[djdato]

Hey guys! We just covered Le Chatelier at school and I get the basic principles. I have a problem, though, with this :

Fizzy drinks contain carbon dioxide gas in equilibrium with dissolvedcarbon dioxide:
CO₂(g) <-----> CO₂(aq) (this is an exothermic reaction)

Please explain to me
a) why bubbles appear when the bottle is opened (using Le Chatelier)
b) and why drinks are usually cooled before they are carbonated?

thnx! 

[xiankai]

(a) first, le chatelier's principle allows for changes in pressure, amount/concentration of reactants/products, temperature to be reduced.

the equation has the same amount of moles on both sides, so the only way the system can be upset is pressure or temperature. what do you think happens such that bubbles form (CO_g is favoured)

(b) cooling them means equilibrium position is shifted to side with less energetic reagents - which side do you think it is?

[djdato]

ok...so for
a) for bubbles to form...there must be more products added...which must come from natural sources in the air. is this right?

for b) since it is exothermic equilibrium postion will be shifted right. This would keep the CO₂ from escaping?

[xiankai]

(a) products = CO₂(aq)

how can you get it from the air?
the only option left is temperature or pressure - which of the two is the factor that upsets equilibrium?

(b) that's right.

[djdato]

pressure will upset the equilibrium from opening the bottle. Opening the bottle will decrease pressure? which will cause a net backward reaction...is this rite?

: CO2 gas released into the air will change the pressure of equilibrium.? therefore, equilibrium will balance itself by making dissolved CO2 react to make more CO2 gas?

but if this is rite..(is it ? )  wouldn't that mean that the equation will never reach equilibrium if the bottle is left open?

[xiankai]

right again.

CO₂ gas released into the air only affects the pressure by a small amount, remember we are dealing with the pressure of air as a whole (the whole wide world, so as to speak!) thus the equation will eventually reach equilibrium.

note that if bottle is closed, CO₂ gas released will increase pressure, and the increased pressure will cause equilibrium to balance itself by dissolving CO₂ gas again, thus it is a dynamic equilibrium.