   Chapter 15, Problem 44GQ

Chapter
Section
Textbook Problem

Ammonium hydrogen sulfide decomposes on heating.NH4HS(s) ⇄ NH3(g) + H2S(g)If Kp for this reaction is 0.11 at 25 °C (when the partial pressures are measured in atmospheres), what is the total pressure in the flask at equilibrium?

Interpretation Introduction

Interpretation:

The total pressure in the given reaction for the decomposition of ammonium hydrogen sulphide has to be given if Kp of the reaction is 0.11.

Concept Introduction:

Equilibrium constant in terms of pressure[Kp]: Equilibrium constant can be expressed in terms of partial pressures in atmospheres.

aA(g)+bB(g)cC(g)+dD(g)KP=PCc×PDdPAa×PBb

The activity of solid substance will not appear in equilibrium and numerically it is considered as one.

Explanation

Given:

NH4HS(s)NH3(g)+H2S(g)KP=0.11

In the given reaction ammonium hydrogen sulphide is in solid state and thus its pressure can be taken as unity. There is no role of ammonium hydrogen sulphide in the equilibrium expression.

Initially only NH4HS is present and the amount of NH3andH2S will be zero.

NH4HS(s)NH3(g)+H2S(g)Initialconcentrationa00IfxmolesofNH4HSdissociatestoformxmolesofNH3andxmolesofH2S,thenEquilibriumconcentrationa-xxx

From this it is clear that, amountofNH3=amountofH2S

Therefore total pressure will be equally distributed among NH3 and H2S

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