Chemical Bonds Electronegativity is an atom’s affinity for electrons. Differences in electronegativity dictate how electrons are distributed in covalent bonds. - nonpolar covalent bonds = equal sharing of electrons - polar covalent bonds = unequal sharing of electrons

Water

Water Chemistry Life on earth evolved in water, and all life still depends on water. All living organisms are dependent on water. At least 80% of the mass of living organisms is water and almost all chemical reactions of life take place in aqueous solution.

Water Chemistry The structure of water is the basis for its unique properties. The most important property of water is the ability to form hydrogen bonds. Within a water molecule, the bonds between oxygen and hydrogen are highly polar.

Water Chemistry Electronegativity is an atom’s affinity for electrons. Differences in electronegativity dictate how electrons are distributed in covalent bonds. Electonegative elements????? Electropositive elements???? polar covalent bonds = unequal sharing of electrons Non-polar covalent bonds=equal sharing of electrons

Water Chemistry Due to the polar covalent bond partial electrical charges develop: water molecules are charged, with the oxygen atom being slightly negative (partially negative) and the hydrogen atoms being slightly positive (partially positive). WHY PARTIAL CHARGES?????

Structure of water

Water Chemistry Polarity unequal charge distribution in a molecule resulting in a – region and a + region Dipole- amphoteric???????? These opposite attract each other, forming hydrogen bonds. These are weak, long distance bonds that are very common and very important in biology.

Water Chemistry Hydrogen bonds are weak attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule. Hydrogen bonds can form between water molecules or between water and another charged molecule.

Water Properties 1.The polarity of water causes it to be cohesive and adhesive. cohesion: water molecules stick to other water molecules by hydrogen bonding, so water has a high cohesion. This explains why long columns of water can be suck up tall trees by transpiration without breaking. Cohesion helps the transport of water against gravity in plants

Water Properties Cohesion also explains surface tension which allows small animals to walk on water. Surface tension is a measure of how hard it is to break the surface of a liquid Surface tension is related to cohesion

Water Chemistry cohesion

Water Chemistry adhesion: water molecules stick to other polar molecules by hydrogen bonding water rises in plants due to its adhesion to capillary walls. Cohesion and adhesion produces a single, polymer-like column of water in xylem, extending from roots to leaves-capillary action

Properties of Water 2.Water has a high specific heat capacity. - A large amount of energy is required to change the temperature of water.(4.2 joules of energy to heat 1g water by 1oC). This means that water does not change temperature easily. This minimises fluctuations in temperature inside cells and means that sea temperature is quite constant.

Water Properties Water absorbs heat from warmer air and releases stored heat to cooler air Water can absorb or release a large amount of heat with only a slight change in its own temperature Water is a temperature buffer it resists changing its temperature because of its high specific heat

Water Properties Water’s high specific heat can be traced to hydrogen bonding Heat is absorbed when hydrogen bonds break Heat is released when hydrogen bonds form The high specific heat of water minimizes temperature fluctuations to within limits that permit life

Properties of Water 3has a high latent heat of vaporization. Evaporation is transformation of a substance from liquid to gas Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas As a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water

Water Properties Water requires a lot of energy to change state from a liquid to a gas, providing a cooling mechanism in animals (sweating and panting) and plants (transpiration). As water evaporates it extracts heats from the surrounding area, cooling the organism - The evaporation of water from a surface causes cooling of that surface.

Properties of Water Solid water is less dense than liquid water. Water is its solid state (ice) is less dense than the liquid state. As the air temperature cools, bodies of water freeze from the surface (top down), forming a layer of ice with the liquid beneath, T his allows aquatic ecosystem to exist in low temperatures.

Water Properties Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense Water reaches its greatest density at 4°C If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth

The three states of water ????????????

Ice, water, vapor

Three states of water

Three states of water

Water: the solvent of life A solution is a liquid that is a homogeneous mixture of substances A solvent is the dissolving agent of a solution The solute is the substance that is dissolved An aqueous solution is one in which water is the solvent

Water Properties Water is a good solvent. Water dissolves polar molecules and ions. It is a very good solvent. Molecules such as salts, sugars, amino acids, alcohols dissolve readily in water (once dissolved they can be transported e.g. glucose in the bloodstream). Water is a good/ universal solvent for charged and polar substances

Water as solvent Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell

Water as a solvent Water can also dissolve compounds made of nonionic polar molecules Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions

Water Properties HYDROPHILIC?????? HYDROPHOBIC??????? Water is a poor solvent for nonpolar substances nonpolar gases aromatic moieties aliphatic chains HYDROPHILIC?????? HYDROPHOBIC??????? AMPHIPHILE/AMPHIPATHIC????????

Properties of Water Water organizes nonpolar molecules. - hydrophilic: “water-loving” A hydrophilic substance is one that has an affinity for water Most biochemical reactions occur in water Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in an aqueous solution Ionization; happens spontaneously

Properties of Water -hydrophobic: “water-fearing” A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds

Properties of Water - Water causes hydrophobic molecules to aggregate or assume specific shapes. This means water is able to transport nutrients and waste material in living organisms . Contributes to the integrity of the cell membrane, nucleic acids and proteins Amphipathic

The Hydrophobic Effect Refers to the association or folding of non-polar molecules in the aqueous solution Is one of the main factors behind: Protein folding Protein-protein association Formation of lipid micelles Binding of steroid hormones to their receptors Does not arise because of some attractive direct force between two non-polar molecules

ADDITIONAL PROPERTIES OF WATER High melting point High boiling point High surface tension High heat of fusion

SUMMARY Many properties of water are emergent properties due to hydrogen bonding. The cohesion of water molecules to each other is exploited by plants and animals. Water resists temperature changes by absorbing lots of heat. Lower density of ice causes it to float & insulate the water below. The polarity of water allows it to dissolve other polar molecules. Non-polar compounds are hydrophobic and not easily dissolved in water.

QUESTION The remarkable properties of water support life on Earth in many ways.Explain

Acids and Bases Hydrogen ion (H+1) is the basis of the pH scale. Greater H+1 concentration --- lower pH (acidic) Lower H+1 concentration --- higher pH (basic)

Acids and Bases Acid: a chemical that releases H+1 ions. pH below 7 Base: a chemical that accepts H+1 ions. pH above 7 Buffer: a chemical that accepts/releases H+1 as necessary to keep pH constant

Acids and Bases

Acids and Bases Most biological buffers consist of a pair of molecules, one an acid and one a base. BUFFER SYSTEM IN HUMAN BLOOD ACID BASE

Acids and Bases