CH05 Chem

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©Professor Alton J. Banks

Chapter 5: The Water We Drink


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 Samuel Taylor Coleridge

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


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.


Different types of Solution

•  Solid in Liquid Sugar water for Ice tea

•  Liquid in Liquid lime juice, dilute HCl, Rubbing alcohol (70%)

•  Gas in Liquid Carbonated drinks, soda •  Liquid in GasSteam in the Bathroom

•  Gas in Gas Air"

How to differentiate solutions from colloidal dispersions

•  The Tyndall effect- in which light photons are deflected by the dispersed particles of the colloid.

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© Professor Alton J. Banks © Professor Alton J. Banks ©Professor Alton J. Banks

Concentration Terms-a reminder Parts per hundred (percent)

Parts per million (ppm)

Parts per billion (ppb)

20 g of NaCl in 80 g of water is a 20% NaCl solution

1 ppm= 1 g solute1,000,000 g water

= 1 mg solute1,000 g water

=1 mg solute1 L water

2 ppb Hg= 2 g Hg1x109 g H2O

= 2x10-6 g Hg1x103 g H2O

=2 mg Hg1 L H2O


Molarity (M) = moles solute liter 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”

A new concentration unit


What is the concentration (in M and mass %) of the resulting solution when you add 5 grams of NaOH to 95 mL of water?

95 mL H2O = 95 g H2O %: 5 g NaOH/100 g solution

95 mL H2O = .095 L = 5% NaOH

Since 1 mol NaOH has a mass of 40g, 5.0 g NaOH

40g NaOH/mol= 0.125 mol NaOH

0.125 mol NaOH0.95 L

= 1.3 M NaOH

© Professor Alton J. Banks

Water, the Universal Solvent Since Water,H2O is a Universal Solvent, the

properties of water , the structure, the bonding of water with itself and other molecules and the electronegativity are greatly studied.

Let’s take a look at some of these properties!

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Different Representations of Water

Lewis structures Space-filling Charge- density

Charge-density

Region of partial negative charge

Regions of partial positive charge


H2 has a non-polar covalent bond.

NaCl NaCl has an ionic bond-look at the EN difference.

Na = 0.9

Cl = 3.0

ΔEN = 2.1

Water molecule is polar – due to polar covalent bonds and the shape of the molecule.

Take home lesson: Polar bonds are those with a Δ EN >0. Note that IONIC bonds are VERY POLAR bonds--as polar as you can get!!

Table 5.3 lists the electronegativities of many representative elements.


NaCl NaCl has an ionic bond-look at the EN difference.

Na = 0.9

Cl = 3.0

ΔEN = 2.1

Take home lesson: Polar bonds are those with a Δ EN >0. Note that IONIC bonds are VERY POLAR bonds--as polar as you can get!!

Table 5.3 lists the electronegativities of many representative elements.

Polarized bonds allow hydrogen bonding to occur.

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-tenth as strong as the covalent bonds that connect atoms together within molecules.

H–bonds are intermolecular bonds. Covalent bonds are intramolecular bonds. Professor Alton J. Banks

Is Hydrogen bonding of concern to Joe and Jane Q. Public?

Here’s one example:http://pubs.acs.org/cen/email/html/cen_86_i41_8641notw4.html

Here’s a blowup of the molecule--and the resulting solid which is frequently fatal to animals:


•  It isn't just soap bubbles that give spheres—as a result of the surface tension. Check out the bubble of milk shown here -->

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A solute you may have heard about

•  http://en.wikipedia.org/wiki/Atrazine

Professor Alton J. Banks

Are there other significant properties associated with Hydrogen bonding? Let’s look at some boiling points - the “hydrides” of Groups 4,6,7

H2O

HF

CH4


Are there other significant properties associated with Hydrogen bonding? - Boiling Points are higher than expected based upon the molecular weight.

-Surface Tension:It’s why raindrops are spherical in shape! -----and why “water striders” float on the surface of water. --What happens when we disrupt the surface tension?

