Questions

[minijumbuk]

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Originally Posted by JasonNg1025

What I'm saying is that before Le Chatelier's principle comes into play, the reactant concentration is already higher than normal. This is the reason for Le Chatelier's principle - to shift equilibrium to counteract some external change. It does not change it from the initial ratio, rather, it reverts it back to the initial ratio which was changed as a result of the pressure change.

What do you mean by "normal"?

And can you explain why it doesn't change the initial ratio? Haven't my previous post showed that there is a change from the initial ratio?

Sorry if I'm not following what you are trying to communicate, but if I interpreted it correctly - that you want to say that pressure doesn't change the ratio of products - then what is the point of adding pressure in equilibrium reactions?

Also, if pressure is completely out of question as a factor, then can you please explain what a 'Pressure-dependent equilibrium constant' is?

[JasonNg1025]

Never heard of it sorry :O

Normal is probably initial... although the problem does arise - what's the point? I will probably think about it further. Back to my first post, this is my hypothesis lol

Hmmmm.... looks like you've shaken my theory...

[minijumbuk]

Haha. The pressure-dependent equilibrium constant isn't even in the syllabus. The K we learn in HSC is the temperature dependent equilibrium constant, so we don't actually need to kill brain cells like this at all =P But just for interest xD

[JasonNg1025]

It's true, jumbuck. This is quite pointless, but it's better than remembering "temperature, temperature temperature..." cause in the HSC we'll suddenly be like "TEMPERATURE OR PRESSURE SOMEONE TELL ME"

Ok, answering my previous post - when pressure is increased, both concentrations increase, but the volume doesn't. But the reactant concentration is way too high so Le Chandalier balances it out, increasing the yield of the product. Notice the volume did not increase before, only concentration. But the point in adding pressure is so that the yield, or volume, increases.

Cheers and hope it makes sense

[minijumbuk]

Originally Posted by JasonNg1025

It's true, jumbuck. This is quite pointless, but it's better than remembering "temperature, temperature temperature..." cause in the HSC we'll suddenly be like "TEMPERATURE OR PRESSURE SOMEONE TELL ME"

Ok, answering my previous post - when pressure is increased, both concentrations increase, but the volume doesn't. But the reactant concentration is way too high so Le Chandalier balances it out, increasing the yield of the product. Notice the volume did not increase before, only concentration. But the point in adding pressure is so that the yield, or volume, increases.

Cheers and hope it makes sense

I don't understand why you correlated the volume with the equilibrium reaction. Volume isn't a direct factor - it's pressure. Volume only contributes to the shifting of equilibrium because it adds/takes away pressure.

So I don't really understand the post xD

Maybe we should just stick to "temperature, temperature, temperature" haha.

[JasonNg1025]

Well, I may not have been concentrating properly... not necessarily volume, but the yield....
Or you could do my other method :P

[Trebla]

Just for your interest, the reason that K changes with temperature only (at least for ideal gases in this case) is because of this equation: (known as van't Hoff's differential equation)

∂ln K / ∂T = ΔH / RT²

This basically means that the derivative of ln K with respect to temperature is dependent on enthalpy (whether reaction is endothermic or exothermic) and temperature. R is just a constant.

So for example, if the forward reaction is endothermic then ΔH > 0, so the derivative is positive. (remember that dy/dx > 0 means y is increasing whenever x is increasing)

So this, means that ln K is increasing as T (temperature) is increasing i.e. K is increasing with higher temperature; which makes sense because when you increase temperature, the reaction favours the forward direction (endothermic) more and you get an increase in products which results in an increase in K.

That's a taste of university chemistry for you....