Question

In Lechatelier Principle , To find the direction of equilibrium when pressure is changed we go with the logic P is directing proportional to the number of moles and proceed further , but when we want to find the direction of equlibrium when Volume is changed , we say P is inversely proprtional to V and proceed further .

My doubt is , Aren't both P and V proportional to the number of moles of the gas according to the Ideal Gas equati if it were so . Please help , I am confused !!

Answer 1

@aaronq , Could you please help me out?

Answer 2

first of all,see in your reaction if provided,you will notice that no.of moles of gas and temperature remains unchanged.

By this,we apply BOYLE'S law,which states that
The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.
so,
\[\large \bf P \alpha \frac{1}{V}\]
\[\large \bf PV=k~where~ k=constant\]

For more information about boyle's law,
SOURCE :- http://en.wikipedia.org/wiki/Boyle's_law

Answer 3

Then,
If in a reaction , if Pressure increases,so volume decreases and the reaction will move in that direction where the NO.OF MOLES OF GAS is less in number.---->Lechatelier Principle

Answer 4

hope you understand. @Dexter810

Answer 5

Agreed .
But , what is wrong in thinking like Volume and Pressure both are proportional to the number of moles , and hence should give same answers regarding Lechatelier Principle .
I agree with what you are saying too , but still do not know why the above mentioned logic is wrong .

Answer 6

see,PV=nRT is a generalized equation or you can say that this equation is valid only for IDEAL GASES not for REAL GASES.

Take a life situation example :-
I think you know that in the gas cylinder,there is full volume comprised of GASES.
and ever you think that how we put gases in the cylinder?

SOLUTION :-
First of all,try this experiment in your home that shake cylinder (gas) left and right you will notice that the sound comes (its obvious) and notice that it is the sound of LIQUID.
and when you clap hands ,the particles in air doesn't create sound,the sound which it created during your clap is your two palms i.e they strike to each other.

So,we can conclude that in gas cylinder there is a liquid like any substance.
and when we open gas stove,the liquid particles move to the high pressure and then when they come in contact with air due to high pressure,they convert into GAS.

So,the question arises is that how can we put liquids in gas cylinder?
The answer is so simple,
We liquiefy gases into liquids.
But HOW IS THIS POSSIBLE?

we do this by applying:-
\[\large \bf HIGH~PRESSURE~so~VOLUME~DECREASES\]

we liquify gases,by applying HIGH PRESSURE,so in result VOLUME DECREASES.that's why the gases convert into liquid and we hear sound of any liquid substance.


But PV=nRT seems to be wrong,it is not wrong it is a equation of IDEAL GAS not REAL GAS and we always use REAL GAS.

Answer 7

hope you understand now very clearly. :) @Dexter810

Answer 8

Thanks a ton . You put a lot of effort in it !
Really , Thanks a lot ! :)

Answer 9

welcome :)..its my pleasure