Intersection 8: Aqueous Chemistry

10/24/05

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Exam 2 (45 points)

• Tuesday, October 31 8-10pm CHEM 1400• Topics: (through 10/27)

– Polarity (Lewis structures, VSEPR)– Intermolecular forces– Thermochemistry– Solutions chemistry (dilution, precipitation,

measuring concentration)

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Mole Day 6:02am-6:02 pm 10/23

Q: What element do moles love to study in chemistry?

http://www.moleday.org/

A: Molybdenum

Q: What do you get when you have a bunch of moles acting like idiots?

A: A bunch of Moleasses

Q: What did Avogadro teach his students in math class?A: Moletiplication

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Outline

• Finish thermochemistry• Revisiting spectroscopy• Aqueous notes/Water Project• Scientific Inquiry

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H2O(s) → H2O(l) H >0

H2O(l) → H2O(g) H >0 H2O(s) → H2O(g) H >0

H2O(l) → H2O(s) H <0

H2O(g) → H2O(s) H <0H2O(g) → H2O(l) H <0

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Problem 5

Suppose that you add a small ice cube to room temperature (25oC) water in a coffee cup calorimeter. What is the final temperature when all of the ice is melted? Assume that you have 200. mL of water and that the ice cube weighs 15.0 g and is at 0oC before being added to the water.

Heat of fusion of water = 333 J/g

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Problem 6

If you want to convert 56.0 g ice (at 0oC) to water at 75.0oC, how many grams of propane (C3H8) would you have to run to supply the energy to melt the ice and then warm it to the final temperature?

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Silicon-Based Life

Space slugs – Star Wars

Is silicon-based life likely to be more stable or less stable than carbon-based life?

J. Chem. Ed. 1988, 65, 414.

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Stability of Silicon vs Carbon

• Consider decomposition of alkanes and silanes to the elements (no oxygen in space for that slug to worry about).

• Consider combustion of silanes and alkanes (will those silicon-based life forms make it on earth?).

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Thermochemical Behavior with Respect to Reversion to the Elements

Si2H6 (g) → 2 Si (s) + 3 H2 (g) C2H6 (g) → 2 C(s) + 3 H2 (g)

Hof Si2H6 (g) = 80.4 kJ/mol Ho

f C2H6 (g) -84.7 kJ/mol

Hrxn = 0 + 0 – 80.4 = -80.4 kJ/mol

Exothermic !

Hrxn = 0 + 0 – (-84.7) = 84.7 kJ/mol

Endothermic!

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Thermochemical Behavior with Respect to Combustion

7/2 O2 (g) + Si2H6 (g) → 2 SiO2 (s) + 3 H2O (g)

Hf =Si2H6 (g) = -80.4 kJ/molSiO2 (s) = -911 kJ/molH2O(g) = -242 kJ/molCO2 (g) = -394 kJ/molC2H6 (g) = -85 kJ/mol

Hrxn = [2(-911) + 3(-242)] – [1(-80.4)] = -2468 kJ

Exothermic !

Hrxn = [2(-394) + 3(-242)] –[1 (-85)] = -1429 kJ

Exothermic!

7/2 O2 (g) + C2H6 (g) → 2 CO2 (s) + 3 H2O (g)

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Silicon-Based Life

Space slugs – Star Wars

Is silicon-based life likely to be more stable or less stable than carbon-based life?

Thermochemical considerations indicate that silicon-based life would tend to decompose exothermically!

Our space slug unlikely to survive.

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Muskov Case Study

How did you determine if the level of nitrate in the well was higher than the MCL?

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NO3- + Zn + 2H+ NO2

- + Zn+2 + H2O

HO3S NH2 + NO2- + 2H+ HO3S N N + 2 H2O

HO3S N N NH2+ HO3S N N NH2 + H+

magenta

diazonium cation

1-naphthylamine

(1)

(2)

(3)

sulfanilic acid

4-[(4-amino-1-naphthalenyl)azo]- benzenesulfonic acid

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Measuring Concentrations of Contaminants

• Utilize spectroscopy if the molecule absorbs light.

