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# Le Chatelier's principle....? Watch

1. Hey guys so I've got a question here which reads

" A and B react to form C and release energy according to the following equation

A(g) + B(g) = 3 C(g)

Using Le Chatelier's principle, state and explain the effect on the position of the equilibrium when:

a) the pressure is increased

b) the temperature is increased"

Now I'm not too sure with these but I've put

a) when the pressure is increased the position of equilibrium would move to the left because there are more molecules on the right hand side of the equation that there are on the left. The position of equilibrium is moving in order to decrease the pressure that has been increased, this is achieved by favouring the reaction which will produce the fewest molecules.

b) when the temperature is increased the position of equilibrium will move to the left in order to favour the reaction which absorbs heat. This movement of the position of equilibrium is an attempt to reverse the increase of heat that has been added to the reaction.

Do those answers sound ok? Or are they wrong? Any help greatly appreciated, I don't mind criticism. These questions were only worth 1 mark so not expecting me to write lots of information. Many thanks for your time.
2. (Original post by Spudhead)
Hey guys so I've got a question here which reads

" A and B react to form C and release energy according to the following equation

A(g) + B(g) = 3 C(g)

Using Le Chatelier's principle, state and explain the effect on the position of the equilibrium when:

a) the pressure is increased

b) the temperature is increased"

Now I'm not too sure with these but I've put

a) when the pressure is increased the position of equilibrium would move to the left because there are more molecules on the right hand side of the equation that there are on the left. The position of equilibrium is moving in order to decrease the pressure that has been increased, this is achieved by favouring the reaction which will produce the fewest molecules.

b) when the temperature is increased the position of equilibrium will move to the left in order to favour the reaction which absorbs heat. This movement of the position of equilibrium is an attempt to reverse the increase of heat that has been added to the reaction.

Do those answers sound ok? Or are they wrong? Any help greatly appreciated, I don't mind criticism. These questions were only worth 1 mark so not expecting me to write lots of information. Many thanks for your time.
You are correct but do not need to write that much, the exam usually gives 2-3 lines for this stuff so just shorten it to
"There are 2 moles on the left and 3 on the right, therefore equilibrium will shift left in order to decreased the pressure.

Temperature is worked out by finding which reaction is exothermic

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3. So for temp jut write
The forward reaction is exothermic/endothermic therefore equilibrium shifts to the left/right

Cross out options as appropriate

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4. Thank you very much!
5. (Original post by Abbieastoria)
So for temp jut write
The forward reaction is exothermic/endothermic therefore equilibrium shifts to the left/right

Cross out options as appropriate

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Maybe a daft question but how do I work out if it is exothermic or endo, I thought exo because it produced more moles?

"the forward reaction is exothermic therefore the equilibrium position shifts to the left"

thats right isn't it?
6. (Original post by Spudhead)
Maybe a daft question but how do I work out if it is exothermic or endo, I thought exo because it produced more moles?

"the forward reaction is exothermic therefore the equilibrium position shifts to the left"

thats right isn't it?
No it should give you a temperature reading for one of the reactions
like -90kjmol-1 or something

If the temperature is negative it is exothermic and if it is positive it is endothermic. Nothing to do with mile production. it is about energy transfer/release

Since there is no temperature on your question sheet ill give an example

S(g) + Y(l) <--> X (s) -120 kjmol-1

This is showing that the forwards reaction is exothermic. So if temperature is increased, the equilibrium will shift left to produce the endothermic products and cool the mixture down

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7. ill give you some tips

Pressure - all you need to say is the number of moles on the left and the number on the right, therefore equilibrium will shift left/right/remain unchanged

Temp- state that the forwards reaction is exothermic/endothermic - you absolutely HAVE to use these words this is what the mark scheme gives the mark for. therefore equilibrium shifts left/right

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8. Ok thanks for the tips, my question sheet definitely doesn't give a heat reading though just "A(g) + B(g) = C(g)"

so do I just put that an increase in this temperature would not effect the reaction because it is neither stated as endo or exothermic?
9. (Original post by Spudhead)
Ok thanks for the tips, my question sheet definitely doesn't give a heat reading though just "A(g) + B(g) = C(g)"

so do I just put that an increase in this temperature would not effect the reaction because it is neither stated as endo or exothermic?
Have you worked out a temperature in a question before that, it could be a multipart question

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10. I'm not sure if it's been mentioned above or not but you need to state that there are more moles of GAS on one side, the gaseous part is very important, because the pressure contribution from solids and liquid is negligible.

edit: Also when told that the temperature is increased, but given no enthalpy of reaction, you can say something like "the rate of both the forward and backward reactions will increase, but the position of equilibrium will not change"

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Updated: April 19, 2013
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