Electrochemistry
This presentation is the property of its rightful owner.
Sponsored Links
1 / 58

Electrochemistry PowerPoint PPT Presentation


  • 183 Views
  • Uploaded on
  • Presentation posted in: General

Electrochemistry. CE 541. Electrochemistry is the relationship between Chemical Phenomena and Electrical Phenomena It is needed in Environmental Engineering to understand: Corrosion Electrochemical oxidation of wastes Analytical procedures Automatic monitoring of waste streams

Download Presentation

Electrochemistry

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Electrochemistry

Electrochemistry

CE 541


Electrochemistry

Electrochemistry is the relationship between Chemical Phenomena and Electrical Phenomena

It is needed in Environmental Engineering to understand:

  • Corrosion

  • Electrochemical oxidation of wastes

  • Analytical procedures

  • Automatic monitoring of waste streams

  • Oxidation-reduction reactions


Current flow in solution

Current Flow in Solution

Current can flow through:

  • Solution of electrolyte

  • Metallic conductors

    Characteristics of current flow through a metal:

  • Chemical properties of metal are not changed

  • Current is carried by electrons

  • Increase in temperature increases resistance


Electrochemistry

Characteristics of current flow through a solution:

  • Chemical change occurs in solution

  • Current is carried by ions

  • Increase in temperature decreases resistance

  • Resistance is normally greater than that with metals


Electrochemistry

Conductivity of Solution

"Is its ability to carry an electrical current"

Conductivity can be measured by a conductivity meter and it is affected by:

  • Number of ions

  • Type of ions

    E = IR

    Where:

  • E = electromotive force (volts)

  • I = current (amperes)

  • R = resistance (ohms)


Electrochemistry

And

Where

  • l = length of conductor

  • A = cross-sectional area of conductor

  •  = specific resistance of conductor (ohm-cm)

  • k = specific conductance (1 / ohm-cm) or siemens (S)

    Specific conductance is conductance afforded by 1 cm3 of an electrolyte solution


Electrochemistry

Conductivity cells are calibrated by determining the resistance of a standard solution (Rs) and the cell constant (C) can be found.

C = ksRs

In such cases 0.0100 N KCl is used in calibrating conductivity cells. For 0.0100 N KCl:

ks = 0.00141185 S = 1411.8 S @ 25 C

So,

Specific Conductance of a Solution = C / R

R needs to be determined


Equivalent conductance

Equivalent Conductance ()

Where

  • N = normality of the solution

  • k = specific conductance

    or,

    and are equivalent ionic conductance of cations and anions, respectively


Electrochemistry

Table 3-3 shows equivalent ionic conductance @ 25 C in S-cm3/equivalent.

Only ions can carry current. Un-ionized species of weak acids or bases will not carry current. Also uncharged soluble organics (ethanol and glucose) can not carry current.

Study Example page 80


Electrochemistry

What is the approximate specific conductance at 25 C of a solution containing 100 mg/l of CaCl2 and 75 mg/l of Na2SO4


Current and chemical change

Current and Chemical Change

Chemical change depends on:

  • Nature of solution (composition)

  • Nature of electrodes

  • Magnitude of electromotive force imposed


Applying a voltage of 1 3 v h 2 evolves at cathode cl 2 evolves at anode

e-

Cl-

Cathode (-)

Anode (+)

H+

HCl

Platinum

Electrodes

Applying a voltage of 1.3 vH2 evolves at CathodeCl2 evolves at Anode


Electrochemistry

To bring about 1 equivalent of chemical change at an electrode:

  • An Avogadro No of electrons must flow through the external circuit

  • This quantity of electrons is called the faraday (F)

  • The rate of flow of electrons gives the current (I) in amperes

  • 1 F is equivalent to an ampere of current flowing for 96500 seconds

  • An ampere is defined as a Coulomb per second

  • 1 F = 96500 Coulomb

    Study Example page 78


Electrochemistry

What weight of silver will pass into solution from a silver anode by the passage of 0.02 A of current through the solution for 24 hours?


