FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 1
Thermochemistry:
The branch of chemistry which deals with study of loss or
gain of heat energy during chemical reactions is known as thermo chemistry.
During a chemical reaction, the bonds among the atoms in reactants break down. This requires energy. This energy is got from the surrounding s of the reactants. When the atoms of reactants molecules separate, they recombine forming new bonds as a result of which products ( new molecules ) are formed. During this process ( bond formation ) , energy is released.
The energy (heat) gained during bond breaking and heat during bond formation are usually different. The net heat of reaction is got by adding the heat lost ( - ve) and heat gained. This net heat is known as “ heat of reaction”
If the heat gained is less then heat lost, then the heat of
reaction will have –ve value and the reaction will be “exothermic” on the other
hand if the heat lost is less than the heat gained then the heat of solution
will have a tire value and the reaction will be “endothermic”
Spontaneous & Non-Spontaneous Changes:
Spontaneous Process:
Any process which occurs on its own without any external
support is known as spontaneous process.
Examples:
Evaporation of water, melting of ice, flow of water from
high place to lower place, cooling of hot tea etc.
Spontaneous chemical reaction:
Any chemical reaction which occurs on its own without any
external support is known as spontaneous chemical reaction.
For example all exothermic chemical reaction are
spontaneous. Some spontaneous reactions like combustion need a starting
support, but once they start, then they proceed on their own and need the external support no
more.
Examples of spontaneous chemical reaction are
Rusting of iron (Fe) is a spontaneous reaction. Rusting of Fe occurs
when Fe reacts with O2 and moisture
(H2O) of air producing Fa(OH)3, 2Fe + 3/2O2+ H2O --------2
Fe(OH)3
W3hen CH4 ( methane gas) burns in the presence of O2 and H2O
a spontaneous chemical reaction
occurs. This process is known as combustion.
CH4(g) + 2O2(g)----------------- CO2(g) + H2O(g)
Non- Spontaneous Process:
A process which doesn’t occur on its own and requires a
continuous external support, is known as Non- spontaneous process.
A non-spontaneous process occurs till the external support
is in work. As soon as the external support stops, the process also stops.
Examples:
Freezing of water, lifting of water from lower to higher
level, filling up of air in a type etc.
Non- Spontaneous Reaction:
A reaction which doesn’t occur on its own and which needs a continuous
support is known as non- spontaneous reaction.
Endothermic reactions are usually non-spontaneous in nature.
Examples :
When rusting of Fe occurs, Fe(OH)3 is formed as a result of
a spontaneous chemical reaction but the Fe(OH)3 doesn’t convert back into the reactants. This the
conversion of Fe(OH)3 back into its reactants is a non spontaneous chemical reaction.
When lime stone (CaCo3) is heated and endothermic reaction
occurs in which CaCO3 decomposes producing Cao ( unislaked lime ) and CO2
CaCO3 + heat-------------CaO + CaO3
This reaction proceeds till heat is provided to it. As soon
as supply of heat is stopped, the reaction stops the formation o f products any more so it is
a non-spontaneous chemical reaction. when line stone ( Ca CO3) Is reacted and endothermic
reaction occurs in which CaCO3 decomposes producing CaO and CO2.
CaCO3 + heat ----------------------- CaO + CO2.
This reaction proceeds till heat is provided to it .as soon
as supply of heat is stopped ,the reaction stops the formation of products any more. So it is
a non-spontaneous chemical reaction.
System & surroundings:
That portion of the universe which is under observation is
known as system. Each and everything of the unirerse (which is not a part of the system )
which are present around the system, are known as surrounding of the system.
In simple words can say that:
System + surrounding -------universe
Examples :
When we are performing a chemical reaction, then the
observation so those reactants are known as system and all other theings present around them are
known suroundings of that system.
The surroundings may influence the system and its properties
like its temperature, pressure,
but these influences can he measured and controlled.
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