Iron(II) chloride (0.025 mol) is added to 1.00 L of 0.500 M NaCN. What is the concentration of Fe 2+ ions at equilibrium? K r for [Fe(CN) 6 ] 4− is 1.0 × 10 35 . (a) 1.0 × 10 35 M (b) 1.0 × 10 −36 M (c) 5.2 × 10 −38 M

Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640

Chapter
Section

Chemistry & Chemical Reactivity

9th Edition
John C. Kotz + 3 others
Publisher: Cengage Learning
ISBN: 9781133949640
Chapter 17.6, Problem 1RC
Textbook Problem
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Iron(II) chloride (0.025 mol) is added to 1.00 L of 0.500 M NaCN. What is the concentration of Fe2+ ions at equilibrium? Kr for [Fe(CN)6]4− is 1.0 × 1035. (a) 1.0 × 1035M (b) 1.0 × 10−36 M (c) 5.2 × 10−38M

Interpretation Introduction

Interpretation:

The concentration of Fe2+ ions present at the equilibrium is to be calculated.

Concept introduction:

Some metal ions when present in an aqueous solution containing anions or neutral species called Lewis base or ligands having a tendency to donate electron pairs to metal ions then complex ion formation will take place.

Example of metal ions that form complex ions includes, Cd2+ , Fe2+ , Zn2+ , Ni2+ etc.

Example of Lewis bases includes, NH3 , OH etc.

The complex ion remains in equilibrium with the metal ion and the ligand called complex ion formation equilibrium and the equilibrium constant is called as formation constant Kf .

A larger value of Kf implies that the complex ion formed is more stable. Kf is the measure of the strength of the interaction between the metal ions and the Lewis base to form the complex ion.

For example for general complex ion formation reaction,

xM+yL[MxLy]

Kf can be given as

Kf=[MxLy][M]x[L]y

Here,

• [MxLy] is the equilibrium concentration of complex ion.
• [M] is the equilibrium concentration of metal ion.
• [L] is the equilibrium concentration of the ligand.
• x and y are the coefficients of metal ion and ligand respectively.

Complex ions are stable and thus formation of these increase the solubility of the salt containing the metal ions same as in complex ions. Effect of complex ion formation on solubility of salt can be explained as below,

AgBr when dissolved in water does not dissolve completely and dissociates as follows,

AgBr(s)KspAg+(aq)+Br(aq) (1)

For this reaction Ksp expression is,

Ksp=[Ag+][Br]

Ag+ ions are capable of forming complex with ligand so when aqueous ammonia solution (strong ligand) is added to the saturated solution of AgBr , Ag+ ions present in the solution form complex with NH3 .

Ag+(aq)+2NH3(aq)Kf[Ag(NH3)2]+(aq) (2)

The expression for formation constant Kf is given as,

Kf=[[Ag(NH3)2]+][Ag+][NH3]2

Complex formation leads to the decrease in the concentration of the Ag+ ions in the solution, as a result, according to Le Chatelier’s principle the equilibrium in equation (1) move in the forward direction producing more of the Ag+ ions and the solubility of the slightly soluble salt AgBr increases.

Adding two equilibrium equation (1) and (2) new overall equilibrium constant can be defined.

Net chemical equation:

Ag+(aq)+2NH3(aq)Knet[Ag(NH3)2]+(aq)+Br(aq) (3)

Net equilibrium constant can be given as,

Knet=(Ksp)(Kf) (4)

Explanation of Solution

The concentration of Ag+ ions present at the equilibrium is calculated below.

Given:

The initial concentration of NaCN is 0.500 M .

The concentration of Iron(II)chloride solution is 0.0025M .

The value of formation constant, Kf , for [Fe(CN)6]4 is 1.0×1035 .

The high value of formation constant for the complex ion [Fe(CN)6]4 implies that the complex formed is very stable. So all the Fe2+ ion from NaCN will react with Iron(II)chloride such the initial concentration of the complex will be equal to the initial concentration of. complex ion then dissociates to give Fe2+ ion and two Iron(II)chloride molecules in solution.

The complex will dissociate as follows,

[Fe(CN)6]4(aq)Fe2+(aq)+6CN-(aq)

The value of dissociation constant, Kd for [Fe(CN)6]4 is given by equation (1).

Kd=1Kf

Substitute 1.0×1035 for Kf .

Kd=11.0×1035=1.0×1035

The concentration of FeCl2 is 0.0025M .

The concentration of FeCl2 is 0.0050M .

The concentration of CN is calculated as below,

[CN-]=orignal concentration of NaCN(2×0.500)=10.50=0.485 M

The ICE table for the dissociation of [Ag(NH3)2]+ is as follows,

Equation[Fe(CN)6]4Fe2++6CN-Initial(M)0.002500.485Change(M)x+x+2xEquilibrium(M)0

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