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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

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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 Phenomena and Electrical Phenomena

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: Phenomena and Electrical Phenomena

  • 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 Phenomena and Electrical Phenomena

"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 Phenomena and Electrical Phenomena

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 ( resistance of a standard solution (R)

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 resistance of a standard solution (R 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 resistance of a standard solution (R C of a solution containing 100 mg/l of CaCl2 and 75 mg/l of Na2SO4


Current and chemical change
Current and Chemical Change resistance of a standard solution (R

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 resistance of a standard solution (R-

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 anode by the passage of 0.02 A of current through the solution for 24 hours?

  • 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: and chemical free energy can be found:

where

  • G is the free energy

  • -zEF is the electrical energy


Electrochemistry

Consider the following reaction: and chemical free energy can be found:

aA + bB  cC + dD

Substituting in:

we get:


Electrochemistry

The value of R in electrical units is: and chemical free energy can be found:

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: free-energy and enthalpy. The values in the table are for a reaction written for 1 mole of e

since

or

log K = 16.9zE

Study Examples page 85


Electrochemistry

Estimate the solubility-product constant for Mg(OH) free-energy and enthalpy. The values in the table are for a reaction written for 1 mole of e2(s) at 25  C from standard electrode potential?


Chemical kinetics
Chemical Kinetics free-energy and enthalpy. The values in the table are for a reaction written for 1 mole of e

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 more complex equations. The unit of k depends on the reaction order and units of concentration of A, B, and C.

    • 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 more complex equations. The unit of k depends on the reaction order and units of concentration of A, B, and C.

    • 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: more complex equations. The unit of k depends on the reaction order and units of concentration of A, B, and C.

    Converting to log10


    Electrochemistry

    k = - slope of line [t versus ln(C/C more complex equations. The unit of k depends on the reaction order and units of concentration of A, B, and C.0)]

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

    Half-life (t1/2)

    In this case,

    t = t1/2 and C = (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 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?

    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

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


    Consecutive reactions
    Consecutive Reactions 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?

    If rates of reactions are 1st order, then:


    Electrochemistry

    Integrating between t = 0 to t = t 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?



    Enzyme reactions
    Enzyme Reactions Engineering

    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: Engineering

    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 Engineering

    The rate of product formation = overall rate of reaction

    Rate of product formation V = k[EcS]


    Electrochemistry

    Then Engineering

    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 Engineering

    "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 Engineering

    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 Engineering


    Adsorption
    Adsorption Engineering

    "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 Engineering

    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 Engineering

    • 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 Engineering

    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 Engineering

    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 Engineering

    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 > C Engineerings 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)