A mixture of H2, S, and H2S is held in a 1.0-L vessel at 90°C and reacts according to the equation
H2(g) + S(s) H2S(g).
At equilibrium the mixture contains 0.46 g of H2S and 0.40 g H2.
(a) Write the equilibrium-constant expression for this reaction (Kc).
I got Kc= [H2S]/[H2]
but the online homework program says its wrong
the reaction is
H2(g) + S(s) <--> H2S(g).
forgot the arrow
Thanks for the arrow. Instead of going through the problem I'm going to take a guess as to what you did wrong. I'll bet you didn't convert g H2S and g H2 to mole/1.0L and substitute those numbers.
The equilibrium constant expression for a chemical reaction is determined by taking the ratio of the products' concentrations to the reactants' concentrations, each raised to their respective stoichiometric coefficient.
For the reaction H2(g) + S(s) ↔ H2S(g), the stoichiometric coefficients are 1 for H2S, 1 for H2, and 1 for S.
Thus, the correct equilibrium constant expression (Kc) would be:
Kc = [H2S]/([H2][S])
Since the reactant sulfur (S) is in the solid state and cannot be included in the equilibrium expression, the expression simplifies to:
Kc = [H2S]/[H2]
To write the equilibrium-constant expression (Kc) for this reaction, you need to consider the stoichiometry of the balanced equation. According to the given equation:
H2(g) + S(s) → H2S(g)
The coefficients in the balanced equation represent the moles of each substance. The equilibrium constant expression for this reaction can be written as:
Kc = [H2S] / ([H2] * [S])
In this expression, [H2S] represents the concentration of H2S at equilibrium, [H2] represents the concentration of H2 at equilibrium, and [S] represents the concentration of S at equilibrium.
However, in this case, the reaction is taking place in a closed 1.0-L vessel, not in a solution with concentrations. Therefore, we need to use the ideal gas law to convert the quantities given (masses) to concentrations (molarities).
To find the molarities of H2S and H2, you need to convert the given masses to moles. The molar mass of H2S is 34.08 g/mol, and the molar mass of H2 is 2.02 g/mol.
For H2S:
moles H2S = mass H2S / molar mass H2S = 0.46 g / 34.08 g/mol
For H2:
moles H2 = mass H2 / molar mass H2 = 0.40 g / 2.02 g/mol
Now, using these calculated moles, you can find the molarities by dividing the moles by the volume of the vessel (1.0 L):
[H2S] = moles H2S / volume = (0.46 g / 34.08 g/mol) / 1.0 L
[H2] = moles H2 / volume = (0.40 g / 2.02 g/mol) / 1.0 L
Now you can substitute these values into the equilibrium-constant expression:
Kc = [H2S] / ([H2] * [S])
Note that the concentration of S is not provided, so you'll need to use an ICE (Initial, Change, Equilibrium) table or use the stoichiometry of the balanced equation to determine the concentration of S at equilibrium. Once you have the value, you can substitute it into the equation to find the correct Kc value.