- Large heat capacity: DEFINITION: Amount of heat energy required to change the temperature of 1 g of water by 1˙C.


When ions (charged particles) are in aqueous solutions, the solutions are able to conduct electricity.

(a)  Pure distilled water (non-conducting)

(b)  Sugar dissolved in water (non-conducting): a nonelectrolyte

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


Substances that will dissociate in solution (and conduct an electric current) 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.

Ions that are themselves made up of more than one atom or element are called polyatomic ions.

Na2SO4 (sodium sulfate) dissociates in water to form:

Na+

Sodium ions

and

Sulfate ions

The sulfate group stays together in solution-as do all polyatomic ions in solution!

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Maximum Contaminant Level Goal (MCLG) and Maximum Contaminant Level (MCL)

TABLE 5.10

MCLG: The level at which a 70kg (154 lbs) person could drink 2L of water with this contaminant level every day for 70 years without suffering ill effects.


A pipe with hard-water scale build up

Hard water contains high concentrations of dissolved calcium and magnesium ions.

Soft water contains few of these dissolved ions.


Ca ions are usually the largest contributors of water hardness. So we frequently express hardness as ppm CaCO3 by mass.

Ca2+(aq) + CO32–(aq) CaCO3(s)

A hardness of 10 ppm indicates that 10 mg CaCO3 could be formed from the Ca2+ ions present in 1 L of water.

A business opportunity •  So the need for

removing these ions from water creates business opportunities


The water softening process


Two water purification techniques:

Distillation Reverse osmosis

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So, what is osmosis?

Water flows from the region of higher concentration (of water) to one of lower concentration. Result is that level of water rises (in right side of container). This is one major mechanism for the rise of water in the xylem of plants/trees.


Reverse Osmosis


Two water purification techniques:

Distillation Reverse osmosis

RO isn’t an exotic method any more!


Access to safe drinking water varies widely across the world


So how does hydraulic fracturing affect the water issue? The 2012 legislative session of the North Carolina General Assembly concluded with legalization of hydraulic fracturing and horizontal drilling for exploration and development of shale gas in North Carolina. Over the past year, North Carolina legislators, policy makers, scientists, independent experts, local governments, state environmental and commerce agencies, and a variety of other stakeholders have studied and debated the potential for shale gas development in North Carolina and potential enabling legislation for hydraulic fracturing and horizontal drilling. This process concluded on July 2, 2012, when the General Assembly voted to override a gubernatorial veto of the enabling legislation, Senate Bill 820 also known as the Clean Energy and Economic Security Act (the “Act”).

This process began in June 2011, when the General Assembly directed the North Carolina Department of Environment and Natural Resources (“DENR”), the Department of Commerce, and the Department of Justice to “study the issue of oil and gas exploration in the State and the use of directional and horizontal drilling and hydraulic fracturing for that purpose.”[1] Further, the 2011 law directed DENR and the other agencies to evaluate the potential shale resource in North Carolina and methods of natural gas extraction and to conduct an analysis of potential economic impacts, environmental impacts, social impacts, consumer protection issues, infrastructure issues, as well as potential oversight and administrative issues related to shale gas development in North Carolina.

These agencies completed the comprehensive study and published the report, North Carolina Oil and Gas Study under Session Law 2011-276, on April 30, 2012. Most notably, the report concluded that “After reviewing other studies and experiences in oil and gas-producing states, DENR has concluded that information available to date suggests that production of natural gas by means of hydraulic fracturing can be done safely as long as the right protections are in place. Production of natural gas by means of hydraulic fracturing can only be done safely in North Carolina if the state adopts adequate safeguards in the form of regulatory standards specifically adapted to conditions in the state and invests sufficient resources in compliance and enforcement.”[2] In light of the DENR report, legislators introduced the Act, when the 2012 legislative session convened in May to legalize hydraulic fracturing and horizontal drilling.

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CH05 Chem