“Water is H 2 O.”
-
Upload
iliana-schmidt -
Category
Documents
-
view
10 -
download
1
description
Transcript of “Water is H 2 O.”
“Water is H2O.”Today, we’ll see that water is the basis of how we measure mass.
Later this week, we will use H2O and some of its component atoms to exemplify molecules, compounds and atoms.
Matter has mass and occupies space.
A solution is a very fine mixture that eventually happens without mechanical agitation.
Definition of Matter: Details
•Matter: has mass and occupies space.
•Mass: how easy is it to accelerate the object. F = ma
•Acceleration: change in speed with time (dv/dt = d2x/dt 2 for you Calculus experts)
•Gravity provides a convenient, relatively constant, acceleration: F = mg where g = 9.8 m-s-2
Mass is measured indirectly• We could define one gram as the mass of
one cubic centimeter (or one cc) of water. • Then see how much a spring is
compressed with this mass on it under Earth’s gravity.
• For a good spring, an object that compresses it 2x more than one cubic cc of water must have 2 grams of mass.
• When you weigh something, you indirectly obtain its mass.
• The thing really being measured is a force, which has other units (kgms-2).
• We’ll practice with compound units soon.
1 cm
1 cm
1 cm
How would we measure mass without
Earth’s gravity?
Or no gravity at all?
http://spaceflight.nasa.gov/gallery/images/station/
View from insidethe spacestation.
Matter Occupies Space
What’s Space?
What does it mean to occupy the space?
In fact, for almost anything you can name, most of the space is not occupied!
Before we can understand that, we have to have ways to measure physical
properties.
Space is volume.
Our unit of length in chemistry is often the centimeter: one-hundredth of a meter.
Volume is length x width x height, or cm3
1 cm
1 cm
1 cm1 cm3 of water is one milliliter of water (one one-thousandth of a liter, and a liter is almost the same volume as a quart.)
S.I. Units* (Appendix C)
*The units have a bizarre history! On some other planet—did you know that there are billions of planets?—perhaps some other form of intelligent life is using altogether different units. Even here in the USA, we still dislike these units. Even most chemists prefer gram to kilogram, centimeter to meter, etc.
Length MeterMass KilogramTime SecondTemperature KelvinsNumber of something MoleElectric current AmpereLuminous intensity Candela
Prefixes—yes, you have to memorize them.
Prefix Example
Example means
Nano Nanometer 0.0000000001 m or 10-9 m
Micro Microliter 0.000001 L or 10-6 L
Milli Milligram 0.001 gram
Centi Centimeter 0.01 meter
Deci Deciliter 0.1 liters (= 100 milliliters, by the way)
Deka Dekameter 10 meters
Hecto Hectogram 100 grams
Kilo Kilometer 1000 meters (=0.6 miles, by the way)
Mega MegaHz 1000 cycles per second, once a very fast computer!
Giga GigaHz 1,000,000 = 106 cycles per second, now an OK computer (barely)
Sm
alle
r th
an
Larg
er
than
Properties with Compound Units
Density is the mass per unit volume. For example, the liquid mercury in a thermometer has a density of 13.6 grams per cubic centimeter.
Units of density: mL
g
cm
g
3
Billiard ball floating in mercury.
http://www.zyra.org.uk/mercury3.htm
Why do we care?1. Density is an
important, easily measured property that lets us gauge the purity of substances.
2. Heavy stuff sinks, so that tells us what we might have to do to, say, clean up a hazardous waste site. The Hanford Nuclear Site: 50
million gallons (more?) of contaminated wastes on the beautiful Columbia River. Pricetag to clean: $40,000,000,000 = $4 x 1010.(I have seen much higher estimates, too).
http://www.whistleblower.org/article.php?did=18&scid=28http://murray.senate.gov/hanfordcleanup/index.cfm
Another property with a compound unit: force.
Defining physical relation
F=ma
Mass in kilograms, kg
Acceleration, m/s2
Resulting physical unit
2sm
kg
Called a Newton, nt
Still more complex: pressure
Defining physical relation
P = F/A
Force, Newtons
Area, m2
Resulting physical unit
22
2
2 smkg
msm
kg
mnt
Called a Pascal, Pa
These aren’t the usual units of pressure here in the USA! 101,325 Pa = atmospheric pressure = 14.7 p.s.i.
Example: most cars use 32 p.s.i. in their tires. What’s that in Europe?
