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
Electrochemistry is the relationship between Chemical Phenomena and Electrical Phenomena
It is needed in Environmental Engineering to understand:
Current can flow through:
Characteristics of current flow through a metal:
Characteristics of current flow through a solution:
Conductivity of Solution
"Is its ability to carry an electrical current"
Conductivity can be measured by a conductivity meter and it is affected by:
E = IR
Specific conductance is conductance afforded by 1 cm3 of an electrolyte solution
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
Specific Conductance of a Solution = C / R
R needs to be determined
and are equivalent ionic conductance of cations and anions, respectively
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
What is the approximate specific conductance at 25 C of a solution containing 100 mg/l of CaCl2 and 75 mg/l of Na2SO4
Chemical change depends on:
To bring about 1 equivalent of chemical change at an electrode:
Study Example page 78
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?
Based on that, a relationship between electrical potential and chemical free energy can be found:
Electrical Energy = EIt
Electrical energy is expressed in Volt-Coulomb or Joule
Electrical energy required to produce one mole of chemical change = zEF
If reaction proceeds (E is +ve), then:
Consider the following reaction:
aA + bB cC + dD
The value of R in electrical units is:
R = 8.314 J / K-mol
At 25 C and converting ln to log
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:
In this case:
log K = 16.9zE
Study Examples page 85
Estimate the solubility-product constant for Mg(OH)2(s) at 25 C from standard electrode potential?
Chemical kinetics deal with speed of reactions. If
then, the rate of reaction could be:
first – order reaction (exponent 1)
second – order reaction (exponent 2)
third – order reaction (exponent 3)
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:
Integrating the above equation:
Converting to log10
k = - slope of line [t versus ln(C/C0)]
k = -slope 2.303 [t versus log10(C/C0)]
In this case,
t = t1/2 andC = (1/2)C0
Applications of 1st order reactions in Environmental Engineering:
Study Example page 89
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?
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.
Ca and Cb are concentrations of A and B, respectively. Integrating (1) and (2), we obtain:
If rates of reactions are 1st order, then:
Integrating between t = 0 to t = t
Examples of consecutive reactions in Environmental 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
Total enzyme concentration in the system = E = [Ef] + [EcS]
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:
The rate of product formation = overall rate of reaction
Rate of product formation V = k[EcS]
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
"rates increase with increase in temperature"
Rate doubles for each 10 C rise.
Using Arrehenius equation:
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.
"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"
Adsorption depends on:
Freundlich found that:
Freundlich isotherm can be expressed as:
Langmuir isotherm is also used to describe adsorption of single layer:
A third isotherm is BET (Brunauer, Emmett, and Teller) which can be used to describe multi-layer adsorption
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)
Study Examples Page 104