Electrochemical cell

Electrochemical Cell

  • two electrodes will be connected by metallic conductor

  • electrons will flow

  • chemical change begins

  • electromotive force (emf) will be generated by the cell

  • this emf is a measure of the driving force of the chemical reaction occurring in the solution

  • the driving force represents the chemical potential or free energy of the reaction


Electrochemistry

Based on that, a relationship between electrical potential and chemical free energy can be found:

Electrical Energy = EIt

  • E = emf in volts

  • I = current in amperes

  • t = time in seconds

    Electrical energy is expressed in Volt-Coulomb or Joule

    Electrical energy required to produce one mole of chemical change = zEF

    Where

  • z = number of electron-equivalent per mole

  • E = emf in volts

  • F = faraday or coulombs per equivalent


Electrochemistry

If reaction proceeds (E is +ve), then:

where

  • G is the free energy

  • -zEF is the electrical energy


Electrochemistry

Consider the following reaction:

aA + bB  cC + dD

Substituting in:

we get:


Electrochemistry

The value of R in electrical units is:

R = 8.314 J / K-mol

At 25 C and converting ln to log


Electrochemistry

The emf can be found in Tables (Table 3-4) just like free-energy and enthalpy. The values in the table are for a reaction written for 1 mole of e- change, such as:

If an electrochemical cell reaches the state of equilibrium, then:

  • no current can flow

  • emf is zero


Electrochemistry

In this case:

since

or

log K = 16.9zE

Study Examples page 85


Electrochemistry

Estimate the solubility-product constant for Mg(OH)2(s) at 25  C from standard electrode potential?


Chemical kinetics

Chemical Kinetics

Chemical kinetics deal with speed of reactions. If

then, the rate of reaction could be:

first – order reaction (exponent 1)

  • kCa

    second – order reaction (exponent 2)

  • kCa2

  • kCaCb

    third – order reaction (exponent 3)

  • kCa3

  • kCa2Cb

  • kCaCbCc

  • Ca, Cb, and Cc = concentrations of A, B, and C, respectively

  • k = rate constant


  • Electrochemistry

    These are simple reaction rates, but in reality there are more complex equations. The unit of k depends on the reaction order and units of concentration of A, B, and C.

    The reaction rates are required in:

    • Microbial growth

    • Aeration

    • Disinfection

    • Radioactive decay


    Zero order reactions

    Zero-Order Reactions

    • They are independent of concentration

    • Most of biological growth occur in linear relationship over a range of concentrations of substance (C).


    First order reactions

    First-Order Reactions

    • The rate is directly proportional to the concentration

    • if we are dealing with a decay or decomposition reaction, then the rate can be expressed as

    • Unit of k is (1/time) and the –ve sign indicates the loss of material with time.


    Electrochemistry

    Integrating the above equation:

    Converting to log10


    Electrochemistry

    k = - slope of line [t versus ln(C/C0)]

    k = -slope 2.303 [t versus log10(C/C0)]

    Half-life (t1/2)

    In this case,

    t = t1/2 andC = (1/2)C0

    Then

    Applications of 1st order reactions in Environmental Engineering:

    • Dissolution of gases in water

    • Removal of gases from water

    • Rate of death of microorganisms

    • Decomposition of organic matters (BOD5 test)

      Study Example page 89


    Electrochemistry

    The radioactive nuclide P32 has a half-life of 14.3 days. How long would a waste containing 10 mg/l of this nuclide have to be stored in order to reduce the concentration to 0.3 mg/l?


    Second order reactions

    Second-Order Reactions

    The rate of reaction is proportional to the square of the concentration of one of the reactants or to the product of concentrations of two different reactants.


