Le Chatlier's Principle

[Sammy1024]

In an exothermic reaction, I understand that as the temperature goes down, the reaction shifts to the right.

I don't understand what it means for products and reactants.

The equation shifts to the right, and so the concentration of reactants is greater than the concentration of products at this point?

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[DrknoSDN]

Shifts right means that the reaction is pushed forwards so that reactants become products.

An exothermic reaction that is releasing heat to the surroundings will be producing more products and thus less reactants.
-That is easy to think about because a exothermic reaction that runs to completion (all products), releases heat to the surroundings.

 

[funtertaining15]

Shifts right means that the reaction is pushed forwards so that reactants become products.

An exothermic reaction that is releasing heat to the surroundings will be producing more products and thus less reactants.
-That is easy to think about because a exothermic reaction that runs to completion (all products), releases heat to the surroundings.

Right. So, if I had X +Y ->Z + heat
that is an exothermic reaction. assuming the rxn is in equil, if I decrease heat, then the rxn is not in equil. (K is not =1) Thus, I have to compensate for that change by restoring the heat. to do that would require me to increase whatever is on the side of where heat is, so if heat increases, Z, or the product increases....

 

[SweetBurger]

If you would like a good example I advise looking at the Haber process. It's really interesting and you can learn a lot about Le Chat's Principle from it.

 

[NextStepTutor_1]

Also as a note, remember that temperature changes the equilibrium constant as given by : dG = RTln(Keq). Thus, when they have problems with heat being added, or removed make sure that the overall temperature doesn't change significantly, because that would change the Keq constant. But if temperature doesn't vary, adding heat to an exothermic reaction will push the reaction to the reactants, and for an endothermic reaction towards the products. For removing heat, the opposite is true.

 

[Alienman52]

Right. So, if I had X +Y ->Z + heat
that is an exothermic reaction. assuming the rxn is in equil, if I decrease heat, then the rxn is not in equil. (K is not =1) Thus, I have to compensate for that change by restoring the heat. to do that would require me to increase whatever is on the side of where heat is, so if heat increases, Z, or the product increases....

This is correct, just think of heat as a reactant/product within the reaction.

 

[edgerock24]

This is correct, just think of heat as a reactant/product within the reaction.

Exactly. Just think of heat being a product in the reaction for exothermic rxn's (and apply Le Chatlier's Princ. accordingly). And vice versa for endothermic reactions (heat being on the reactant side).

 

[Czarcasm]

In an exothermic reaction, I understand that as the temperature goes down, the reaction shifts to the right.

I don't understand what it means for products and reactants.

The equation shifts to the right, and so the concentration of reactants is greater than the concentration of products at this point?

Think of it this way. If they tell you it's an exothermic reaction, think of "heat" as a product (because in the progress of reaction, heat is released). If they tell you it's an endothermic reaction, think of "heat" as a reactant (because in the progress of reaction, heat is consumed).

Now consider what happens when we change the temperature. For starters, Keq will change (as T is the only thing that can alter the equilibrium constant). But LeChat's Principle tells us which way the reaction will shift to establish equilibrium. Think of temperature as "heat." If we reduce the temperature for an exothermic reaction, what will happen? Do we have fewer reactants or products? In this case, because it's an exothermic reaction, reducing temperature = less products. What does LeChat's Principle tell us then? Well, we need to make up for that lost heat, so the reaction will shift to products to compensate for the heat lost. Hope this makes sense.