Differences between physisorption and chemisorption

Use of colorimeter for the determination of the concentration of a solution

Absorbance is important in determining concentration of a substance in a sample through colorimeter analysis. Colorimeter measures the intensity of colour and light transmittance by the sample to achieve the concentration. When a beam of light passes through a coloured solution, the amount of light absorbed depends on the nature of the molecules absorbing the light, their concentration and thickness (path length) of the solution. The ratio of transmitted intensity to original intensity s known as the “transmittance”, T.

Transmittance (T) = I/I₀

I = intensity of the transmitted light

I₀ = intensity of incident light

The Beer- Lambert law states that there is a logarithmic dependence between the transmittance and the absorbance. Therefore the transmittance is expressed in terms of absorbance;

Absorbance (A) = -log₁₀ T

                             = -log₁₀ (I/I₀)

According to this, the absorbance becomes linear with concentration considering;

A = ℰ ℓ C

ℰ = Molar absorbance coefficient

ℓ = path length

C = concentration of the solution

Therefore in dilute solution,

A = -log₁₀ (I/I₀) = ℰ ℓ C

Molar absorbance coefficient indicates the absorbance under a standard set of conditions, i.e. the light travelling 1cm through a solution of 1moldm^-3. In a material with a low absorption coefficient, light is poorly absorbed and vise versa. This depends on the material and on the wave length of the light.

When using the colorimeter the path length i.e. the width of the glass cell is constant. Also the concentration of one solution used at one specific wave length. Therefore ℰ is also constant through the measurements. This shows out clear relationship between the absorbance and the concentration.

A ∝ C as ℓ and ℰ are constant

The glass cell/ container with plane parallel faces are transverse by monochromatic radiation in the colorimeter. If the glass cell is filled with non absorbing solution, there is 100% transmittance; therefore the absorption would be zero.

Colorimeter applies only in relation to the visible region. Also Beer- Lambert law is applicable for 0.800-0.200 absorbances.

In the experiment, firstly the absorbance reading of the colorimeter should be zeroed using distilled water as distilled water is used to prepare the solutions. 

Also before taking the measurements of the absorbance value in each solution, the glass cell should be washed with distilled water in order to prevent interferences to the reading. It is important not to touch the two smooth surfaces of the glass cell and wipe out the additional drops remain on the surfaces of the glass cell, using a tissue. Otherwise the beam of the radiation would be scattered incorrectly and interfere the accuracy of the reading.

When filling the cell, air bubbles should not be remained inside the cell as it would decrease the absorbance value.

When refilling a glass cell with a different solution, small amount of the new solution should be used to rinse the cell before filing as it would give more accurate results.

Spectrophotometer also uses a monochromatic light to pass though a solution and measure its absorbance. The principle of spectrophotometer and colorimeter is same but a colorimeter can only use one wavelength at a time and have a fixed number of wavelengths that can be used. Also they have to be in visible range only.

A spectrophotometer on the other hand can not function like a colorimeter but take a spectrum of a solution across the entire wave spectrum especially in UV – IR. Therefore use of spectrophotometer is beneficial than a colorimeter and useful to determine concentration of unknown solutions.

Fundamentals of thermodynamics:- 1st law, 2nd law and 3rd law short notes

·         Deals with energy and the energy changes.

Thermodynamics can only give information about a system when it is at equilibrium state; a time-invariant state.

When it is based on the concept of equilibrium it’s known as “Equilibrium thermodynamics”.

When it is based on the concept of time-invariant state it’s known as “Thermodynamics of Steady state” or “non-equilibrium thermodynamics”.

Equilibrium thermodynamics: - Only with closed and isolated systems.

System

·         Open: Both matter and energy can transfer between system and surroundings.

·         Closed: Only energy can transfer.

·         Isolated: Neither energy nor matter can transfer.

·         Homogenous system: Consists of a single phase.

·         Heterogeneous system: Consist of two or more phases.

Isothermal: Constant T

Isobaric: Constant P

Isochoric: Constant V

Adiabatic: No heat transfer between the system and the surroundings.

Heat and work do not belong to system and are NOT properties of thermodynamics.

They are operations which performed on the system to alter its energy.

Properties

·         Extensive: Describe and depend on the size of the system. (Mass, volume, pressure…)

·         Intensive: Does not depend on the size of the system. (Molar volume, Molecular weight, Temperature…)

First law

Introduces the concept of internal energy.

·         DQ = U + W

From 1^st law;

When isothermal: DU= 0

                              DQ = DW

At constant volume: DW = 0

                                  DQ = DU

W = - PDV   it is a minus value for a closed system in expansion

W=nRTln.V_f / V_i   Can be taken for an isothermal expansion of a gas

Heat capacity

C=dQ/dT

At constant P: C_p= (dH/dT)_p

At constant V: C_v= (dU/dT)_v

To know how the reaction proceeds we need to know;

·         Enthalpy-H

·         Entropy-S

Enthalpy

DH^o  = å n H^o_products  - å n H^o_reactants

DH = U + PV

DH = C_pdT (as above mentioned in heat capacity)

DH = mCDǾ

Second Law

Describes entropy. Entropy is an idea of randomness in a reaction.

S>0 reaction is spontaneous

S<0 reaction is non spontaneous

S=0 reaction is at equilibrium

·         DS  =  DQ/T

At constant pressure

DS  =  DH/T

Also;

DS_universe  =  DS_system  +  DS_surroundings

DS^o  =  å n S^o_products  -  å n S^o_reactants

Third law

Absolute Entropy, S, =  0  at 0  Kelvins for a perfect crystal of a pure substance.

DG,  Gibbs Free Energy

The maximum amount of energy available to do useful work on the surroundings.

·         DG  = D H – T D S

DG  <  0  (-) Spontaneous

DG  >  0 (+) Spontaneous in the opposite direction       DG  =  0  equilibrium

·         DG^o  =   DH^o  -  T DS^o

DS (+), DH (-) Spontaneous at all temperature

DS (+), DH (+) Spontaneous at high temperatures (where exothermicity is relatively unimportant)     

    

DS (-), DH (-) Spontaneous at low temperatures

            (where exothermicity is dominant)

DS(-), DH (+)  Process not spontaneous at any temperature (reverse process is spontaneous at all temperatures)

·         DG  = DG^o  +  RT ln Q

                       Q  =  Reaction Quotient.

Free energy at equilibrium

·         G  =  0

·         So DG^o  =  -RT ln Q_equilibrium

              Q_equiliibrium  =  K_p (gases)

                                 =  K_c  (solution)