Lecture 9: Water for Life

SCCH 100 Integrated Chemistry

Instructor: Assist. Prof. Pasit Pakawatpanurut

Department of Chemistry

Faculty of Science, Mahidol University

Email: pasit.pk@gmail.com

Textbook:

Chemistry in Context: Applying Chemistry to Society,

7th edition, C.H.Middlecamp (ed.), 2012.

Water has never lost its mystery. After at least two and a half

millennia of philosophical and scientific inquiry, the most vital of

the worlds substances remains surrounded by deep uncertainties.

Without too much poetic license, we can reduce these questions to

a single bare essential: What exactly is water?

Philip Ball, in Lifes Matrix: A Biography of Water,University of California Press,

Berkeley, CA, 2001, p. 115

Do you know where your drinking water comes from?

Do you know if your drinking water is safe to drink?

How would you know?

Different Representations of Water

Lewis structures Space-filling

EN Values assigned by Linus Pauling,

winner of TWO Nobel Prizes.

Electronegativity is a measure of an atoms attraction for

the electrons it shares in a covalent bond.

On periodic

table, EN

increases

HH

O

A difference in the electronegativities

of the atoms in a bond creates a

polar bond.

Partial charges result from

bond polarization.

A polar covalent bond is a

covalent bond in which the

electrons are not equally shared,

but rather displaced toward the

more electronegative atom.

H HH2 has a nonpolar

covalent bond.

NaCl

NaCl has an ionic

bond look at the

EN difference.

Na = 1.0

Cl = 2.9

DEN = 1.9

A water molecule is polar due to

polar covalent bonds and the shape of

the molecule.

Polarized bonds

allow hydrogen

bonding to occur.

Hbonds are intermolecular bonds.

Covalent bonds are intramolecular bonds.

A hydrogen bond is an electrostatic attraction between an atom

bearing a partial positive charge in one molecule and an atom

bearing a partial negative charge in a neighboring molecule. The

H atom must be bonded to an O, N, or F atom.

Hydrogen bonds typically are only about one-fifteenth as strong

as the covalent bonds that connect atoms together within

molecules.

The boiling point is the temperature at which the (equilibrium) vapor

pressure of a liquid is equal to the external pressure.

The normal boiling point is the temperature at which a liquid boils when

the external pressure is 1 atm.

Maximum Density

40C

Ice is less dense than water

Density of Water

Water is a Unique Substance

Hydrogen Bonding

in Water

The hydrogen-bonded lattice structure of the common form of ice. Notice the open channels

between layers of water molecules.

Water Footprint

Water is necessary to produce food:

Water Footprint

Water is necessary for products:

International Water Footprint (per capita)

Where Does Potable

(fit for consumption)

Drinking Water Come From?

Surface water: from lakes, rivers, reservoirs

Ground water: pumped from wells drilled into

underground aquifers

Much of our clean water comes from

underground aquifers.

The Ogallala Aquifer is shown in dark blue.

While normally free of pollutants, groundwater can be contaminated

by a number of sources:

Abandoned mines

Poorly constructed landfills and septic systems

Runoff from fertilized fields

Household chemicals poured down the drain or on the ground

The average American uses

almost 100 gallons of water a day.

Nearly of the water entering

our homes goes down the drain.

Access to safe drinking water varies widely across the world.

A solution is a homogeneous mixture of uniform composition.

Solutions are made up of solvents and solutes.

Substances capable of

dissolving other substances

usually present in the greater

amount.

Substances dissolved in a

solvent usually present in

the lesser amount.

When water is the solvent, you have an aqueous solution.

