1. Engineering Chemistry CHM 406
Water
Engineering Chemistry
CHM 406

2. Properties of water
Excellent solvent: ionic & polar compounds, biological molecules, gases, etc.
Very high boiling point, melting point, dielectric constant, surface tension, latent heat of fusion, heat of evaporation, heat capacity, etc.
Crystal structure of ice such that ice is less dense than liquid water.
Water can form more hydrogen bonds than any other comparable molecule.

3. Structure of water
Bent; bond angle = 105o. Hence very polar.
H-bond: a non-covalent interaction between a H bonded to N, O, or F, and an electron pair on another N, O, or F. About 5% as strong as a covalent bond. Many properties of water are due to H-bonding.

4. Acidity and water dissociation
Water dissociates very slightly as follows:
H2O H+ + OH-
[H+][OH-] = Kw (a constant at a given T)
At 25oC, Kw = x 10-14
The higher [H+], the lower [OH-], and vice versa.
[H+] measured on a logarithmic scale, as pH.
pH = - log [H+]
[H+] can range from ~10 mol L-1 (pH = -1) to 1 x mol L-1 ([OH-] = 10 mol L-1, pH = 15).
pH usually in the range 0 – 14, can be directly measured using a pH meter and a glass electrode.

5. Reactions in water Various reactions of biomolecules Redox reactions
Acid – base reactions
Reactions in which one product is insoluble in water:
Precipitation reactions
Gas-forming reactions
Complex – forming reactions

6. Acidic oxides Many non-metal oxides react with water to produce acids.
H2O + CO2 → H2CO H+ + HCO3-
(carbonic acid)
H2O + SO2 → H2SO H+ + HSO3-
(sulphurous acid)
H2O + SO3 → H2SO4
(sulphuric acid)
H2O NO2 → HNO3 + HNO2
(nitric acid) (nitrous acid)

7. Precipitation reactions
When two aqueous salts are mixed such that one cation-anion combination is insoluble, that combination precipitates out.
E.g.,
NaNO3(aq) + KCl(aq) → NaCl(aq) + KNO3(aq)
(No real reaction has occurred)
AgNO3(aq) + KCl(aq) → AgCl(s) + KNO3(aq)
Ag+(aq) + Cl-(aq) → AgCl(s)
(Net ionic equation; the NO3- and K+ ions are called spectator ions.)

8. Important insoluble salts
Silver halides: AgCl, AgBr, AgI
Most carbonates:
CaCO3: limestone, marble, calcite.
MgCO3.CaCO3: dolomite
Barium sulphate: BaSO4
Most phosphates.
Salts of Group 1 metals, nitrates, most chlorides are soluble.

9. Consumption of water Agriculture
Industry: mostly used for cooling, steam generation, waste disposal; may be special needs depending on application.
Domestic consumption: clean, free of chemical and biological contaminants, ideally fit for drinking.
Recreation

10. Sources of water (Earth’s crust)
Oceans and seas %
Saline lakes and groundwater 0.95%
Glaciers, ice-caps, snow 1.74%
Fresh groundwater %
Ground ice and permafrost 0.02%
Rivers and fresh lakes 0.01%
Balance: soil moisture, atmospheric moisture, etc.

11. Water treatment and purification
Water treatment has one of three objectives:
Purification for domestic use
Treatment for a specialised industrial application
Treatment of wastewater for re-use or release into the environment.
Type and extent of treatment depends on source, quality, and intended use.

12. Municipal water treatment
Water supply typically contains
Bacteria
Volatile compounds and gases, including products of bacterial metabolism (bad-smells)
Fine suspended particles (colloids – hazy appearance)
Depending on the source there may also be
Metal ions (Ca2+, Mg2+, Fe2+/Fe3+, etc.)
Anions such as Cl-, F-, SO42-, HCO3-, HPO42-, etc.
Agricultural run-off, industrial pollutants.

13. Municipal water treatment (contd.)
Coagul-
ant
Water
Supply
Aeration
Flocculation
Air
Sludge
Clean
water
Filtration
Disinfection
Cl2

14. Aeration Air bubbled through the water.
Removes volatile compounds, oxidises typically bad smelling sulphur-containing compounds.
Oxidises Fe2+ to more insoluble Fe3+.

15. Flocculation
pH of water adjusted to slightly basic (~8), by addition of CaO or Ca(OH)2.
Coagulant (flocculating agent) added: Fe2(SO4)3 or Al2(SO4)3.
Fe OH- → Fe(OH)3
Fe(OH)3 and Al(OH)3 are gelatinous precipitates; colloidal (fine, hazy) particles are trapped in the precipitate and removed as “sludge.”
If necessary, pH is adjusted back to neutral by bubbling CO2 through the water.

