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”