   Chapter 15, Problem 31GQ

Chapter
Section
Textbook Problem

Suppose 0.086 mol of Br2 is placed in a 1.26-L flask and heated to 1756 K, a temperature at which the halogen dissociates to atoms.Br2(g) ⇄ 2 Br(g)If Br2 is 3.7% dissociated at this temperature, calculate Kc.

Interpretation Introduction

Interpretation: The equilibrium constant for the given reaction has to be determined.

Concept introduction:

• Equilibrium constant: At equilibrium the ratio of products to reactants has a constant value. And it is represented by the letter K.

For a general reaction, aA+bBcC+dD

The equilibrium constant Kc = [C]c[D]d[A]a[B]b, where a, b, c and d are the stoichiometric coefficients of reactant and product in the reaction. Concentration value for solid substance is 1.

If the value of Kc and the concentration of any of the reactant of a reaction is known then the concentration of product can be determined by multiplying Kc with the concentration of reactant.

• Concentration =AmountofsubstanceVolume
• Amountofsubstance=Concentration×Volume
• ICE (reaction initial concentration equilibrium) table is mainly used to calculate the value of K for a reaction. This table contains the concentration of reactant and product in various stage of reaction.
Explanation

The balanced chemical equation for the given reaction is.

Br2(g)2Br(g)

3.7%ofBr2 is dissociated in this reaction. The initial amount of Br2 placed in a 1.26L flask is 0.086mol

From the percentage of dissociation the change in concentration of Br2 at in equilibrium stage can be calculated as follows,

Amount of Br2 = 0.086mol

Volume = 1.26L

Change in concentration of Br2 (x) =0.086mol×3.7100=0.0032mol/L

By using this concentration ICE table for this reaction can be constructed as follows,

 Reaction Br2(g)             ⇄             2Br(g) Initial concentration(mol/L) 0.086 0 Change in concentration (mol/L) −0

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