3Chemical Equilibrium
All the tables and figures (except were noted) taken from:
Snoeyink, V.L . and Jenkins, D., 1980. Water Chemistry, John Wiley &Sons
Benjamin, M.M., 2014. Water Chemistry, Waveland Press Inc.
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
explain equilibrium Chemical Equlibrium balancing equation
Typology: Lecture notes
2020/2021
1 / 23
This page cannot be seen from the preview
Don't miss anything!
Related documents
Partial preview of the text
Download Chemistry power point and more Lecture notes Physical Chemistry in PDF only on Docsity!
3 Chemical Equilibrium
All the tables and figures (except were noted) taken from:
Snoeyink, V.L. and Jenkins, D., 1980. Water Chemistry, John Wiley & Sons
Benjamin, M.M., 2014. Water Chemistry, Waveland Press Inc.
Chemical Equilibrium
This Section will allow us to answer the questions:
- Will this reaction proceed?
- In which direction?
If a reaction is impossible, there is no sense in attempting in it at a water or wastewater process
This, without having to perform an experiment!
Will calcium carbonate tend to precipitate or dissolve in water?
Can I oxidize sulfide with nitrate?
๏ It is faster to do a calculation than a lab experiment!
These calculations come from Thermodynamics
Thermodynamics
๐บ๐บ = ๐ป๐ป โ ๐๐๐๐ Assume closed system G = Gibbs free energy (kcal) conservation of mass T = Absolute temperature (หK) energy can flow across boundaries (energy available for โuseful workโ) Constant T and P
0หC = 273หK 25หC = 298หK โ standard conditions (25หC and 1 atmosphere of pressure)
H = Enthalpy (kcal) ๏ Is the total energy of an element or compound
S = Entropy (kcal/หK) ๏ Internal energy โ degree of internal order/disorder ๏ Highly structured materials (e.g. crystals) โ low entropy ๏ Random systems (e.g. gases) โ high entropy
Thermodynamics
GT = Total free energy
๏ For closed systems (at constant pressure and temperature) the criterion for equilibrium is that the total free energy of the system (GT ) is minimum
๏ Reaction proceeds spontaneously only if GT decreases (โ)
Thermodynamics
Since total free energy is difficult to measure, usually we determine the change in Gibbs free energy (โG)
If โG < 0 โ reaction proceeds spontaneously as written
If โG > 0 โ reaction can not proceed spontaneously as written, but proceeds spontaneously in the opposite direction
If โG = 0 (GT is minimum)โ reaction is at equilibrium and will not proceed in either direction
โ๐บ๐บ = โ๐บ๐บ ๐๐^ + ๐ ๐ ๐๐ ๐๐๐๐
T = Absolute temperature (หK)
โGห = change in free energy at standard conditions
ห โ means at standard conditions
R = 8.31x10-3^ KJ/mol หK = 1.987x10-3Kcal/mol หK
(1 KJ/mol = 4.184 Kcal/mol)
Equation 2
Thermodynamics
At equilibrium โG=0 (change in Gibbs free energy) and equation 2 transforms into:
[๐ถ๐ถ]๐๐[๐ท๐ท]๐๐
[๐ด๐ด]๐๐[๐ต๐ต]๐๐
From equation 1: [๐ถ๐ถ]๐๐[๐ท๐ท]๐๐ ๐พ๐พ = [๐ด๐ด]๐๐[๐ต๐ต]๐๐
Substituting equation 4 into equation 2
Equation 3
Equation 4
โ๐บ๐บ = โ๐ ๐ ๐๐๐๐๐๐๐พ๐พ + ๐ ๐ ๐๐ ๐๐๐๐ Equation 5
[๐ถ๐ถ]๐๐[๐ท๐ท]๐๐
[๐ด๐ด]๐๐[๐ต๐ต]๐๐
Thermodynamics
โ๐บ๐บ ๐๐^ = Gibbs free energy of formation:
(the energy necessary to produce one mole of a substance) By convention:
โ๐บ๐บ ๐๐ ๐๐ (^) = 0 for elements in their most stable form (including H+ )
โ๐บ๐บ ๐๐ ๐๐ (^) = 1 for pure solids
๐๐
Chemical Equilibrium
Thermodynamics
Example 3-1 from Snoeyink (pg 68)
a) Determine the equilibrium constant (K) for the reaction in which water dissociates into H+^ and OH-^ at 25หC.
๐ป๐ป 2 ๐๐ โ ๐ป๐ป+^ + 0๐ป๐ปโ
b) Will this reaction proceed as written when ๐ป๐ป+^ = 10โ6^ ๐๐ and (^) ๐๐๐ป๐ปโ^ = 10โ8๐๐?
Thermodynamics
Exercise 3.1 from Appelo and Postma (pg 47)
A water sample contains 10 ppm of Ca+2^ and 5.5 ppm of F -. Is the sample saturated with respect to fluorite ( CaF 2 )? Assume standard conditions.