See notes for harder problems—a little like the team homeworks.
p.s.i. = pounds per square inch
Today’s Announcements
Chancellor’s Distinguished Lecture Series
“The Dark Side of the Universe: Beyond the Stars and the Starstuff We Are Made Of”
Professor Michael TurnerUniversity of Chicago5:00 p.m. 130 Nicholson Hall
Daily Evaluations
We made the quota last Thursday; bonus points will be added.
Biggest concern: Teams! PLEASE GIVE IT A CHANCE.
ConversionsFigure out the smaller unit. You will need a larger number of those. Example: Q. How many deciliters in 10 Liters? A. Deciliters is smaller, so you will need
more of them. Ten deciliters for each liter times 10 liters = 100 deciliters.
Or, you can do it very prim and proper (Appendix C of your
text)
deciliter 1Liter 1.0
1
1Liter 10
deciliter 1
.
1. Write the given: 10 liters2. Write the conversion: 1 deciliter = 0.1 liter3. Divide both sides of the conversion by right side: 4. Divide both sides of the conversion by left side:
5. Multiply the “given” from step 1 with whicheverform of the number one from steps 3-4
cancels the undesired unit.
deciliters 100Liter 0.1
deciliter 1Liter 10
Try to learn to line things out
deciliters 100Liter 0.1
deciliter 1Liter 10
Start by putting what you want at the left. Arrange conversion factors so as to cancel out the unit you do not want!
Despite the metric system, you must still to be able to convert between
different unit systems. Example: the smallest MOPAR V8 engine of 1965 was 273 cubic
inches. Today, we’d quote that in liters, but how many liters?
Reason it out: A liter is like one quart, a cubic inch is like your thumb.You will need a smaller number.
1 inch = 2.54 centimeters (Appendix C of text)
L 4.5mL 1000
L 1cm 1mL 1
in 1cm 2.54
in 273 3
33
You may find it much easier at first to do these problems one
step at a time.
33333 cm 16.4cm (2.54) inch 1
For example, just compute the conversion factor for in3 to cm3 first.
Cube both sides
cm 2.54 inch 1
Now what?
Problem for your team: estimate the volume of this
room in milliliters.
This will also teach us a bit about scientific notation.
Mapping: conversions that involve more than
multiplication & division. Example:
Centigrade to Fahrenheit
1.832F
Co
o
oC
oF
0
32
Slope = 1.8
32C1.8F oo
Kelvin temperature
273.15CK o
oC
K
0
273.15 Slope = 1
Converting oC to oF in your head
Double the oC temperature.
Subtract 10%.
Add 32.
16 oC = 61 oF July in Duluth or January in
Baton Rouge
98.6 oF = 37 oC Body Temperature or
Summer in Dallas
- 40 oF = - 40 oC Winter in Duluth, but hardly
the coldest day
100 oFRectal temperature of the
cow
77 oC = 25 oC Room Temperature
77 K = -196.15 oC = -321 oF Liquid N2
Facts of Life
Stop here?
Elements
If all the atoms in a given sample of "material" are
"the same" then the chemical is said to be an
elemental form. Actually, almost the same!
Another definition of elements
Difficult to purify further by the means available to a 1930’s chemist.
Example: •It is easy to break water down into oxygen and hydrogen. •It gets a lot harder after that. •Oxygen is an element. •Hydrogen is another element.
There are about 118 elements….some of them man-made.
This will become more and more clear asthe semester progresses.
Atoms: Trust me (for now)
The word means “indivisible” Atoms are NOT indivisible! Much effort went into cutting them
up to figure out how they work. We will cover that fascinating history
later.
We think atoms look like this
All the mass is here!
The light red spaceis mostly empty!
That is why almost everything is really empty!
Some elements exist as single atoms.
Example: Xe, Ne
www.theroadwanderer.net/ 66NMex/gallneon.htm
Some elements exist as arrays of atoms with undefined size.
Example: Carbon as diamond
http://www.phy.mtu.edu/nue/CubeHover_small.htmlhttp://www.phy.mtu.edu/nue/CubeHover_small.html
The 45.52 carat Hope Diamond
Example: Carbon as graphite (the little disk floating above
the cubical magnets, same stuff as pencil “lead”!)
Of huge interest to chemists, and almost nobody else, some elements
exist as molecules.Example: Hydrogen as H2
Molecules?
