Chemistry Review & Water Chapters 2 & 3

YOU MUST KNOW The 3 subatomic particles and their significance The types of chemical bonds and how they form The importance of hydrogen bonding to the properties of water Four unique properties of water and how each contributes to life on Earth How to interpret the pH scale How changes in pH can alter biological systems The importance of buffers in biological systems

Bonding Review Covalent Bonds Atoms share electrons Can have single, double, triple bonds Relatively strong bonds Many different types for specific kinds of molecules (ex. peptide, glycosidic, phosphodiester) Can be polar or non-polar (are electrons shared equally?)

Van de Waals Interactions Ionic Bonds Atoms gain/lose electrons, resulting in charges which attract them to each other Van de Waals Interactions Weak connections, break and reform, as a result of asymmetrical distribution of electrons (aka intermolecular forces)

WATER Polarity: Uneven distribution of charges around a molecule Hydrogen side of water molecule being slightly positive while oxygen side is slightly negative Attraction between positive side of one water molecule and negative side of next = Hydrogen bond

1. Cohesion Water molecules are attracted to other water molecules Water forms into droplets (which join together when close) https://www.youtube.com/watch?v=btD84ESfTJM Water has strong surface tension (remember penny lab?) https://www.youtube.com/watch?v=OAliTzF8lyc Water can move as a column in plants https://www.youtube.com/watch?v=f8TBxPOwjvs (1:55) http://www.youtube.com/watch?v=Oe3St1GgoHQ

Adhesion Attraction between water molecules and other types of molecules Also at play in movement of water in vascular plants (capillary action) Once adhesion moves water up the vascular tissue, adhesion keeps it from dropping back down

Less dense in solid form High Specific Heat The amount of heat required to raise or lower the temperature of a substance by 1 degree C Makes temperature of Earth stable (ocean temps stable) Helps plants and animals maintain temperatures Evaporative cooling Less dense in solid form Ice floats – insulates large bodies of water to keep them from freezing entirely (life can survive the winter)

4. Solvent Capable of dissolving an abundance of other molecules Makes it extremely valuable in transporting molecules in your body Water being transported by cohesion and adhesion in plants is carrying dissolved substances everywhere it goes Blood is composed mainly of water – carries dissolved substances around your body (glucose, ions, amino acids) Also makes it an excellent medium for metabolic reactions *Remember Solvent = substance doing the dissolving Solute = substance that gets dissolved Together they make a solution

Hydrophilic molecules – molecules that dissolve in water Generally polar (polar solvent-water-dissolves polar solutes) Ex. glucose and other carbohydrates, ions such as sodium (Na+, Ca++, Cl-) Hydrophobic molecules – don’t dissolve in water Generally non-polar (charges evenly distributed around molecule, atoms share electrons equally) Ex. Lipids, methane

pH Buffers Weak acids/bases that minimize changes in pH Ex. Carbonic acid (H2CO3) – in living systems Moderates pH changes in blood plasma

Molarity (M) Number of moles (6.022 x1023 molecules) per liter of solution How we quantify the concentration of a solution Sample Problem: How many grams of sucrose (C12H22O11) would you add to 0.75L of water to make a 0.6M aqueous solution?

Step 1: Determine the molecular mass of sucrose. C12H22O11 - (12 molecules of carbon x 12) + (22 molecules of hydrogen x 1) + (11 molecules of oxygen x 16) = (144) + (22) + (176) = 342g/mol Step 2: Solve for number of moles in the solution Molarity (M) = m (moles)/V (volume in liters) 0.6M = m/0.75L m = 0.45 moles Step 3: Determine g in 0.45 moles 342 g/mol = X/0.45 mol X = 154g