MCAT question: examkrackers 1001 chem LaChatelier's Principle

[anotheremail]

I know the rules for this but the examkrackers seem to have contradictory answers.

For question #289, the reaction is exothermic, but the answer is the reaction rate will increase. The explanation is that even if the reaction is exothermic, increasing temperature will increase the reaction rate.

Then on question #291, a similar question comes: The answer says "all reaction rates increase with temperature unless the reaction has heat as a product--in that case the reaction will go towards the left(reactants)"

Are the two problems contradicting each other? I think I am simply confused. If heat is in the product side it follows LaChateliers Principle, but if heat is not and the reaction is not exothermic, it doesn't follow LaChateliers?

Thanks.

edit: oops I did not submit this thread in the right subforum. Can a mod move it?(I dont know how to delete threads)

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[Rabolisk]

I don't have the EK book, but Le Chatelier's principle has to do with thermodynamics and equilibrium, which is separate from kinetics. Increasing temperature always increases the rate of a reaction, both forwards and backwards. The only time a temperature increase won't increase the rate is when a catalyst is destroyed or inhibited by high temperature (e.g. denaturation of enzymes under high temperature). The effect of temperature increase on equilibrium concentrations of products or reactants, or the yield can be deduced using Le Chatelier's principle. It can be advantageous to increase the rate of a reaction even if this would result in less yield at times.

 

[dougkaye]

The easiest way to think about heat and leChat's is just to include heat as a product or reactant in the reaction. If it's exothermic, put heat on reaction side and do your equilibrium analysis (the see-saw approach). If rxn is endothermic, put heat on product side and balance away. It gets a bit more complicated with pressure and volume changes which is what i have to review.

As for contradictory answers, i agree it's sort of confusing the way they say it. The whole temperature concept (as EK explains it) works much better in one-way reactions that only have one possible product (i.e. a simple gen-chem rxn versus an orgo rxn). In a one-way reaction, assuming (as you should) that heat is not deactivating an enzyme, etc., increasing temp will GENERALLY increase reaction rate. When it's a two-way reaction (one w/ a significant equilibrium) then you have to consider leChat's in determining the effect of temp changes. If passage says something about enzyme deactivation, etc. then obviously you can't ignore that information.

This "rule" that EK is trying to be so adamant about also won't apply to orgo rxns where changes in temp can make one rxn more or less favorable over another... so take EK's "always" with a grain of salt.

 

[ilovemcat]

The easiest way to think about heat and leChat's is just to include heat as a product or reactant in the reaction. If it's exothermic, put heat on reaction side and do your equilibrium analysis (the see-saw approach). If rxn is endothermic, put heat on product side and balance away. It gets a bit more complicated with pressure and volume changes which is what i have to review.

As for contradictory answers, i agree it's sort of confusing the way they say it. The whole temperature concept (as EK explains it) works much better in one-way reactions that only have one possible product (i.e. a simple gen-chem rxn versus an orgo rxn). In a one-way reaction, assuming (as you should) that heat is not deactivating an enzyme, etc., increasing temp will GENERALLY increase reaction rate. When it's a two-way reaction (one w/ a significant equilibrium) then you have to consider leChat's in determining the effect of temp changes. If passage says something about enzyme deactivation, etc. then obviously you can't ignore that information.

This "rule" that EK is trying to be so adamant about also won't apply to orgo rxns where changes in temp can make one rxn more or less favorable over another... so take EK's "always" with a grain of salt.

You have that backwards. An exothermic reaction would include heat as a product (ie. it releases heat during the conversion of reactants to products). An endothermic would require the input of heat for the reaction to proceed - hence it would be included on the reactant side.

Now if you "heat" something up - you have to ask yourself is the reaction Endothermic or Exothermic BEFORE figuring out how the reaction would proceed.

If the reaction is exothermic, you're adding (more) heat to the product side so the reaction would shift to the left (towards reactants). If the reaction is endothermic, you're adding (more) heat to the reactant side so to use up that excess heat -the reaction will proceed towards the products.

 

[BestDoctorEver]

You have that backwards. An exothermic reaction would include heat as a product (ie. it releases heat during the conversion of reactants to products). An endothermic would require the input of heat for the reaction to proceed - hence it would be included on the reactant side.

Now if you "heat" something up - you have to ask yourself is the reaction Endothermic or Exothermic BEFORE figuring out how the reaction would proceed.

If the reaction is exothermic, you're adding (more) heat to the product side so the reaction would shift to the left (towards reactants). If the reaction is endothermic, you're adding (more) heat to the reactant side so to use up that excess heat -the reaction will proceed towards the products.

This explanation is excellent...Le Chatelier's principle is a bit more complicated than I thought.

 

[NYR56]

As for contradictory answers, i agree it's sort of confusing the way they say it. The whole temperature concept (as EK explains it) works much better in one-way reactions that only have one possible product (i.e. a simple gen-chem rxn versus an orgo rxn). In a one-way reaction, assuming (as you should) that heat is not deactivating an enzyme, etc., increasing temp will GENERALLY increase reaction rate. When it's a two-way reaction (one w/ a significant equilibrium) then you have to consider leChat's in determining the effect of temp changes. If passage says something about enzyme deactivation, etc. then obviously you can't ignore that information.

This "rule" that EK is trying to be so adamant about also won't apply to orgo rxns where changes in temp can make one rxn more or less favorable over another... so take EK's "always" with a grain of salt.

To clarify a bit, as Rabolisk said, increased temperature will just about always increase the rate of reaction for the MCAT, regardless of whether it is endo or exothermic. LeChatlier's principle discusses where the equilibrium will lie, which has nothing to do with kinetics. It doesn't matter if the reaction is "one-way or two-way". The only time things may change if you have multiple competing reactions going on, which is not common for the MCAT.