La Chatelier's Princple - Heat

[Merissa87]

I am so confused with the addition/subtraction of heat in a reaction. How does this alter the equilibrium and which reaction will be favored? Forward or reverse? Also can someone please explain how endothermic and exothermic come into play??

Gracias

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[Revilla]

I am so confused with the addition/subtraction of heat in a reaction. How does this alter the equilibrium and which reaction will be favored? Forward or reverse? Also can someone please explain how endothermic and exothermic come into play??

Gracias

Here's the way I understand it. If heat is on the reactants' side, the equilibrium will shift to the products. If heat is on the products' side, the equilibrium will shift to the reactants. The way you judge this is by the temperature. If it's being increased, the reaction shifts to the reactants to compensate for it. If it's being decreased, the reaction shifts to the products.

As for exothermic and endothermic, I think (someone correct me if I'm wrong), if a reaction is endothermic, the reaction shifts to the reactants and if it's exothermic, it shifts to the products. I'm not positive on that though.

 

[bluemonkey]

Keep in mind that the equilibrium does not change. Rather, the system adjust in such a way to return to equilibrium.

So exothermic reactions release heat. You can think of heat as being a product of the reaction. Thus if you increase the temperature, you are adding "product" and the formation of reactants is favored. If, on the other hand, you lower the temperature, you are essentially removing "product" and the formation of products is favored.

Endothermic reactions are just the opposite. Here heat is behaving like a "reactant."

 

[Revilla]

Thanks for making that easier to understand, Bluemonkey. I knew there was something I wasn't quite grasping which made it difficult to explain.

 

[tncekm]

Here's the way I understand it. If heat is on the reactants' side, the equilibrium will shift to the products. If heat is on the products' side, the equilibrium will shift to the reactants. The way you judge this is by the temperature. If it's being increased, the reaction shifts to the reactants to compensate for it. If it's being decreased, the reaction shifts to the products.

As for exothermic and endothermic, I think (someone correct me if I'm wrong), if a reaction is endothermic, the reaction shifts to the reactants and if it's exothermic, it shifts to the products. I'm not positive on that though.

Yep, that's the gist of it. Exothermic reactions NEED to release heat. If the temperature differences are such that heat transfer cannot occur outward (remember, a temperature gradient is what allows "heat", or transfer of thermal energy so the temperature of the environment needs to be less than the system so heat will transfer out), then the reaction will not favor products. If the reaction is endothermic, it NEEDS heat input. If the temperatures are low, its very unlikely that enough heat will be transferred to the reactants to form products.

So:

Exothermic:
A + B --> C + Heat (&#916;H < 0)

Endothermic:
A + B + Heat --> C (&#916;H > 0)

Using the visual method of Le Chatlier's principle you can see that adding something to the right will drive the reaction left and adding something to the left will drive the reaction right. Or, removing something from the right will drive the reaction right, and removing something from the left will drive the reaction left.

edit: looks like people beat me to it

 

[Revilla]

LOL, I think everyone here welcomes as many explanations as we can get!

 

[Zerconia2921]

Think of like this:

if heat is on the product side its an exothermic rxn. for example

A + B ---> C +{heat}

now when we add heat to the rxn

reaction builds up too much heat must go to left side to relax.