Chemistry Thermodynamics Math Question 4

Question

Calculate the equilibrium constant for the steam reforming reaction of methane, CH₄(g) + H₂O(g) ↔ CO(g) + 3 H₂(g) at 1120.0 K. Assume that ΔH and ΔS are independent of temperature. 

Answer 1

Zoey,

Doing this problem (and going into your exam), you should have at least four equations at your fingertips (memorized):

1) The Equilibrium Constant is related to the Standard Gibbs Energy of Reaction:

ΔG^ο_rxn = -RT ln(K_eq)

2) The Standard Enthalpy of Reaction and the Standard Entropy of Reaction are related to the Standard Gibbs Energy of Reaction:

ΔG^ο_rxn = ΔH^ο_rxn - T ΔS^ο_rxn

3) The Standard Enthalpy of Reaction can be calculated from the Standard Enthalpies of Formation of the products and the reactants thinking that the Standard Enthalpy of Reaction is the heat absorbed during a reaction going from reactants to products:

Reactants + Heat Absorbed <==> Products

or Heat Absorbed = Heats of Formation of Products - Heats of Formation of Reactants

You can and should look up the Heats of Formation for the products and the reactants (from the elements in their standard states) in your reaction in a table online or in your book. You won't find the Heats of Formation for Elements (from the elements in their standard states) in the tables because they are zero. (There is nothing to change!)

4) The Standard Entropy of Reaction can be calculated from the Standard Entropies of Formation of the products and the reactants thinking that the Standard Entropy of Reaction is the entropy absorbed during a reaction going from reactants to products:

Reactants + Entropy Absorbed <==> Products

or

Entropy Absorbed = Entropies of Formation of Products - Entropies of Formation of Reactants

You can and should look up the Entropies of Formation for the products and the reactants (from the elements in their standard states) in your reaction in a table online or in your book. You won't find the Enthalpies of Formation for Elements (from the elements in their standard states) in the tables because they are zero. (There is nothing to change!)

Calculate the Standard Entropy of Reaction, the Standard Enthalpy of Reaction, the Standard Gibbs Energy of Reaction, and the Equilibrium Constant in this order, and you have your answer.

Michael.