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Tuesday, August 17, 2010

Acids, Bases and Buffers

Tuesday, August 17, 2010 - 0 Comments

Electrolyte is a substrate that conducts electricity when in solution, such as sodium chloride (NaCl). Many fluids contain strong electrolytes that break down or ionize into ions. Most acid and bases are electrolytes. An acid is a substrate that releases hydrogen ions (H+), when dissolved in water. Hydrogen atom without its electron is only a proton, an acid can be described as a proton donor. One molecule of Hydrogen chloride dissolves in water to produce hydrochloric acid which dissociates into one Hydrogen ion and one chlorine ion.
HCl ====> H+ + Cl-
In contrast, a base (or alkali) is a substrate that releases hydroxyl ions (OH-) when dissolved in water one molecule of sodium hydroxide dissolves in water to produce one sodium ion and one hydroxyl ion.
NaOH ====> Na+ + OH-
When a base dissolves in water it removes free protons (H+) from water and it is proton acceptor
OH- + H+ ====> H2O
pH: Measuring acidity and alkalinity:
Hydrogen and hydroxyl ions often affect the chemical reactions involved with life processes. Therefore the concentrations of these ions in body fluids are important. Higher the concentration of hydrogen ions (H+), more acidic the solution be.
If the concentration of hydroxyl (OH+) ions is higher in a solution, more basic or alkaline the solution is. A solution is neutral when the number of hydrogen ions equals the number of hydroxyl ions.
The pH scale runs from 0 to 14, measures acidity and alkalinity. Acidic solutions have pH less than 7 and basic (alkaline) solutions have pH above 7. A pH of 7 is neutral. Each whole number on pH scale represents a tenfold change (logarithmic) in acidity; therefore a solution with pH of 3 is 10 times more acidic than a solution with pH of 4 and pH of 9 is 10 times more basic than a pH of 8. Actually pH value is equal to negative logarithm of hydrogen ion concentration.
pH = –log [H+] or pH = Log (1/[H+])

pH: Control with Buffers:
Maintenance of stable internal environment in an animal requires constant pH of body fluids. A strong acid or base can destroy cell stability. Like wise sudden change in pH, may also be destructive. Fluid systems of most animals contain chemical substances that help, regulate the acid base balance. These substances called buffers resist changes in pH by accepting H+ ions when they are in excess and donating H+ ions when they are depleted. Most important buffers are bicarbonates, phosphates and organic molecules such as amino acids and proteins.

Carbonic acid – Bicarbonate ion System:
It is important in buffering the blood of many vertebrates. Carbonic acid dissociates to form Hydrogen ion and bicarbonate ion.
H2CO3 <====> H+ + HCO3-
In this example if H+ ions are added to the system, they combine with HCO3 to form H2CO3. This reaction removes H+ ions and keeps pH from changing.

Molecules of animals:
The chemicals that enter into or are produced by metabolic reactions can be divided into two large groups, organic molecules that contain carbon and inorganic molecules without carbon atoms. Most important characteristics of organic molecules depend on properties of key element carbon, the indispensable element for all life. Carbon atom has four electrons in the outermost shell, also requires four electrons to fill its outer orbits. Carbon atoms may share one or two pairs of electrons. When they share one pair of electron, single covalent bond is formed and still leaving carbon atoms to bond with other atoms. In case of ethane C2H6, two Carbons form covalent bond among themselves and the remaining three electrons share with three hydrogen atoms to form additional three single covalent bonds. In case of ethane C2H4, two covalent bonds are formed between two carbons and the remaining two electrons are shared with two hydrogen atoms.

H – C = C

     H     H
       |      |
H - C = C

     H     H
       |      |
H - C - C - H
       |      |
      H    H

The ability of carbon to bond with other carbon atoms helps to develop carbon chains or rings of variable length and sizes. Bonds between Carbon and Hydrogen are also source of energy for living organisms Hydrocarbons are organic molecules that contain only carbon and hydrogen. Carbons are bended with each other in a linear fashion. Hydrocarbons form the frame work of all organic molecules.
Carbon chain or ring of many organic molecules provides relatively inactive molecular backbone to which reactive group or atoms are attached. These functional groups of molecules are responsible for molecules unique chemical properties and behaviour.

