BISC 101 Lecture Notes - Lecture 1: Ionic Compound, Chemical Reaction, Weak Base

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BISC 101 – Lecture 1 - Water
The Properties of Water
Life is based on water – It is the biological medium on Earth
All living organisms require water more than any other substance
Water constitutes 70 - 90% of the cell’s volume
You cannot live more than 3 or 4 days without drinking
Water is the only common substance that can exist in all 3 states of matter: sold,
liquid, gas
The Polar Covalent Bonds of Water Molecules Results in Hydrogen Bonding
In a water molecule, two hydrogen atoms form single polar covalent bonds with
an oxygen atom
Oxygen is more electronegative than hydrogen
oElectronegativity: A measure of the tendency of an atom to attract a
bonding pair of electrons
oThe more electronegative an atom, the more strongly it pulls electrons
towards itself
oIn water, the region around oxygen atom has a partial negative charge
while the regions round the two hydrogen atoms have a partial positive
1 water molecule can hydrogen bond up to 4 neighboring water molecules
Polar Covalent Bond: Covalent bond where one atom holds the shared electrons
in a covalent bond more tightly than do the other atoms
oAtoms have very different electronegativities
oElectrons are NOT shared equally between both atoms
oOxygen is the most electronegative element
oIt attracts covalently bonded electrons more strongly than any other atom
oNitrogen is slightly lower than oxygen
oCarbon and hydrogen have low and equal electronegativity
oEx: O – H, N – H, F – H, O – C, N – C, CL – C, CL – H
Nonpolar Covalent Bond: Covalent bond that is symmetrical; The electrons are
shared equally between the two hydrogen atoms
oAtoms have the same or similar electronegativities
oEx: H2, NH3, C – H, CH4
Covalent bonds are more stable and stronger than non-covalent bonds
oThey require more energy to break
Four Emergent Properties of Water Contribute to Earth’s Suitability for Life
1. Cohesion, Adhesion, and Surface Tension
Cohesion: Force of attraction between similar molecules
oWater is cohesive
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oHydrogen bonds hold water molecules together
Adhesion: Force of attraction between different molecules
oWater adheres to surfaces that have any polar or charged components
oAdhesion is usually regarded to interactions between a liquid and solid
Cohesion and adhesion is important to explain how water moves from the roots of
plants up to their leaves – defying gravity
Surface Tension: A measure of how hard it is to break the surface of a liquid
oSurface tension is related to cohesion
oWater resists any force that increases its surface area
oAny force that depresses a water surface meets with resistance
oThis resistance makes a water surface act as if it has an elastic
oAll liquids have surface tension but water’s is extremely high and strong
due to hydrogen bonding between molecules that resist stretching or
breaking the surface
oLight objects like insects will not break the surface of water
2. Moderation of Temperature
Water has a high capacity for absorbing energy
Water stabilizes air temperatures by absorbing heat from warm air and releasing
heat to cool air
Water can absorb or release large amounts of heat with only a slight change in its
own temperature
Specific Heat: Amount of energy required to raise the temperature of 1 gram by
Water has a high specific heat: 1 calorie/gram oC
oWater’s high specific heat allows it to minimize temperature fluctuations
to a range that permits life
It takes a large amount of energy to change the temperature of molecules that
have hydrogen bonding
Water absorbs or releases a large quantity of heat for each degree of temperature
When energy hits the specific heat level, hydrogen bonds must break before heat
can be transferred for the water molecules to begin moving faster
oHeat is absorbed when hydrogen bonds break
oHeat is released when hydrogen bonds form
Heat of Vaporization: Energy required to change 1 gram of water from a liquid to
oHigher than most molecules that are liquid at room temperature
oWater has to absorb a huge amount of heat to evaporate
Hydrogen bonds make it difficult for water molecules to escape the liquid state
Water molecules absorb energy from your body before they evaporate so you
lose heat
Evaporation Cooling: As a liquid evaporates, its remaining surface cools
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