16. Disinfection and filtration
Microorganisms are destroyed by bubbling Cl2 through the water (chlorination).
Cl2 + H2O → HOCl + HCl
Occasionally, other oxidising agents such as ozone (O3) may be used.
Filtration (usually through a bed of sand) removes the last traces of particulate matter.
Other steps may be included for removing, e.g., excess Ca2+, Mg2+ or Fe3+ ions (typically prior to flocculation).

17. Hard water
Water that has been in contact with soil or rock contains dissolved anions and cations.
Cations: Ca2+, Na+, K+, Mg2+,Fe2+/Fe3+, Mn2+, etc.
Anions: Cl-, SO42-, HCO3-.
Water containing relatively high concentrations of Ca2+ and/or Mg2+ ions is called “hard water.”
Other divalent cations may also contribute to hardness.

18. Formation of hard water
Ca2+ and Mg2+ exist in minerals present in rocks / soil.
Gypsum: CaSO4.2H2O
Limestone, calcite: CaCO3
Dolomite: CaCO3.MgCO3
CaSO4 is slightly soluble and will dissolve to a limited extent: permanent (non-carbonate) hardness.
CaCO3 and MgCO3 react with water containing dissolved CO2.

19. Formation of hard water (contd.)
CaCO3(s) + H2O + CO2 → Ca(HCO3)2(aq)
This is known as temporary (carbonate) hardness.
The above reaction is reversed upon heating:
Ca(HCO3)2(aq) CaCO3(s) + H2O + CO2(g)
The resulting CaCO3 and MgCO3 is deposited in industrial boilers and household kettles, water-heaters, etc.

20. Consequences of water hardness
Soap is a soluble Na+ salt of certain organic acids. The Ca2+ and Mg2+ salts are insoluble. Hard water results in the formation of an insoluble “soap scum.”
Synthetic detergents do not form insoluble compounds, but their performance is affected.
Insoluble deposits of CaCO3 and MgCO3 are formed inside boilers (“scale”), reducing heating efficiency, clogging up pipes, and damaging the boiler metal.

21. Measurement of water hardness
Hardness = [Ca2+] + [Mg2+]
However, the Ca2+ and Mg2+ concentrations are not reported in mol L-1, but as equivalent amounts of CaCO3 in mg L-1 (parts per million, ppm).
Molar mass of CaCO3 is 100 g mol-1.
Thus, a hardness of mol L-1 (Ca2+, Mg2+, or both combined) is equivalent to 100 mg L-1 (ppm) of CaCO3: 100 mg L-1 of hardness.
Hardness scale (mg L-1):
Soft 0 – 60
Moderately hard 61 – 120
Hard
Very hard > 180

22. Determination of water hardness
Usually determined by titrating with ethylenediaminetetraacetic acid (EDTA).
Titrating means reacting with a solution of EDTA of known concentration and measuring the volume required.
EDTA is a complexing agent. It forms strong complexes with dications such as Ca2+ and Mg2+ by wrapping around them, effectively taking them out of solution and blocking their cationic behaviour.

24. Water softening
Lime softening: Used on a large scale for municipal water.
Ion exchange; Used in household water softeners as well as on a large scale.
Reverse osmosis: passing the water through a membrane which is permeable to water, but not to electrolytes (like filtering on a very small scale).
Use of sequestering agents.

25. Lime softening
Addition of Ca(OH)2 (just before flocculation in municipal water treatment).
Ca(HCO3)2 (aq) + Ca(OH)2 (aq) → 2 CaCO3 (s) H2O
Mg2+(aq) OH-(aq) → Mg(OH)2 (s)
Permanent hardness (CaSO4) is treated by adding Na2CO3
Ca2+(aq) + CO32-(aq) → CaCO3 (s)
The pH is later adjusted back to neutral by bubbling CO2 through the water.

26. Ion exchange
The water is passed through a tube containing particles of a synthetic (polystyrene) or natural (zeolite) substance with a negatively charged (anionic) surface.
The surface is covered with Na+ cations.
When the water passes over the particles, the Ca2+ and Mg2+ cations are replaced by Na+.
When the ion exchanger is used up, it can be recharged by passing a strong solution of NaCl through it. This washes away the Ca2+ and Mg2+ and replaces them again with Na+.

27. Sequestering agents
These are strong complexing agents like EDTA and polyphosphate. They complex to (wrap themselves around) the Ca2+, Mg2+, etc., and make them ineffective.
Often added to detergent formulations.
(triphosphate)