Le Chateliers's principle question

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Normally Im very good at these...but I got this Q wrong and I don't understand why.

N204 (g) ----> <---- 2NO2 (g) delta H=58.0 kJ

Which will favor the formation of N2O4.

Answer: Decrease the temperature.


I eliminated that choice because I figured lowering the temp would push the RXN to the right in effort to increase the amount of heat given off? So that it would still be 58.0 kJ

Why would it go to the left?

Thanks.

 

[grburst]

Normally Im very good at these...but I got this Q wrong and I don't understand why.

N204 (g) ----> <---- 2NO2 (g) delta H=58.0 kJ

Which will favor the formation of N2O4.

Answer: Decrease the temperature.


I eliminated that choice because I figured lowering the temp would push the RXN to the right in effort to increase the amount of heat given off? So that it would still be 58.0 kJ

Why would it go to the left?

Thanks.

The forward reaction is endothermic. Therefore, the reverse reaction is exothermic (&#916;H of forward = -&#916;H of reverse).

Adding heat to an endothermic reaction (the forward reaction) favors the formation of its products (NO2). Subtracting heat from an exothermic reaction (the reverse reaction) favors the formation of its products (namely, N2O4).

 

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The forward reaction is endothermic. Therefore, the reverse reaction is exothermic (&#916;H of forward = -&#916;H of reverse).

Adding heat to an endothermic reaction (the forward reaction) favors the formation of its products (NO2). Subtracting heat from an exothermic reaction (the reverse reaction) favors the formation of its products (namely, N2O4).

Thanks so much for your help.

I thought the forward reaction was exothermic...got my signs backwards. Figured +H means producing heat. Think I confused it with work done on a system vs by a system.

-H = exo
+H = endo

 

[grburst]

Thanks so much for your help.

I thought the forward reaction was exothermic...got my signs backwards. Figured +H means producing heat. Think I confused it with work done on a system vs by a system.

-H = exo
+H = endo

Think of enthalpy as a measure of the internal energy of the system. The fact that &#916;H is positive means the products have a higher internal energy than the reactants. You somehow have to "bring up" the low-energy reactants up to the level of the high-energy products. You do this by adding heat. The more heat you add, the more low-energy products can "climb the internal energy hill" and become high-energy reactants.