Section 13.1 Uniquely Water. Objectives Describe the uniqueness of water as a chemical substance...
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Transcript of Section 13.1 Uniquely Water. Objectives Describe the uniqueness of water as a chemical substance...
Section 13.1 Uniquely Water
ObjectivesDescribe the uniqueness of water as a
chemical substanceModel the 3-D geometry of a water moleculeRelate the physical properties of water to its
molecular structure
Water has many different usesAverage use in the U.S is 300 L/ person /day Water is essential to life- 60% of our body
weight is water Almost every chemical rxn that happens in
our bodies happens in a water environment
What is the Lewis Dot Structure of water?Is water a polar or non polar molecule?Explain?
Physical PropertiesWater has a simple formula- H2OMolecular mass = 18 u (g/mol)
Has unique properties from its electron distribution and 3-D arrangement Water is most often thought of as a liquid. But solid water (ice) and gaseous water (steam or vapor) also exist in large quantities on Earth Most substances tend to be more dense as solids than they are as liquids – water is an exception The density of solid water is less than liquid water
Water MoleculeLarge EN difference between the covalently
bonded H and O in water (e- shared unequally)
Bent shape, the poles of the positive and negative charge do not cancel – the water molecule is polar
Demo
Interparticle ForcesWater molecules will orient themselves with
the opposite poles of other water molecules O atoms from water molecules attract to H
atoms on other water molecules The connections between these molecules
are not full covalent bonds, but they are fairly strong -they are called hydrogen bonds
HYDROGEN BONDINGA connection or interparticle force between
atoms in one molecule and a highly EN atom (O, F or N) in another.
A hydrogen bond is an attractive force only, NOT a full covalent bond.
Hydrogen bonds are not strong, are easily broken
In pure water, each water molecule may form H-bonds with four other water molecules
GRAPHING Activity:1) Calculate the molar mass of each of the following substances, and enter into your data table. 2) Make a graph of molar mass (x-axis) versus boiling point (y-axis).3) Label both axes, and give your graph a title.4) Connect the plotted points in order of increasing molar mass value.5) Label the element/compound that represents each coordinate on the graph. (See sample below. This is only a sample, and NOT your final graph.)6) Answer the questions that follow. Formula B.P. Molar Mass (g/mol)H2 - 253°CNe - 246°CCO - 192°CCO2 - 78.5°CHBr - 67.0°CCCl4 76.0°CH2O 100°CCBr4 189°C
Graphing (cont)Example
Graphing (cont)1) What general trend seems to exist?2) Are there any substances out of place?3) Why is the boiling point so high? 4) What does it tell you about the nature of
interactions between particles?5) Pick three other substances and calculate ∆EN. 6) What types of bonds exist between the elements of these three substances? 7) How might you have predicted this?
Properties of waterHigh MP and high BP (for a small molecule) Liquid at RT (most small molecules are
gases)Solid water has less density than liquid water
(floats)Volume increases as density decreasesWater is the universal solvent – it can
dissolve a wide variety of solutes.
States of waterFor water to boil, the temp. must be increased,
adding enough energy to overcome the hydrogen bonds that are holding water molecules together
As water cools, its volume decreases and its
density increases – water molecules move less rapidly
Below 4ºC, water molecules begin to approach
the solid state which is highly organized
SURFACE TENSIONForce need to overcome intermolecular
attractions and break through the surface of a liquid
A water molecule forms a drop because of
surface tension
The higher the surface tension, the more
resistant the liquid is to having its surface brokenDemo…
CAPILLARITYWater has capillarity (capillary action) - rising of
liquid in narrow tubes or graduated cylinders.
A curved meniscus results: Water is attracted to itself and water is attracted to glass
Capillarity results from the competition between
interparticle attractive forces between molecules of liquid and attractive forces between the liquid and the tube that contains it
Water molecules can form H-bonds to O atoms in
the silicon dioxide that makes up glass (attracted to glass)Cohesion- water molecules attracted to each otherAdhesion – water molecules attracted to other
molecules (i.e. glass)
SPECIFIC HEATThe amount of heat (joules) needed to raise the
temperature of 1 g of substance by 1ºC Water has a high specific heat – 4.18 J/gºC To raise the temperature of 1 g of water by 1ºC,
you must add 4.18 J of heat To lower the temperature of 1 g of water by 1ºC,
you must remove 4.18 J of heatBecause water has a high specific heat – it takes
a long time for bodies of water to change temperature.
Water moderates Earth’s temperatures
Vaporization/ CondensationVaporization- change of state from liquid to a gas The amount of heat required to vaporize a liquid is called the heat of vaporization An endothermic process – energy-absorbing Condensation- change of state from a gas to a liquidAn exothermic process – energy-releasing
Water absorbs a great deal of heat when vaporized - has high heat of vaporizationloses a great deal of heat when it condenses Evaporation is vaporization from the surface of a liquid
Example: body regulates temp. by evaporating water from the surface of skin (sweat)
Water as a solventMost water on Earth is not pure, it is present
in solutions Water is difficult to keep pure because it is
an excellent solvent – important physical property
Water is called the “universal solvent”