Textbook Focus 5J #13

[Sarah Wang 1I]

Hi! In the textbook, problem 13 in Focus 5J says:

A gaseous mixture consisting of 2.23 mmol N2 and 6.69 mmol H2 in a 500. mL container was heated to 600. K and allowed to reach equilibrium. Will more ammonia be formed if that equilibrium mixture is then heated to 700. K? For N2(g) + 3H2(g) <--> 2NH3(g), K=1.7x10^-3 at 600. K and 7.8x10^-5 at 700. K.

At first, I assumed more ammonia would be formed since the reaction is heated, and therefore endothermic which would shift the reaction towards the products. However, I also noticed that the K at 700 K was smaller than the K at 600 K, which would indicate a shift to the reactants. Why do they contradict each other, and which direction does the reaction actually shift?

[AudreyQian1J]

Hello,
N2(g) + 3H2(g) <--> 2NH3(g) is an exothermic reaction, because it is a combination reaction that breaks the bonds of the diatomic gases and forms new bonds to make ammonia, ultimately resulting in a larger release of energy. Since this reaction is exothermic, adding more heat (from 600 K to 700 K) would make the reaction favor the reactants and go in the reverse direction. This concept coincides with why the Ka value at 700 K is less than that at 600 K.

I hope this helps.

[Ethan Mai 1D]

Hello! I think for this problem it would also be possible to look at just the K value alone. If the value of K is lower at a higher temperature, then raising the temperature would shift equilibrium towards the reactants. Also, endothermic is not necessarily used to describe the increase or decrease of heat, but more so whether a reaction tends to absorb to increase heat. Yes, heat is being added to the system, which means heat is being absorbed. However, that does not necessarily mean heat is being absorbed in the forward reaction. I hope this helps!