Compounds and Molecules as Aggregates of Atoms

In addition to being an element, a substance can also be a compound. Compound is composed of atoms of two or more elements chemically united in fixed proportion. For example water has two H atoms and one O atom. This composition does not change that is a compound cannot be separated into its pure components, the atoms of the elements present, except by chemical methods. When atoms interact chemically, electrical forces called chemical bonds hold the atoms together and form molecules like H2, N2, H2O, CO2 etc.

Chemical Bonds:
There are three types of chemical bonds, Covalent, Hydrogen and Ionic which are responsible in making all types of molecules whether smaller or larger, organic or inorganic in nature.

Covalent bonds sharing electron pairs:
When atoms share outer shell electrons with other atoms, the chemical bond that forms is covalent bond. In covalent bonding electrons are always shared in pairs. When a pair of electron is shared from each atom, single bond forms as in hydrogen [H – H] molecule. When two pairs of electrons are shared, double bond forms as in oxygen [O O] molecule and when three pairs of electrons are shared, a triple bond forms e.g. nitrogen [N N] molecule. In a molecule like H2, the electrons spend as much time orbiting one nucleus as the other. The distribution of charges is symmetrical and the bond is called non polar covalent bond. Due to this equal sharing, the molecule is electrically balanced and the molecule as a whole is neutral.

Hydrogen Bonds:
In molecules in which hydrogen combines with certain other atoms like O, N or Fe, Hydrogen electron is drawn toward another atom leaving a proton behind. As a result, Hydrogen atom gains slight positive charge. Remaining proton is attracted to negatively charged atoms of oxygen in nearby molecules. When this happens a weak attraction called Hydrogen bond, forms. Hydrogen atom in one water molecule forms hydrogen bond with oxygen atom in another water molecule and so forth until many molecules bond.

Hydrogen bonds and water Droplets:
Hydrogen bonds form when oxygen atoms of different water molecules weakly join by the attraction of electronegative oxygen for the positively charged hydrogen. Because of the arrangement of electron orbital and the bonding angles between oxygen and hydrogen, the molecule as a whole is polar as it carries a slight negative charge at one end and slight positive charge at the other end. This polarity is responsible for many important properties of water. In other molecules such as H2O where two hydrogen atoms combine with one oxygen atom the electrons spend more time orbiting oxygen nucleus than Hydrogen nuclei. As the electrical charge from the cloud of moving electrons is asymmetrical, the bond is called Polar covalent bond. Such a bond leaves oxygen atom with slightly negative charge and Hydrogen with slightly positive charge, even though the entire molecule is electrically neutral. Shape of water molecule reflects this polarity; rather than the linear arrangement H – O – H, two hydrogen are at one end, a bit like the corners of a triangle. This shape and the polarity can lead to the formation of another kind of chemical bond – the hydrogen bond.

Ionic Bonds: When an atom either gains or loses electrons, it acquires electrical charge and is called an ion. When atom loses one or more electrons, it becomes positively charged because more positively charged protons are now in the nucleus than negatively charged electrons surrounding the nucleus. This positive charge is shown as ‘plus’ sign and is known as cation e.g. Sodium (Na+), Potassium (K+), Hydrogen (H+), Calcium (Ca2+) and iron (Fe3+).
When atom gains one more electrons it becomes negatively charged and is shown as one or more ‘minus’ signs and such charged particle is called anion e.g. chlorine (Cl-), Hydroxyl (OH-), Bicarbonate (HCO3-), Sulphate (So42-), Phosphate (Po43-) and Carboxyl (CO3-).

Ionic Bonds: Ionic bonds form when an atom or group of atoms develop on electrical charge due to loss or gain of electrons. Due to opposite charges these atoms or group of atoms attract each other. A molecule of sodium chloride, the common table salt is formed charge when sodium atom and chlorine atom come together, sodium atom donates an electron to chlorine atom. This electron transfer changes the balance between protons and electrons in each of the two atoms. Sodium atom ends up with one more proton than it has electrons and chlorine atom with one more electron than it has Protons. Sodium atom is left with net charge of +1 (Na+) and net charge of chlorine atoms is -1 (Cl-). These opposite charges attract each other and form ionic bonds.

