## Calculating Equilibrium constant

$\Delta G^{\circ}= \Delta H^{\circ} - T \Delta S^{\circ}$

$\Delta G^{\circ}= -RT\ln K$

$\Delta G^{\circ}= \sum \Delta G_{f}^{\circ}(products) - \sum \Delta G_{f}^{\circ}(reactants)$

Kira_Maszewski_1B
Posts: 17
Joined: Wed Sep 21, 2016 2:57 pm

### Calculating Equilibrium constant

The standard reaction free energy for H2(g)+I2(g) <-->2HI(g) is DeltaG = -21.1 kj at 500K. In a sealed reaction container, the equilibrium partial pressures of the gases are PH2= 1.2 bar, PI2 = 1.5 bar, and PHI is unknown.

a) Calculate the value of the equilibrium constant for this reaction.
b) Calculate the equilibrium partial pressure pressure of HI(g).

Please help, I'm all together confused how to do this problem.

Cherry_Deng_1K
Posts: 12
Joined: Wed Sep 21, 2016 2:55 pm

### Re: Calculating Equilibrium constant

Hello,

For part (a), I would start by using the equation ΔG°= -RT ln K. Since the standard reaction free energy is already given, you can just directly substitute ΔG°, R, and T to find the equilibrium constant, K. Since ΔG° is given in kJ and the unit for R is J/(K mol), don't forget to convert kJ to J to make sure the calculations are consistent! For part (b), since you have just calculated the equilibrium constant K and you are given the equilibrium partial pressures for H2 and I2, you can directly substitute it into the general equation K = P(HI)^2 / [P(I2) x P(H2)] (you learned this in Chem 14A). If you do your calculations correctly, you should be able to find the partial pressure of HI. Hope this helps!

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