Chapter 13 Water and Its Solutions Section 13.1 Uniquely Water

Objectives Describe the uniqueness of water as a chemical substance Model the 3-D geometry of a water molecule Relate the physical properties of water to its molecular structure

Water has many different uses Average 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 Properties Water has a simple formula- H2O Molecular 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 Molecule Large 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 Forces Water 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 BONDING A 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°C Ne - 246°C CO - 192°C CO2 - 78.5°C HBr - 67.0°C CCl4 76.0°C H2O 100°C CBr4 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 water High 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 decreases Water is the universal solvent – it can dissolve a wide variety of solutes.

States of water For 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 TENSION Force 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 broken Demo…

CAPILLARITY Water 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 other Adhesion – water molecules attracted to other molecules (i.e. glass)

SPECIFIC HEAT The 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 heat Because water has a high specific heat – it takes a long time for bodies of water to change temperature. Water moderates Earth’s temperatures

Vaporization/ Condensation Vaporization- 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 liquid An exothermic process – energy-releasing Water absorbs a great deal of heat when vaporized - has high heat of vaporization loses 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 solvent Most 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”