Atoms and Elements as Building Blocks

Matter occupies space and has mass. It includes solid, liquids and gases of the environment as well as those in the bodies of all forms of life. Mass is the amount of matter in an object. Matter is composed of elements; chemical reactions cannot break down into simpler units. O is symbol for element oxygen, H is symbol for hydrogen, and Na is symbol for sodium. Recently scientists recognise 92 elements occurring in nature. Fifteen elements are found in large quantity in most animals and four of these (carbon, hydrogen, oxygen and nitrogen) account for the majority (97%) of animals’ body weight. Remaining 3% of animals’ weight consists primarily of calcium, phosphorus and potassium. Elements present in trace amount include sodium, sulphur, manganese, magnesium, copper, iodine, iron and chlorine. Elements are composed of units of matter called atoms. Atom is smallest part of an element that can enter into chemical reaction. Atoms vary in size, weight and the diverse ways they interact with each other. For example some atoms can combine with atoms like themselves or with dissimilar atoms, others lack this ability.

Structure of Atom: Atoms have two main parts, a central core called a nucleus and the surrounding electron cloud. Nucleus contains two major particles, positively charged protons (P+) and uncharged neutrons (n). Surrounding the nucleus are negatively charged particles called electrons (e-). Any one electron moves so rapidly around the nucleus that it cannot be found at any given point at any particular movement in time. Therefore its location is given as an electron cloud. Because the number of negatively charged electrons outside the nucleus is equal to the number of positively charged protons, the atom is electrically uncharged or neutral.
Atomic number: The number of protons and neutron in the nucleus and the number and arrangement of electrons in the electron cloud determine an atoms’ chemical and physical properties. The atomic number of an element is the number of protons in the nucleus of one of its atoms. Elements are identified by their atomic number. For example, if an atom has one proton, it is hydrogen; if it has six it is carbon; if it has seven, it is nitrogen and if an atom has eight protons it is oxygen.

Valence: Another measure of an atom is its valance. The valance is the number of bonds an atom will usually form equal to the number of electrons required to complete the outermost that is the valence electron shall.
Atomic mass: Another measure of an atom is its atomic mass. Atomic mass is equal the number of neutrons and protons in the atoms nucleus. Because carbon contains six protons and six neutrons, its atomic mass is 12 and is symbolized with a super script preceding the elements. Symbol 12C and is read as “Carbon – 12”.
Isotopes: All atoms of given element have same number of protons in the nucleus but some have different number of neutrons and thus different atomic masses. These different form with the same atomic number but different atomic masses are isotopes. Most common form of carbon atom has six protons and six neutrons in the nucleus and atomic mass of 12 (12C). A carbon isotope with six protons and seven neutrons has atomic mass of 12 (12C) while carbon isotope with six protons and eight neutrons has atomic mass of 14(14C). Some isotopes like 12C and 13C are stable and do not break down. Other isotopes like 14C are unstable and tend to break down, decay or decompose by periodically emitting small particles and energy. These unstable isotopes are named as radio isotopes or radio active isotopes. Hydrogen has three isotopic forms, each isotope has one proton in each nuclear but have no, one or two neutrons. Hydrogen with no neutron is denoted by 1H; Hydrogen with two neutrons is deuterium and represented by 3H and is radio isotope. Oxygen, iron, cobalt, iodine and phosphorus are examples of elements that have radio active isotopes.
Energy level shells: Electrons of an atom are distributed around its nucleus in orbital called energy level shells or clouds of electrons and are numbered as K, L, M, N. Seven Energy level shells are possible. Each shell can hold only certain number of electrons. The shell nearest to nucleus never has more than two electrons. Second and third shells can each have not more than 8 electrons. Larger number fills the more distant shells. When the shell of an atom holds the maximum number of electrons possible, the shell is complete and stable.

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