PPo

bsample

A = log10 P0 / PA = log10 1 / T A = log10 100 / %TA = 2 - log10 %T 

 

Transmittance, T = P / P0

% Transmittance, %T = 100 T

Absorbance,

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What kind of relationship do absorbance and concentration have? Describe why and how you would use absorbance to find concentration? (use an equation in your description)

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Beer-Lambert law: A=bc A absorbance (no units, since A = log 10 P0 / P )

molar absorptivity with units of L mol-1 cm-1. Molar absorptivity is a constant unique to every colored molecule at a particular wavelength.

b is the path length of the sample, that is, the length that the light must travel through the sample expressed in centimeters.

c is the concentration of the molecule in solution, expressed in mol L-1 (M)

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A compound with a high molar absorptivity is very effective at absorbing light (of the appropriate wavelength); a very low concentration of this compound will have a high absorption. A compound with a low molar absorptivity absorbs less light per unit concentration, so it will take a higher concentration of this compound to reach the same absorbance.

Molar Absorptivity

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Path Length

The Beer-Lambert Law implies that the fraction of the light absorbed by each layer of solution is the same.

cm 0 0.2 0.4 0.6 0.8 1.0

Abs 0.0 0.3 0.6 0.9 1.2 1.5

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One note on case study: Anti-bacterial Soaps

• “Normal usage" (according to directions) of these products will destroy some beneficial bacteria. Fortunately, the normal bacteria population within the septic system is sufficient and adequate to quickly recover. Significant treatment problems, with conservative use, should not occur. Excessive use of these products in the home can cause significant and even total destruction of the bacteria population.”

• University of Arizona http://ag.arizona.edu/pubs/water/az1258.html

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Aqueous Chemistry

Picture from: http://www.twingroves.district96.k12.il.us/Wetlands/WaterLilies/WaterLily.html

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Why is water important?

• Support life (cook, food, heat/cool, economy)

• Clean• Transportation• Recreation• More plants and animals live in water than

on land

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What are common water issues?

• Pollution/health• Ownership• Who polices it• Who cleans it up (who pays)

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Distribution of Drinking Water on EarthSource Volume

(thousands of km3)Percent of total water

Oceans 1,320,000 97.3

Icecaps and Glaciers 29,200 2.14

Groundwater 8350 0.61

Freshwater lakes 125 0.009

Saline Lakes and Inland Seas

104 0.008

Soil Moisture 67 0.005

Atmospheric water 13 0.001

Rivers 1.25 0.0001

From: Kegley, S. E., Landfear, D., Jenkins, D., Gross, B., Shomglin, K. Water Treatment: How Can We Make our Water Safe to Drink? Norton: 2004.

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Define the desirable characteristics of drinkable water.  

Is the best water supply composed of 100% H2O?

What is the water source for our community?

How many of you have lived someplace where there was a septic system? Where well water was used as the drinking source?

Drinkable Water

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What’s in water that is harmful?

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Drinking Water StandardsCongress passed the Safe Drinking Water Act in 1974. Amended in 1986 and 1996, the law requires the EPA to set standards for contaminant levels such that the concentrations of these species do not threaten public health and safety.

1) Primary contaminants: those substances that may cause serious health problems above certain concentrations.

2) Secondary contaminant: substances that are not toxic but detract from water quality, affecting such characteristics as appearance, smell, and taste.

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EPA StandardsMCL maximum contaminant level enforceableMCLG maximum contamination level goals

MCL (mg/L) MCLG (mg/L)

CN- 0.2 0.2Hg 0.002 0.002

As 0 0.10 (as of 1/23/06)

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Question 1Figure 1 represents a 1.0 L solution of sugar dissolved in water.

The dots in the magnification circle represent the sugar molecules. In order to simplify the diagram, the water molecules have not been shown.

Which response represents the view after 1.0 L of water was added (Figure 2).

12 dots

12 dots7 dots6 dots 24 dots

What’s added to water intentionally?

• What may you want to be there?– Bubbles– Caffeine– Vitamins– Soy– Sugar– Flavors– Fluoride

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Bottled Water• According to the Beverage Marketing Corporation,

manufacturers sold more than $7.7 billion worth of bottled water in the United States in 2002.

