<< Chapter < Page Chapter >> Page >
Determine the Value of an Equilibrium Constant by Complex Ion Formation by reacting Fe(III) with the thiocyanate ion

Determine the Value of an Equilibrium Constant by Complex Ion Formation

Objectives

In this laboratory you will:

  • Use MicroLab to take colorimetric measurements
  • Use Beer’s Law to measure the equilibrium concentration of a complex ion
  • Review Le Chatelier’s Principle
  • Calculate the equilibrium constant for the formation of a complex ion

 grading

Your grade will consist of the following:

  • Pre-lab (10%)
  • Correctness and thoroughness of your observations and the answers to the questions on the report form (80%)
  • TA evaluation of lab procedure (10%)

Before Coming to Lab . . .

  • Complete the pre-lab exercise
  • Read the introduction and any related materials provided to you

Introduction

When two reactants are mixed, the reaction typically does not go to completion. Rather, they will react to form products until a state is reached whereby the concentrations of the reactants and products remain constant at which point the rate of formation of the products is equal to the rate of formation of the reactants. The reactants and products are then in chemical equilibrium and will remain so until affected by some external force. The equilibrium constant K c size 12{K rSub { size 8{c} } } {} for the reaction relates the concentration of the reactants and products.

In our experiment we will study the equilibrium properties of the reaction between iron (III) ion and thiocyanate ion:

Fe 3 + ( aq ) + SCN ( aq ) FeSCN 2 + ( aq ) size 12{ ital "Fe" rSup { size 8{3+{}} } \( ital "aq" \) + ital "SCN" rSup { size 8{ - {}} } \( ital "aq" \) rightarrow ital "FeSCN" rSup { size 8{2+{}} } \( ital "aq" \) } {} Equation 1

When solutions containing Fe 3 + size 12{ ital "Fe" rSup { size 8{3+{}} } } {} ion and thiocyanate ion are mixed, the deep red thiocyanatoiron (III) ion ( FeSCN 2 + size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} ) is formed. As a result of the reaction, the starting concentrations of Fe 3 + size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and SCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} will decrease: so for every mole of FeSCN 2 + size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} that is formed, one mole of Fe 3 + size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and one mole of SCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} will react. The equilibrium constant expression K c size 12{K rSub { size 8{c} } } {} , according to the Law of Chemical Equilibrium, for this reaction is formulated as follows:

[ FeSCN 2 + ] / [ Fe 3 + ] [ SCN ] = K c size 12{ \[ ital "FeSCN" rSup { size 8{2+{}} } \] / \[ ital "Fe" rSup { size 8{3+{}} } \]\[ ital "SCN" rSup { size 8{ - {}} } \] =K rSub { size 8{c} } } {} Equation 2

Remember, square brackets ([]) are used to indicate concentration in mol/liter, i.e. , molarity (M).

The value of K c size 12{K rSub { size 8{c} } } {} is constant at a given temperature. This means that mixtures containing Fe 3 + size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and SCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} will react until the above equation is satisfied, so that the same value of the K c will be obtained no matter what initial amounts of Fe 3 + size 12{ ital "Fe" rSup { size 8{3+{}} } } {} and SCN size 12{ ital "SCN" rSup { size 8{ - {}} } } {} were used. Our purpose in this experiment will be to find K c size 12{K rSub { size 8{c} } } {} for this reaction for several mixtures made up in different ways, and to show that K c size 12{K rSub { size 8{c} } } {} indeed has the same value in each of the mixtures.

The reaction is a particularly good one to study because K c size 12{K rSub { size 8{c} } } {} is of a convenient magnitude and the red color of the FeSCN 2 + size 12{ ital "FeSCN" rSup { size 8{2+{}} } } {} ion makes for an easy analysis of the equilibrium mixture using a spectrophotometer. The amount of light absorbed by the red complex is measured at 447 nm, the wavelength at which the complex most strongly absorbs. The absorbance, A, of the complex is proportional to its concentration, M, and can be measured directly on the spectrophotometer:

A = kM Equation 3

We know it as the Beer-Lambert law which relates the amount of light being absorbed to the concentration of the substance absorbing the light and the pathlength through which the light passes:

Questions & Answers

do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
AMJAD
what is system testing
AMJAD
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Good
Berger describes sociologists as concerned with
Mueller Reply
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, General chemistry lab spring. OpenStax CNX. Apr 03, 2009 Download for free at http://cnx.org/content/col10506/1.56
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'General chemistry lab spring' conversation and receive update notifications?

Ask