   Chapter 15, Problem 45GQ

Chapter
Section
Textbook Problem

Ammonium iodide dissociates reversibly to ammonia and hydrogen iodide if the salt is heated to a sufficiently high temperature.NH4I(s) ⇄ NH3(g) + HI(g)Some ammonium iodide is placed in a flask, which is then heated to 400 °C. If the total pressure in the flask when equilibrium has been achieved is 705 mm Hg, what is the value of Kp (when partial pressures are in atmospheres)?

Interpretation Introduction

Interpretation:

The Kp for the reaction when the dissociation of ammonium iodide into ammonia and hydrogen iodide has to be calculated.

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:

NH4I(s)NH3(g)+HI(g)Totalpressure=705mmHg=0.927atm=0.93atm

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

Initially only NH4I is present and the amount of NH3andHI will be zero.

NH4I(s)NH3(g)+HI(g)Initialconcentrationa00IfxmolesofNH4Idissociates

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