Water as a Polar Water as a Polar MoleculeMolecule

TAKS: Objective 4TAKS: Objective 4

TEKS: 8DTEKS: 8D

The Polarity of WaterThe Polarity of Water

Water has a simple Water has a simple molecular structure. It molecular structure. It is composed of one is composed of one oxygen atom and two oxygen atom and two hydrogen atoms. Each hydrogen atoms. Each hydrogen atom is hydrogen atom is covalently bonded to covalently bonded to the oxygen via a the oxygen via a shared pair of shared pair of electrons. electrons.

RecallRecall

What is electronegativity????What is electronegativity????

chemical property that describes the ability chemical property that describes the ability of an atom (or, more rarely, a functional of an atom (or, more rarely, a functional group) to attract electrons (or electron group) to attract electrons (or electron density) towards itself density) towards itself

Looking at water, which elLooking at water, which element has aement has a higher electronegativity?higher electronegativity?

OXYGENOXYGEN

Since oxygen has a higher electronegativity Since oxygen has a higher electronegativity than hydrogen, the electrons of the than hydrogen, the electrons of the molecule tend to group closer to the molecule tend to group closer to the oxygen than to the hydrogen atoms. This oxygen than to the hydrogen atoms. This means that even though the molecule as a means that even though the molecule as a whole is neutral, the oxygen side has a whole is neutral, the oxygen side has a slight negative charge while the hydrogen slight negative charge while the hydrogen side has a slightside has a slight positive charge. positive charge.

Therefore, water is said to be a "polar" molecule, which means that there is an uneven distribution of electron density.

The electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond

Hydrogen bond [H-bond] Hydrogen bond [H-bond] A A hydrogen bondhydrogen bond is a non-covalent bond is a non-covalent bond

between a partial negative charge and a between a partial negative charge and a partial positive charge. partial positive charge.

Hydrogen bondsHydrogen bonds tend to be weak. tend to be weak. Hydrogen bondsHydrogen bonds tend to be transient. tend to be transient. Hydrogen bonds Hydrogen bonds are very numerous which are very numerous which

somewhat offsets their weak and transient somewhat offsets their weak and transient nature. nature.

On average each On average each water moleculewater molecule in liquid in liquid waterwater is is hydrogen bonded hydrogen bonded to 3.4 other to 3.4 other water water molecules.molecules.

Many other unique properties of Many other unique properties of water are due to the hydrogen bonds. water are due to the hydrogen bonds.

For example, ice floats because hydrogen bonds For example, ice floats because hydrogen bonds hold water molecules farther apart in a solid than hold water molecules farther apart in a solid than in a liquid, where there is one less hydrogen bond in a liquid, where there is one less hydrogen bond per molecule. Therefore, the density decreases.per molecule. Therefore, the density decreases.

High Boiling PointHigh Boiling Point

On average, each waterOn average, each water molecule interacts withmolecule interacts with about four others. about four others.

In water vapor, the molecules are too far In water vapor, the molecules are too far apart for hydrogen bonding to occur. This apart for hydrogen bonding to occur. This means that in order for us to boil water we means that in order for us to boil water we must break all the hydrogen bonds in must break all the hydrogen bonds in liquid water. Breaking those bonds takes liquid water. Breaking those bonds takes energy, thus the high boiling point for energy, thus the high boiling point for water.water.

Cohesion Cohesion

The attraction of one The attraction of one water moleculewater molecule to to another resulting from hydrogen bonding. another resulting from hydrogen bonding.

By placing a drop of By placing a drop of waterwater on a surface you on a surface you can directly observe cohesion in the can directly observe cohesion in the resistance that resistance that waterwater droplet shows to droplet shows to wettingwetting, i.e., , i.e., waterwater clumps up in a pile clumps up in a pile despite being a liquid, rather than spreading despite being a liquid, rather than spreading out over the surface. out over the surface.

AdhesionAdhesion Similar to cohesion except Similar to cohesion except adhesionadhesion

involves the attraction of a involves the attraction of a water moleculewater molecule to to a non-a non-waterwater molecule. molecule.

How is adhesion takingHow is adhesion taking

place in this image??place in this image??

Surface TensionSurface Tension The water molecules The water molecules

on the surface have on the surface have partners for hydrogen partners for hydrogen bonding only within the bonding only within the liquid; above the water liquid; above the water surface there are no surface there are no more molecules more molecules available for hydrogen available for hydrogen bonding. This means bonding. This means that molecules at the that molecules at the surface experience a surface experience a net force pulling them net force pulling them inward. inward.

Properties of WaterProperties of Water 11. . a. Draw the structure of water. Include the a. Draw the structure of water. Include the

partial charges of each atom.partial charges of each atom. b. Why is water considered to be a polar b. Why is water considered to be a polar

molecule?molecule? 2. a. What enables neighboring water molecules to 2. a. What enables neighboring water molecules to

hydrogen-bond to one another?hydrogen-bond to one another? b. How many hydrogen bonds can each water b. How many hydrogen bonds can each water

molecule form?molecule form? 3. Explain the difference between adhesion and 3. Explain the difference between adhesion and

cohesion. Give an example of each.cohesion. Give an example of each. 4. What is surface tension? Give an example.4. What is surface tension? Give an example. 5. Explain why water is considered to have a high 5. Explain why water is considered to have a high

boiling point.boiling point.