NS FL #2 Biology Q50

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sdaswani
Posts: 37
Joined: Sat Jan 26, 2019 5:32 pm

NS FL #2 Biology Q50

Post by sdaswani » Wed Jan 30, 2019 7:35 am

Hi! I'm curious as to why adding water will change the ratio of products and reactants for this reaction at equilibrium. Since water is pure liquid, wouldn't we always include it as 1 in the Keq equation, implying that adding or removing water wouldn't change much?
I feel like I'm missing something here, would appreciate any clarification.
Thanks much!
NS_Tutor_Will
Posts: 766
Joined: Fri May 25, 2018 9:15 am

Re: NS FL #2 Biology Q50

Post by NS_Tutor_Will » Wed Jan 30, 2019 8:46 am

Remember, adding any product or reactant to an equilibrium mixture won't actually change the equilibrium ratio, just the current state of the system. If you add a product, you've tipped the equilibrium toward the product side. In response (and according to Le Chatelier's principle) the balance will shift toward the reactants in order to restore the equilibrium balance.

You are right that pure liquids and pure solids should not be included in equilibrium expressions. But, in this case for instance, when a liquid is part of the reaction (and is on one side or the other) the rules related to Le Chatelier's principle still hold. The equilibrium balance won't change in the long run, we've just temporarily altered it and the system reacts to restore the equilibrium ratios.

So your thinking is definitely close, it just requires that small update!
sdaswani
Posts: 37
Joined: Sat Jan 26, 2019 5:32 pm

Re: NS FL #2 Biology Q50

Post by sdaswani » Wed Jan 30, 2019 12:13 pm

Got it, so we're talking about a "temporary" change here. Adding products/reactants will affect the ratio momentarily, until equilibrium is established. Since we don't know the effect of heat, Temp will thus, not affect this momentary ratio in a predictable away. However, in the long term, increasing T could change Keq altogether, while adding products/reactants wouldn't affect that. Did I understand this correctly? Thanks!!
NS_Tutor_Will
Posts: 766
Joined: Fri May 25, 2018 9:15 am

Re: NS FL #2 Biology Q50

Post by NS_Tutor_Will » Wed Jan 30, 2019 1:54 pm

Yes: temperature could change the Keq depending on whether heat is a product or reactant! If heat were a "product" (exothermic), then increasing the temperature would favor the reactants. If heat were a "reactant" (endothermic), then increasing temperature would favor the products.

So temperature will change Keq predictably provided we know whether the reaction is endothermic or exothermic.
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