Question

± Relating Different Forms of the Equilibrium Constant

For chemical reactions where all reactants and products are in the gas phase the amount of each gas in the vessel can be expressed either as partial pressures or as concentrations. As such the equilibrium constant for a gas phase reaction can also be expressed in terms of concentrations or pressures. For the general reaction,

aA(g)+bB(g)⇌cC(g)+dD(g)

Kp=(PC)c(PD)d(PA)a(PB)b and Kc=[C]c[D]d[A]a[B]b.

It is possible to interconvert between Kp and Kc using

Kp=Kc(RT)Δn

where R=0.08314 L bar mol−1 K−1 and Δn is the difference in stoichiometric coefficients between gaseous products and gaseous reactants.

If we assume that the gaseous reactant and products are “ideal” and the partial pressures of the reactants and products were expressed in bar, then the thermodynamic equilibrium constant, K is the magnitude (or numerical value) of Kp.

Part A

For the reaction

2CH4(g)⇌C2H2(g)+3H2(g)

Kc=0.160 mol2 L−2 at 1621 ∘C. What is Kp for the reaction at this temperature?

Enter your answer numerically.

Part B

For the reaction

N2(g)+3H2(g)⇌2NH3(g)

Kp=3.80×10−3 bar−2 at 265 ∘C. What is Kc for the reaction at this temperature?

Enter your answer numerically.