Water, Polar molecules, and an Intro to Solutions (p264 -280) Lesson 1 December 5th, 2011

Review of bond types Intermolecular Force Bonding Model Ionic Involves electron transfer, forms cations and anions Cations and anions are attracted to each other

Review of bond types Intermolecular Force Bonding Model Polar Covalent Involves unequal sharing of pairs of electrons by atoms of two different atoms Bonds can involve 1, 2, or 3 pairs of electrons.

Review of bond types Intermolecular Force Bonding Model Covalent Involves equal sharing of pairs of electrons Bonds can involve 1, 2, or 3 pairs of electrons.

Polar Molecules If a molecule contains polar covalent bonds, the entire molecule may have a positive end and a negative end. This would then be classified as a polar molecule.

Polar molecules Not all molecules that contain polar covalent bonds are polar molecules. This can be due to the shape of the molecule.

Guidelines for predicting polar and nonpolar molecules Type Description Examples Polar AB HAx AxOH OxAy NxAy Diatomic compounds Any molecule with a single H Any molecule with a single OH at one end Any molecule with an O at one end Any molecule with an N at one end CO(g) HCl(g) C2H5OH(l) H2O(l), OCl2(g) NH3(g), NF3(g)

Guidelines for predicting polar and nonpolar molecules Type Description Examples Non polar Ax CxAy All elements Most carbon compounds (including organic solvents, fats and oils) Cl2(g), N2(g) CO2(g), CH4(g)

Intermolecular Forces Van der Waals forces Dipole dipole – attractive force between polar molecules London dispersion force – attractive force between all molecules

Hydrogen bonds A strong dipole-dipole force between a positive hydrogen atom of one molecule, and a highly electronegative atom (N, O, F) in another molecule.

Water: Essential for Life 70% of the earth is covered in water yet only 3 % of it is fresh water and only 1 % of this is water is in a liquid state. Canada has the most abundant supply of fresh water in the world but this doesn’t mean that we shouldn’t learn how to conserve water.

The water cycle is one method of natural purification The water cycle is one method of natural purification. Energy from the sun causes water to evaporate, dissolved contaminates are left behind as it rises. When the water condenses it returns to earth as precipitation.

Physical properties of water Pure water is colourless, odourless, and tasteless. Whether or not an object sinks or floats relates to its density compared to water. Pure water density = 1.0 g / mL

Water is unique in the fact that its density decreases when it becomes a solid, this is what causes ice to float. This in turn acts a blanket which prevents the water underneath from freezing and killing all life.

Heat capacity is a measure of how much heat it takes to heat one gram of a substance 1 ° C. Water requires 4.18 J of heat to increase the temperature of 1 gram 1 ° C.

Chemical Properties of Water Water is a polar molecule that consists of oxygen and hydrogen molecules. The oxygen has a larger electronegativity and so it creates a partial – charge as it draws hydrogen’s electrons closer to it. Draw this

The large difference in charge allows for Hydrogen bonding to occur between molecules. This provides water with its unique surface tension, high boiling point and ice that is less dense. (see page 87 fig 7 for diagram)

The polarity of water also allows water to dissolve other polar molecules. Like dissolves Like – polar dissolves polar and non-polar dissolves non-polar

Ionic substances disassociate into charged ions when dissolved in water form electrolytic solutions – conduct electricity Draw this

Substances that stay electronically neutral when dissolved in water are called nonelectrolytes. Ex glucose

Due to waters great ability to be a solvent it can become contaminated easily.

What’s in Polluted Water? As the amount of human activity increases the amount of contaminants found in nearby water also increase. Contaminants are classified into three types.

1. Physical Contaminants Objects that do not dissolve in water. Ex – oil, plastic, tree branches, leaves, peat, silt.

2. Biological Contaminants Biological contaminants include bacteria and viruses which may make the water unsafe to consume.

3. Chemical Contaminants Chemicals that are soluble in water. Ex – metal ions, pesticides, fertilizers. These are not visible but can lead to visible effects such as an algae bloom

Solutions Most substances contain water because it is so good at dissolving other substances. AKA the “Universal Solvent” Technically it is not universal because water does not dissolve non-polar substances. Only non-polar can dissolve non-polar. All aqueous solutions use water as the solvent and are transparent.

A solution is a homogeneous mixture of substances composed of at least one solute and one solvent that are uniform throughout the sample. This can be in a liquid or gas state.

Solute – A substance that is dissolved in a solvent Solvent – The medium in which a solute is dissolved.

Properties of Aqueous Solutions Compounds can be classified as either electrolytes or non-electrolytes. Electrolytes – solutes that form aqueous solutions that conduct electricity.

Non-electrolytes – solutes that form aqueous solutions that do not conduct electricity.

Most molecular compounds are nonelectrolytes, except for acids. Solutions can also be classified as acids, bases, or neutral. These will be covered more in the next unit. Questions – P 276 figure 6- predict solubility, p 269 # 8, p 271 # 4, 5, 6, p 277 # 4,5, p 279 # 9-12, p280 # 2-4