Molecules
Normal Oxygen, O2
Ozone, O3
Molecules can involve different types of atoms.
H2
O
A molecule is a compound, but not all compounds are
molecules.Molecule of water: Barring total destruction, as in electrolysis,these two hydrogens and the oxygen will stay with each other forever.
Sodium ion, Na+
Chloride ion, Cl-
A tiny chunk of the compound, sodium chloride. Not a molecule.
Learn to draw it yourself: http://www.chemguide.co.uk/atoms/structures/ionicstruct.htmlhttp://www.chemguide.co.uk/atoms/structures/ionicstruct.html
The difference between molecules and compounds is like the difference between marriage and dating.
Molecules = Atomic Marriage: you stay with a particular atom no matter what (barring real torture, such as electrocution—i.e. electrolysis—or something equally painful, like a chemical reaction).
Compounds = Atomic Dating: it’s a lot easier to break up and start seeing other atoms. Later, you might get back together with the original atom….but probably not. Thus, sodium chloride can be easily dissolved. If you dry out that saltwater solution, the sodium and chloride atoms will attach to different partners.
Different Representations of Molecules
A good time to check out the ACS
Molecule of the Week
ACS is the world’s largest scientific society: 118,452 members.
Daily Evaluation/Amusement
Conversions!Cows! Notes. Meanwhile, download
lectures!Do we need to know the m/s velocity
stuff?
Physical States of Matter: Physical vs. Chemical
Transformations •Big 3 are: Liquid, Solid, Gas•Other, more nebulous states exist: glass, liquid crystal •Ice to water to steam are physical transformations: •no real change in chemical formula.•Water electrolysis to Hydrogen and Oxygen gas
is a chemical transformation: not only does
the formula change, but a molecule is broken. •In many practical operations, both physical and
chemical transformations take place.
Polymer liquid crystal: J. Chem. Phys., 111(4), 1746-1752 (1999).
Isomers (Iso=same; mers=parts)
Chemical EquationsChemists are good at converting one compound to another. We have discovered a few rules about doing this:
1. The total mass is never changed.
2. Atoms are not changed or lost.
3. How the atoms are arranged, and which partners they choose,
can change a lot.
Balancing Chemical Equations
1 H2(g) + 1 Cl2(g) 2 HCl(g)
H atoms on left: 1x 2 = 2Cl atoms on left: 1 x 2 = 2
H atoms on right: 2 x 1 = 2Cl atoms on right: 2 x 1 = 2
Burning Hydrogen
2H2 + O2 2H2O
Ooops…The blue H atoms should be smaller than the orange O atoms.
Another Example
The decomposition of potassium chlorate to potassium chloride and oxygen:
KClO3 KCl + O2
KCl Problem
Checkit, Dammit!2 KClO3 ----» 2 KCl + 3 O2
AtomOn left
sideOn right
sideK 2 x 1 = 2 2 x 1 = 2
Cl 2 x 1 = 2 2 x 1 = 2
O 2 x 3 = 63 x 2 = 6
The Secret of Balancing (and life)
Do it a lot! DRILL!(You have lots of homework like
this.)Keep trying! PERSIST!
It is trial and error, so GUESS!
Try This Now!
___CH4 + ___O2 ___H2O + ___CO2
Compute the force in Newtons exerted by your body mass when you stand on a scale.
Blank Screen
Spare Stuff from Appendix
Precision vs. Accuracy
It is possible to be consistently wrong.
Significant FiguresWrong Right Why
1.867 + 2 = 3.867 1.867 + 2 = 4 Addition/Subtraction: use least precise places
1.867 + 2.000 = 4 1.867 + 2.000 = 3.867 "
1.867 x 2 = 3.734 1.867 x 2 = 4 Multiply/Divide: use smallest number of significant figures
1.867 / 2.000 = 1 1.867/2.000 = 0.9355 "
What does this read?
Exponential (Scientific) Notation
Avogadro’s number (the number of H2O molecules in 18 grams of
water):602,000,000,000,000,000,000,000 = 6.02 x 1023
Age of universe in seconds (somebody’s guess):300,000,000,000,000,000 s = 3 x 1017
Learn to type exponents efficiently into your calculator! (The E or EEX button)
Sample problems
How many clock cycles does a 3GHz Pentium computer go through in one day?
How many molecules of water in a nanogram of water?