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Chemistry power point, Lecture notes of Physical Chemistry

explain equilibrium Chemical Equlibrium balancing equation

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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.
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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?

๏ƒ˜ It is faster to do a calculation than a lab experiment!

These calculations come from Thermodynamics

Thermodynamics

๐บ๐บ = ๐ป๐ป โˆ’ ๐‘‡๐‘‡๐‘†๐‘† Assume closed system G = Gibbs free energy (kcal) conservation of mass T = Absolute temperature (หšK) energy can flow across boundaries (energy available for โ€œuseful workโ€) Constant T and P

0หšC = 273หšK 25หšC = 298หšK โ†’ standard conditions (25หšC and 1 atmosphere of pressure)

H = Enthalpy (kcal) ๏ƒ˜ Is the total energy of an element or compound

S = Entropy (kcal/หšK) ๏ƒ˜ Internal energy โ†’ degree of internal order/disorder ๏ƒ˜ Highly structured materials (e.g. crystals) โ†’ low entropy ๏ƒ˜ Random systems (e.g. gases) โ†’ high entropy

Thermodynamics

GT = Total free energy

๏ƒ˜ For closed systems (at constant pressure and temperature) the criterion for equilibrium is that the total free energy of the system (GT ) is minimum

๏ƒ˜ 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 (โˆ†G)

If โˆ†G < 0 โ†’ reaction proceeds spontaneously as written

If โˆ†G > 0 โ†’ reaction can not proceed spontaneously as written, but proceeds spontaneously in the opposite direction

If โˆ†G = 0 (GT is minimum)โ†’ reaction is at equilibrium and will not proceed in either direction

โˆ†๐บ๐บ = โˆ†๐บ๐บ ๐‘œ๐‘œ^ + ๐‘…๐‘…๐‘‡๐‘‡ ๐‘™๐‘™๐‘›๐‘›

T = Absolute temperature (หšK)

โˆ†Gหš = change in free energy at standard conditions

หš โ†’ means at standard conditions

R = 8.31x10-3^ KJ/mol หšK = 1.987x10-3Kcal/mol หšK

(1 KJ/mol = 4.184 Kcal/mol)

Equation 2

Thermodynamics

At equilibrium โˆ†G=0 (change in Gibbs free energy) and equation 2 transforms into:

[๐ถ๐ถ]๐‘๐‘[๐ท๐ท]๐‘‘๐‘‘

[๐ด๐ด]๐‘Ž๐‘Ž[๐ต๐ต]๐‘๐‘

From equation 1: [๐ถ๐ถ]๐‘๐‘[๐ท๐ท]๐‘‘๐‘‘ ๐พ๐พ = [๐ด๐ด]๐‘Ž๐‘Ž[๐ต๐ต]๐‘๐‘

Substituting equation 4 into equation 2

Equation 3

Equation 4

โˆ†๐บ๐บ = โˆ’๐‘…๐‘…๐‘‡๐‘‡๐‘™๐‘™๐‘›๐‘›๐พ๐พ + ๐‘…๐‘…๐‘‡๐‘‡ ๐‘™๐‘™๐‘›๐‘› Equation 5

[๐ถ๐ถ]๐‘๐‘[๐ท๐ท]๐‘‘๐‘‘

[๐ด๐ด]๐‘Ž๐‘Ž[๐ต๐ต]๐‘๐‘

Thermodynamics

โˆ†๐บ๐บ ๐‘œ๐‘œ^ = Gibbs free energy of formation:

(the energy necessary to produce one mole of a substance) By convention:

โˆ†๐บ๐บ ๐‘“๐‘“ ๐‘œ๐‘œ (^) = 0 for elements in their most stable form (including H+ )

โˆ†๐บ๐บ ๐‘“๐‘“ ๐‘œ๐‘œ (^) = 1 for pure solids

๐‘œ๐‘œ

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 25หšC.

๐ป๐ป 2 ๐‘‚๐‘‚ โ†” ๐ป๐ป+^ + 0๐ป๐ปโˆ’

b) Will this reaction proceed as written when ๐ป๐ป+^ = 10โˆ’6^ ๐‘€๐‘€ and (^) ๐‘‚๐‘‚๐ป๐ปโˆ’^ = 10โˆ’8๐‘€๐‘€?

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.

๐พ๐พ ๐‘“๐‘“๐‘™๐‘™๐‘œ๐‘œ๐‘ข๐‘ข๐‘Ÿ๐‘Ÿ๐‘–๐‘–๐‘ก๐‘ก๐‘’๐‘’ = 10โˆ’10.57^ @25ยฐ๐ถ๐ถ ๐‘ค๐‘ค๐‘–๐‘–๐‘ก๐‘กโ„Ž ๐‘ก๐‘กโ„Ž๐‘’๐‘’ ๐‘“๐‘“๐‘œ๐‘œ๐‘™๐‘™๐‘™๐‘™๐‘œ๐‘œ๐‘ค๐‘ค๐‘–๐‘–๐‘›๐‘›๐‘”๐‘” ๐‘Ÿ๐‘Ÿ๐‘’๐‘’๐‘Ž๐‘Ž๐‘๐‘๐‘ก๐‘ก๐‘–๐‘–๐‘œ๐‘œ๐‘›๐‘›: