Water as a Polar Molecule TAKS: Objective 4 TEKS: 8D

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

Electronegativity  What is electronegativity and how does it affect an atom of an element??? With your shoulder partner, discuss the answer to the question and be ready to share with the class…….

Electronegativity Chemical property that describes the ability of an atom (or, more rarely, a functional group) to attract electrons (or electron density) towards itself in a compound  Looking at water, which element has a higher electronegativity? OXYGEN OXYGEN

Since oxygen has a higher electronegativity than hydrogen:  Electrons in molecule group closer to oxygen than with hydrogen  The oxygen side has a slight or partial negative charge  The hydrogen side has a slight or partial positive charge.

What does this do the water molecule????  The partial positive and negative ends of the molecule make it a “polar” molecule  There is an uneven distribution of electron density in the molecule

Hydrogen bond: Electrostatic attraction between partial positive charge near hydrogen atoms and partial negative charge near oxygen atom Weak bonds Non-covalent bonds Effective solvent

PROPERTIES OF WATER

Brainstorm!!! This polar condition is a great characteristic that all water molecules have…….why do you think????? With your shoulder partner discuss how is this characteristic of water beneficial to us? Be prepared to share with the class……

Polarity of Water   Makes it an effective solvent Polar attracts polar and dissolves (“like dissolves like”)   Dissolves other polar compounds Ionic compounds dissolve, they break up into positive and negative ions Example: Water molecules surround sodium ions and chloride ions, thus dissolving the salt

Many other unique properties of water are due to the hydrogen bonds.  For example, ice floats because hydrogen bonds hold water molecules farther apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. Therefore, the density decreases.

Application   Because ice is less dense than water, lakes begin to freeze from the top down.   The layer of ice that forms on a surface of a lake helps shields the water underneath from cold air temperatures.   This tends to keep lakes from freezing solid and killing the organisms that live there.

High Boiling Point  On average, each water molecule interacts with molecule interacts with about four others. about four others.  In water vapor, the molecules are too far apart for hydrogen bonding to occur.  All hydrogen bonds in liquid water must be broken in order to boil water  Breaking those bonds takes energy, thus the high boiling point for water.

Cohesion The attraction of one water molecule to another resulting from hydrogen bonding. The attraction of one water molecule to another resulting from hydrogen bonding.hydrogen bondinghydrogen bonding By placing a drop of water on a surface you can directly observe cohesion in the resistance that water droplet shows to wetting, i.e., water clumps up in a pile despite being a liquid, rather than spreading out over the surface. By placing a drop of water on a surface you can directly observe cohesion in the resistance that water droplet shows to wetting, i.e., water clumps up in a pile despite being a liquid, rather than spreading out over the surface.

Adhesion  Similar to cohesion except adhesion involves the attraction of a water molecule to a non-water molecule. How are adhesion and cohesion taking place in this image??

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

Surface Tension  The result of attraction between molecules of a liquid which causes the surface of the liquid to act as a thin elastic film under tension

Cohesion, Adhesion, and Surface Tension Capillary Action

Cohesion, Adhesion, and Surface Tension Water adhesion to the walls of the tube is greater than water cohesion Surface tension increases and causes the meniscus Water begins to go up through the tube until it can no longer ascend due to the pull of gravity (capillary action)

 Properties of Water Quiz  1. Draw the structure of water. Include the partial charges of each atom.  2. Explain how a water molecule becomes a polar molecule?  3. How does a hydrogen bond form?  4. Explain the difference between adhesion and cohesion.  5. Explain how capillary action involves cohesion, adhesion, and surface tension of water.