For the reaction 2HI(g) H2(g) + I2(g), Kc = 0.290 at 400 K. If the initial conce

Question

For the reaction 2HI(g) H2(g) + I2(g), Kc = 0.290 at 400 K. If the initial concentrations of HI, H2, and I2 are all 1.50 103 M at 400 K, which one of the following statements is correct?
Answer
A.
The concentrations of HI and I2 will increase as the system is approaching equilibrium.
B.
The concentration of HI will increase as the system is approaching equilibrium.
C.
The concentrations of H2 and HI will decrease as the system is approaching equilibrium.
D.
The concentrations of H2 and I2 will increase as the system is approaching equilibrium.
E.
The system is at equilibrium.

Explanation / Answer

For the reaction 2HI(g) H2(g) + I2(g), Kc = 0.290 at 400 K. If the initial concentrations of HI, H2, and I2 are all 1.50 * 10^-3 M at 400 K, which one of the following statements is correct?

First, define Q

Q = [H2][I2] /([HI]^2)

Q = 1

Note that if Q = 1, and K = 0.29

this implies Q > K

then

If Q > Keq; this has much more products than reactants, therefore expect products to form more reactants in order to achieve equilibrium.

Then, the products will start decreasing in order to form more reactatns, HI, until Q = Kc

A.
The concentrations of HI and I2 will increase as the system is approaching equilibrium. FALSE, I2 decreases, HI incrases
B.
The concentration of HI will increase as the system is approaching equilibrium. TRUE, it will increase
C.
The concentrations of H2 and HI will decrease as the system is approaching equilibrium. FALSE, H2 decreases, HI increases
D.
The concentrations of H2 and I2 will increase as the system is approaching equilibrium. FALSE, they must decrease
E.
The system is at equilibrium. False, since Q = K is not true

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