HW 1 #10

[mya warner 2C]

Calculate the equilibrium concentrations of N2O4 and NO2 after the extra 1.00 mol NO2 is added to 1.00 L of solution.

For this last portion of number 10 can someone explain why the change portion of the ICE box would be +X for N2O4 and -2X for NO2? (specifically the positive and negative aspects because I thought that products have a positive X and reactants have a negative.)

[Hannah Choi 1K]

In this case, NO2 has been added to the compound that had been at equilibrium, and according to Le Chatelier's principle, the reaction will try to minimize the effects of the addition (try to get back to equilibrium). So now that there is more NO2 than N2O4, you have to consider the reverse reaction of the given chemical equation. This basically means that NO2 will become the reactant R, and N2O4 will become the product P in your Kc calculation. Hope this helps!

[Nigah Fatima]

You are correct, that it is more common for the products to have +x and the reactants to have -x. However, since an additional 1.00 mol NO2, the product, is added to 1.00 L of the solution, this disrupts the equilibrium. LeChatelier's principle states that when a system in dynamic equilibrium is disturbed, the equilibrium will shift to minimize the effects of the disturbance. In this case, the addition of the product, NO2, will cause the equilibrium to shift towards the reactant to reestablish dynamic equilibrium. As previously determined, the reaction will shift to form more of the reactant N2O4 with the addition of NO2. That is why the +x is in the reactant side, while the -x is in the product side for this question because the reaction is going in the reverse direction due to the addition of NO2 (product). Hope this helps!

[Nithya Narapa Reddy]

For this specific case since the concentration of the product 2No2 was increased the equilibrium shifts to mitigate the effects of the change which is why the ice table has a -2x on the products side.

[Hanyi Jia 3B]

Before the 1 mol of NO2 is added, the reaction is at equilibrium.
As the NO2 is added, the product (right side) of this equilibrium increase in concentration. Now, the equilibrium quotient (Q) is larger than the equilibrium constant, as Q=[NO2]_f^2/[N2O4], K_c=[NO2]_i^2/[N2O4], and [NO2]_f > [NO2]_i.
Therefore, the reaction shift left, which means the reverse reaction rate increases, and [product] decreases, while [reactant] increases. This is why for Change in the ICE table, product has -2x, while reactant has +x.

Hope this helps.