    Electrochemistry

    Ca and Cb are concentrations of A and B, respectively. Integrating (1) and (2), we obtain:


    Consecutive reactions

    Consecutive Reactions

    If rates of reactions are 1st order, then:


    Electrochemistry

    Integrating between t = 0 to t = t


    Electrochemistry

    Examples of consecutive reactions in Environmental Engineering


    Enzyme reactions

    Enzyme Reactions

    Are used to describe the rate of biological waste treatment. The relationship between Substrate (S) and the rate of utilization per unit mass of enzyme or bacteria (V/E)

    Michaelis – Menton Relationship

    • Ef = free enzyme

    • S = substrate

    • EcS = enzyme-substrate complex

      Total enzyme concentration in the system = E = [Ef] + [EcS]


    Electrochemistry

    The rate of formation of enzyme-substrate complex is:

    The rate of complex formation  rate of overall reaction. Therefore, d[EcS] / dt can be considered as ZERO when overall reaction rate is required to be determined. So:


    Electrochemistry

    Or

    The rate of product formation = overall rate of reaction

    Rate of product formation V = k[EcS]


    Electrochemistry

    Then

    Where

    • k is the maximum rate

    • Ks is the substrate concentration at arte = (1/2)k

    • Ks is called the "half velocity" constant

      V/E  k'S when S<< Ks (1st order with respect to S)

      V/E  k when S >> Ks (zero order with respect to S)

      Study Example page 95


    Temperature dependence of reaction rates

    Temperature Dependence of Reaction Rates

    "rates increase with increase in temperature"

    Rate doubles for each 10 C rise.

    Using Arrehenius equation:

    Where

    • T = temperature,  K

    • R = universal gas constant

    • Ea = constant


    Electrochemistry

    Integrating

    k2 and k1 are rate constants at T2 and T1. In environmental engineering processes, the range of temperature is small. So T2T1 can be assumed constant.


    Electrochemistry

    Therefore


    Adsorption

    Adsorption

    "sorption is the concentration or movement of contaminants from one place to another"

    "adsorption involves partitioning of contaminants from one phase to another"

    "adsorption is the process by which ions or molecules present in one phase tend to condense and concentrate on the surface of another phase"


    Electrochemistry

    Adsorption

    Physical

    • Weak

    • multi-layers

    • free moving

    • reversible

      Chemical

    • Strong

    • mono-layer

    • no movement

    • non-reversible in most cases

      Ion exchange

    • electrical attraction

    • smaller particles

    • have stronger attraction

    • trivalent have stronger attraction than monovalent ions


    Electrochemistry

    Activated Carbon

    • 1 gram has surface area of 1000 m2

    • Pore size ranges between 10 to 1000 A

    • Adsorption of gas increases wit the increase I pressure

      Adsorption depends on:

    • Nature of materials

    • Concentration

    • Temperature


    Freundlich isotherm

    Freundlich Isotherm

    Freundlich found that:

    Where

    • q = mass of contaminant per unit weight of the adsorbent

    • C = concentration of solute after adsorption

    • K and n = constants (they should be evaluated for each soluble and temperature)

      Freundlich isotherm can be expressed as:


    Langmuir isotherm

    Langmuir Isotherm

    Langmuir isotherm is also used to describe adsorption of single layer:

    • qm = maximum adsorption that can take place in grams of adsorbate per gram of adsorbent

    • a = constant


    Bet isotherm

    BET Isotherm

    A third isotherm is BET (Brunauer, Emmett, and Teller) which can be used to describe multi-layer adsorption

    Assumptions

    • Multi-layers of adsorbent accumulate at the surface of adsorbent

    • Each layer can be described by Langmuir isotherm

    • Cs = saturation concentration for the adsorbate in solution


    Electrochemistry

    If C > Cs then the solute precipitates or condenses from solution as solid or liquid and concentrates on the surface

    BET equation can be put in this form:

    If we have data, then we have to find the best isotherm that can be used to describe the data (get straight line)


    Electrochemistry

    Study Examples Page 104


  • Login