be obtained from different sources such as river, ocean,
well, canal, tube well, rain or by the chemical combination of
hydrogen and oxygen. If different samples of water are analyzed, it
will have two elements, hydrogen and oxygen and the ratio between
their massis 1:8.
Law of Multiple Proportions
Statement
This law is defined as:
"When two elements combine to give more than one compounds, the
different masses of one element, which will combine with the fixed
mass of other element, will be in simple whole number ratio."
Two different elements can combine to form more than one compound.
They cando so by combining in different ratios to give different
compounds.
Example
Hydrogen and oxygen combine with one another to form water (H2O) and
hydrogen peroxide (H2O2). In water and hydrogen oxide 2 g of hydrogen
combine with 16g and 32g of oxygen respectively. According to law of
multipleproportions, the different masses of oxygen (16g and 32g)
which have reacted with fixed mass (2g) of hydrogenwill have a simple
ratio between each other i.e. 16:32 or 1:2. It means that hydrogen
peroxide contains double the number of oxygen atoms than water. This
law proves this point of Dalton's Atomic Theory that atoms do not
break in a chemical reaction.
Law of Reciprocal Proportions
Statement
This law is defined as:
"When two element A, B combine separately, with the mixed mass of the
third element E, the ratio in which these elements combine with E is
either the same or simple multiple of the ratio in which A and B
combine with each other."
Example
Hydrogen and Nitrogen separately combine to form ammonia (NH3) and
dinitrogen oxide (N2O), in these compounds, fixed mass of nitrogen is
14g and combines with 8 g of oxygen and 3 g of hydrogen. The ratio
between the mass of oxygen and hydrogen is 8:3. Hydrogen and oxygen
also combine withone another to form water (H2O). The ratio between
hydrogen and oxygen in water is 16:2. These ratios are not same. Let
us observe whether these ratios are simple multiple to each other or
not following mathematical operation is carried out.
8:3 ::16:2
8/3 : 16/2
or
8/3 x 2/16
or
1/3
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