• The average U.S. consumer drank 21 gallons of bottled water,

• A 1997 report published in International Journal of Sports Nutrition found that in 52 healthy nonsmoking adults, who exercised 30 minutes two to four times a week, drinking flavored water actually increased fluid consumption by 20 percent.

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Water Project

Friday 11/3 Proposal Meeting Mon 11/13, 11/20 Lab Work Days       Wed 11/29            Poster Session Planning Mon 12/11  Poster Session

Paper Due 

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ProposalHypothesis: Use one to two sentences to describe a testable

scientific hypothesis that will be the focus of your mini-proposal.

Significance: Use two to three sentences to say why testing this hypothesis is significant.

Planned Experiment: Write a concise paragraph that describes your planned experiment. This should include details of obtaining field samples, laboratory experiments planned, instrumental analysis planned etc.

Anticipated Results: Write several sentences describing the likely positive and negative outcomes of your experiment.

Possible Conclusions: Write several sentences describing the conclusions you could reach based upon the experiments you are proposing.

Instruments and Materials Needed: List the instruments you need to use and the materials you need.

See course website for template

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Proposals

Proposal Meetings during studio, Friday 11/3– 12 minutes– 1 hard copy of proposal for us to keep (in

template format)– No more than 5 minutes to present proposal

orally– Questions and discussion

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Other Facets of “Scientific Research”

• Does trial and error fit into scientific research?

• Does creativity fit into scientific research?

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Complexity: Molecules vs. Systems vs. People

• What are the differences?

• Do you still use the scientific method?

• Do you understand the consequences of your actions/results?

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Characteristics of Good Inquiry

• Investigations are conducted for different reasons: – Exploring new phenomena– Checking previous results– Test how well a theory predicts– Compare different theories

• Sometimes scientists can control conditions in order to obtain evidence. When that is not possible for ethical reasons, they try to observe as wide a range of natural occurrences as possible in order to discern patterns.

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Characteristics of Good Inquiry

• There are different traditions in science about what is investigated and how, but they all have in common certain beliefs about the value of evidence, logic, and good arguments. There is agreement that progress in all fields of science depends on intelligence, hard work, imagination, and even chance.

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Characteristics of Good Inquiry

• Scientists in any one research group tend to see things alike, so even groups of scientists may have trouble being entirely objective about their methods and findings. For that reason, scientific teams are expected to seek out the possible sources of bias in the design of their investigations and in their data analysis. Checking each other’s results and explanations help, but there is no guarantee against bias.

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Bias• Symphony trials• Unconscious bias (Reskin, B. Regional

Review, 2005, 33-37.)– Women, minorities, elderly, overweight http://implicit.harvard.edu/

• Bias even in scientific results…how?• Is bias an ethical issue?

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From: http://microbiology.georgetown.edu/education/bioscipoladv.html

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Science Policy

• How does research impact decisions about life/funding?

• Should research be published and open to everyone?– (milk/terrorism)

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Science Policy: BudgetM

Proposed Total Federal Budget, FY2006

Fiscal year 2006 runs from October 1, 2005 to September 30, 2006. The Bush Administration, in its proposed budget, did not include spending on the wars in Iraq or Afghanistan.

Other includes general science, space and technology; energy; agriculture; commerce and housing credit; community and regional development; general government; allowances; and undistributed offsetting receipts.

Source: Office of Management and Budget, Budget of the U.S. Government, FY2006, Historical Tables, Table 5-1.

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National Science Foundation (NSF)

The National Science Foundation (NSF) budget was cut in 2005

In 2006, there is a proposed increase of 2.4% to $5.6 billionMost increase allotted to facilities fundingMost programs receive a 1% increase (but still would be

below 2004 budget)Cuts in education and trainingTotal number of research grants funded in 2006: 63101 in 5 applications funded

From: http://www.aaas.org/spp/rd/prel06pr.htm

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Science Policy: Laws

• Environmental controls• Speed limits• BAC

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Arsenic

• MCL (maximum contamination limit) • 1976 initial MCL set to 50 ppb• Jan 16, 2001 set to 10 ppb• Feb 2001 10 ppb merits further review;

NAS study recommends 3 ppb the limit achieved using current treatment technologies

• Oct 31, 2001 set to 10 ppb

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MSF

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