chemistry

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2 NO2(g) N2(g) + 2 O2(g)

The ΔH° for the reaction above is -66.4 kJ. The system is initially at equilibrium. What happens if N2 is added to the reaction mixture at constant temperature and volume?
The reaction absorbs energy.
The reaction releases energy.
[NO2] increases.
[NO2] decreases.
[NO2] remains constant.
[N2] increases.
[N2] decreases.
[N2] remains constant.
[O2] increases.
[O2] decreases.
[O2] remains constant.

I am so confused about how to do this. I put releases heat, NO2 increases, N2 increases, and O2 decreases.

• chemistry -

What happened to the arrow. How do we know the reactants from the products. If you mean this,
2 NO2(g)==> N2(g) + 2 O2(g) + heat

Adding N2 shifts the equilibrium to the left.
(NO2) increaes.
(O2) decreases.
heat is absorbed.
(N2) will be larger.

• chemistry -

Would you spend a few minutes telling me what you don't understand about Le Chatelier's Principle? This is such a simple concept but students get confused. Perhaps if you tell me what is so daunting, I can understand and pitch my answers to help you understand. Just giving the answers isn't helping.

• chemistry -

I don't understand how you decide what is increasing or decreasing and how you tell.

• chemistry -

Le Chatelier's Principle is simple. It says this. When a system is at equilibrium and we do something to change it (adding N2 in this case), the reaction will shift in such a way so as to undo the change. SO, adding N2 means the reaction will move to undo the addition which means the reaction will move to the left. If it moves to the left, everything on the product side gets smaller (except for N2 which the problem states is increased) and everything on the reactant side gets larger. If heat is added the reaction tries to undo the added heat so it moves to the left. If NO2 is added the reaction moves to the right so as to use up NO2. Does that help?

• chemistry -

yes, it really does help. How did you decided that it absorbed heat? I though exothermic reactions released it.

• chemistry -

You are right. Exothermic does mean it gives off heat. The reaction is exothermic so it gets hotter. BUT, if we ADD heat to a system in equilibrium, the reaction must undo the added heat; therefore, the reaction moves to the left so as to use up the heat. (Moving to the right means it gets hotter; moving to the left means we use up some of the heat we added.)

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