Unit 1.1 Properties of Water

Unit 1.1 Properties of Water Chemistry Review Matter  Anything that takes up space and has mass o Rocks, metals, oils, gasses, organisms, etc. are all forms of matter Element  A substance that cannot be broken down into other substances by chemical reactions o 92 elements occur in nature o Periodic table Compound  A substance consisting of two or more different elements combined in a fixed ratio o H2O o NaCI Essential elements: of the 92 naturally occurring elements 20-25% are essential to survive and reproduce. CHON make up 96% of living matter. Trace elements: of the 92 naturally occurring elements, these are required by an organism in very small quantities CHON- Carbon, hydrogen, oxygen, and nitrogen Group: elements in the same vertical column have the same number of valence electrons Period: elements in the same horizontal row have the same total number of electron shells Bohr Model  Show electrons orbiting the nucleus of an atom  Electrons are placed on shells around the nucleus  Each shell is a different energy level and can hold up to a certain number of electrons o 1st shell: 2 eo 2nd shell: 8 eo 3rd shell: 18 e- Examples: Lithium Lewis Dot Model  Simplified Bohr diagrams o Does not show energy levels  Only shows electrons in the valence shell (outermost shell)  Electrons are placed around the element symbol Elements want to be stable How do elements achieve this? bonds ⇨ Atoms with incomplete valence shells can interact ⇨ Form chemical Chemical Bonds: an attraction between two atoms, resulting from the sharing or transferring of valence electrons Electronegativity: the measure of an atom’s ability to attract electrons to itself Sharing of electrons by two atoms  Form molecules and compounds Single bond: 1 pair of shared e Double bond: 2 pair of shared e Triple bond: 3 pairs of shared e - ⇨ Nonpolar covalent Electrons are shared equally between two atoms  O2 Double bond: 2 pairs shared e- ⇨ Polar covalent Electrons are not shared equally between two atoms  H2O Unequal sharing of e - results in partial charges on oxygen and hydrogen Ionic Bonds The attraction between oppositely charged atoms (ions)  Forms ionic compounds and salts o NaCI o LiF Transfer of electrons from one atom to another atom forms ions  Cation: positively charged ion  Anion: negatively charged ion o The attraction between the anion and cation forms the ionic bond Hydrogen Bonding The partially positive hydrogen atom in one polar covalent molecule will be attracted to an electronegative atom in another polar covalent molecule  Intermolecular bond: bond that forms between molecules Why does this happen? When a hydrogen atom is bonded to an electronegative atom (usually O, N, or F) the electrons are not being shared equally between atoms (remember: this is a polar covalent bond)  This causes the hydrogen to have a partial positive charge and the electronegative atom to have a partial negative charge Example: hydrogen bonds between water molecules Properties of Water Polarity Unequal sharing of the electrons make water a polar molecule Cohesion Attraction of molecules for other molecules of the same kind  Hydrogen bonds between H 2O molecules hold them together and increase cohesive forces  Allows for the transport of H2O and nutrients against gravity in plants  Responsible for surface tension  Surface tension: property allowing liquid to resist external force Cohesion: H2O molecules stick together Adhesion The clinging of one molecule to a different molecule  Due to the polarity of H2O  In plants, this allows water to cling to the cell walls to resist the downward pull of gravity Adhesion: H2O molecules stick to the xylem wall Capillary Action The upward movement of water due to the forces of cohesion, adhesion, and surface tension  Occurs when adhesion is greater than cohesion o Important for transport of water and nutrients in plant Capillary action occurs moving water upwards Temperature Control High Specific Heat H2O resists changes in temperature How?  Hydrogen bonds! Heat must be absorbed to break hydrogen bonds, but heat is released when hydrogen bonds form 1. Moderates air temperature o Large bodies of water can absorb heat in the daytime and release heat at night 2. Stabilizes ocean temp o Benefits marine life 3. Organisms (which are made primarily of H2O) can resist changes in their own internal temp Temp control properties of H2 O stem from these hydrogen bonds Evaporative Cooling Water has a high heat of vaporization (same reasons as high specific heat) The molecules with the highest kinetic energy leave as gas  Moderates Earth’s climate  Stabilizes temp in lakes and ponds  Prevents terrestrial organisms from overheating (think sweating in humans)  Prevents leaves from becoming too warm in the sun Floating Ice As water solidifies it expands and becomes less dense  Due to the hydrogen bonds o When cooled, H2O molecules move too slowly to break the bonds  Allows marine life to survive under floating ice sheets Hydrogen bonds cause water molecules to form a crystalline structure Quick! Think, Pair, Share Imagine the 3D crystalline structure of ice. How many hydrogen bonds can one molecule of water make with its neighboring water molecules? Answer: four Solvent Water is a versatile solvent: its polar molecules are attracted to ions and other polar molecules it can form hydrogen bonds with Solution: homogenous mix of 2+ substances Solvent: dissolving agent in a solution Solute: substance that is dissolved  “Like dissolves like” o Water can interact with sugars or proteins containing many oxygen and hydrogen  Hydrogen bonds ⇨ Water will form hydrogen bonds with the sugar or protein to dissolve it Ionic compounds  The partially negative oxygen in water will interact with a positive atom  The partially positive hydrogen in water will interact with a negative atom  Dissolves ions Na+ surrounded by oxygen Cl- surrounded by hydrogen a. There are two hydrogen bonds in the diagram, both are represented by the dashed lines. b. For both hydrogen bonds shown, the hydrogens would have a partial positive charge and the oxygen would have a partial negative charge.