Water

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Water
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Introduction

• All matter is made up of atoms.
• Atoms consist of a nucleus of positively charged
  protons and neutral neutrons. Negatively charged
  electrons orbit the nucleus.
• Two or more atoms bonded together form a
  molecule.
• The arrangement of electrons determines what kind
  of chemical bonds the atom makes.

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Oxygen Atom
Hydrogen Atom
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Types Of Chemical Bonds

• Ionic bond one or more of the orbiting electrons
  is transferred from one atom to the other.
• Covalent bond orbiting electrons are shared
  between atoms.
• If the electrons are shared equally between
  the two atoms, then the bond is nonpolar
  covalent.
• If one of the two atoms tends to pull the
  shared electrons towards it more than the other
  atom, then the bond is polar covalent.
• Non Covalent bonds including the following
• hydrogen bonds Hydrogen bonds form between the
  positive end of a polar covalently bonded
  molecule and the negative end of another polar
  covalently bonded molecule.
• Electrostatic interactions They occur between
  oppositely charged atoms or groups..
• Van der waals forces They are a class of
  transient electrostatic interaction.
• Hydrophobic interactions Non polar molecules or
  groups tend to cluster together in water.

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Water

• Importance functions
• Water is essential for life.
• Human body is comprised of over 70 water
• Water is a major component of many body fluids
  including blood, urine, and saliva.
• Water is the solvent for most biological
  molecules within the body
• Water participates in a variety of biochemical
  reactions.
• Relative deficiency or excess of water impair the
  function of tissues and organs.
• Water regulates body temperature
  

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Water

• Molecular structure of water
• Water has a simple molecular structure.
• It is composed of one oxygen atom and two
  hydrogen atoms.
• Each hydrogen atom is covalently bonded to the
  oxygen via a shared pair of electrons.
• Oxygen also has two unshared pairs of electrons.
• Thus there are 4 pairs of electrons surrounding
  the oxygen atom, two pairs involved in covalent
  bonds with hydrogen, and two unshared pairs on
  the opposite side of the oxygen atom
• Water is a "polar" molecule, meaning that there
  is an uneven distribution of electron density.

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Properties of Water

• 1-Thermal properties of water
• Water actually melts at 0o C and boils at 100o
  C.
• Hydrogen bonding is responsible for thermal
  properties of water.
• The maximum number of hydrogen bonds are four,
  which form when water has frozen into ice.
• Energy is required to break these bonds. When ice
  is warmed to its melting point, approximately 15
  of the hydrogen bonds break.
• As the temperature rises, the movement and
  vibration of the water molecules accelerate, and
  additional hydrogen bonds are broken.
• When boiling point is reached, the water
  molecules free from one another and vaporize.
• 2- Water is a good solvent
• The polar nature of water, with its partial
  positive and partial negative dipole, allows it
  to dissolve charged molecules (ions) easily.
• Water is thus an excellent solvent for charged
  compounds.
• The positive side of water surrounds negatively
  charged molecules, and the negatively charged
  side of water surrounds positively charged
  molecules.

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Properties of Water contd,

• 3- Ionization and pH of water
• Sometimes the hydrogen of one water molecule will
  "jump" to another water molecule
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  H2O H2O H3O OH-
• This proton hopping is called the ionization of
  water from one water molecule to another
• H2O H OH-.
• The equilibrium constant, Keq describes the
  ionization equilibrium of water
• Keq HOH-
• For the value for the Keq of water must remain
  constant. For neutral water, the Keq is 1 x 10-14
  M and the concentrations of H and OH- are
  each 1 x 10-7 M 0.0000001 M
• This is obviously a very small number. A more
  manageable way to discuss small numbers such as
  this is to take the negative logarithm.
• For the concentration of H, this is called the
  pH. In this case -log(0.0000001 M) 7
• The pH of a solution is simply the negative
  logarithm of H
• The pH of a solution describes the acidity of a
  solution. Acidic solutions are those with a pH of
  less than 7 and basic solutions have a pH greater
  than 7.
• A solution, like H2O, with a pH 7 is neutral.
• pH pOH 14 For any substance dissolved in water

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Buffers

• Buffers are solutions that resist changes in
  their pH when moderate amounts of acids or bases
  are added.
• Buffers are made up of a mixture of a weak acid
  with its conjugate base or a weak base with its
  conjugate acid. Remember that an acid donates a
  H. A weak acid does not donate its H as easily.
  Similarly, a weak base will not accept a H as
  well as a strong base.
• Composition and types There are mainly two
  types
• A weak acid and its salt with strong
  base, for example
• Acetic acid / Na acetate mixture
  
• Carbonic acid / Na Bicarbonate
  mixture.
• A weak base and its salt with strong
  acid , for example
• Ammonium hydroxide / Ammonium
  chloride mixture.
• Mechanism of action
• Addition of strong acid as HCl to
  carbonic / bicarbonate system.
• NaHCO3 HCl
  NaCl H2CO3
• Addition of strong base as NaOH to
  carbonic bicarbonate system.
• H2CO3 NaOH
  NaHCO3 H2O