Concentration Terms

Parts per hundred (percent)

Parts per million (ppm)

Parts per billion (ppb)

20 g of NaCl in 100 g of water is a 20% NaCl solution

2 ppb Hg2 g Hg

1109 g H 2O

210-6 g Hg

1103 g H2O

2 g Hg1 L H 2O

Molarity (M) = moles soluteliter of solution

1.0 M NaCl solution

[NaCl] = 1.0 M = 1.0 mol NaCl/L solution

Also this solution is 1.0 M in Na+ and 1.0 M in Cl

[Na+] = 1.0 M and [Cl] = 1.0 M

[ ] = concentration of

How to prepare a 1.00 M NaCl solution:

Note you do NOT add

58.5 g NaCl to 1.00 L of

water.

The 58.5 g will take up

some volume, resulting in

slightly more than 1.00 L

of solution and the

molarity would be lower.

mol soluteL of solution

M =

What is the concentration (in M and mass %) of

the resulting solution when you add 5 grams of

NaOH to 95 mL of water?

What is the molarity of glucose (C6H12O6) in a solution

containing 126 mg glucose per 100.0 mL solution?

When ions (charged particles) are in aqueous solutions,

the solutions are able to conduct electricity.

(a) Pure distilled water (nonconducting)

(b) Sugar dissolved in water (nonconducting): a nonelectrolyte

(c) NaCl dissolved in water (conducting): an electrolyte

+ 1 e-Na Na

Na atom Na+ ion

Forming ions

+ 1 e-

Cl atom

Cl- ion

ClCl

Some atoms form more than one stable ion.

Naming simple ionic compounds is easy.

Name the metallic element (cation) first, followed

by the nonmetallic element (the anion) second, but

with an ide suffix.

MgO Mg is the metal; O is the nonmetal.

magnesium oxide

NaBr Na is the metal; Br is the nonmetal.

sodium bromide

Ions that are themselves made up of more than one

atom or element are called polyatomic ions.

NaSO4 (sodium sulfate) dissociates in water to form:

Na+

Sodium ions

and

Sulfate ions

The sulfate group

stays together in

solution.

Naming polyatomic ionic compounds is also easy.

Name the cation first, followed by the anion second.

MgOH Mg+ is the cation; OH is the anion.

magnesium

hydroxide

NH4Br NH4+ is the cation; Br is the anion.

ammonium

bromide

Substances that will dissociate in

solution are called electrolytes.

Dissolution of NaCl in Water

The polar water molecules stabilize the ions

as they break apart (dissociate).

Ions are simply charged

particles atoms or groups of

atoms.

They may be positively

charged cations.

Or negatively charged

anions.

NaCl(s) Na+(aq) + Cl(aq)H2O

Simple generalizations about ionic compounds

allow us to predict their water solubility.

*Insoluble means that the compounds have extremely low solubility in water (less than 0.01 M).

All ionic compounds have at least a very small solubility in water.

Covalent molecules in solution

A sucrose molecule when dissolved in water,

sugar molecules interact with and become

surrounded by water molecules, but the sucrose

molecules do not dissociate like ionic

compounds do; covalent molecules remain

intact when dissolved in solution.

They will not conduct electricity; they are

nonelectrolytes.

Maximum Contaminant Level Goal (MCLG)

and Maximum Contaminant Level (MCL)

Schematic drawing of a typical municipal water treatment facility.

Making freshwater from saltwater

Desalinization a process that removes ions from saltwater

Making freshwater from saltwater continued

Distillation a separation process in which a liquid solution is

heated and the vapors are condensed and collected

Either perform distillation in laboratory (left) or use solar power (right).

Making freshwater from saltwater continued

Osmosis the passage of water through a semipermeable membrane from a

solution that is less concentrated to a solution that is more concentrated

Reverse Osmosis uses pressure to force the movement of water through a

semipermeable membrane from a solution that is more concentrated to a

solution that is less concentrated

LifeStraw created for developing countries to remove bacteria,

viruses, and parasites from water to use for drinking

Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.

And every tongue, through utter drought,

Was withered at the root;

We could not speak, no more than if

We had been choked with soot.

The Rime of the Ancient Mariner, excerpt

Samuel Taylor Coleridge