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How does it WORK?
Colour in organic materials is the result of light absorption by certain chemical configurations called chromophores in molecules. C=C and C=O bonds are examples of chromophores. A chromophore, eg C=C and C=O, is a part of a molecule that is able to absorb UV or visible light and producing colour in organic compounds
Oxidising bleaches break up these double bonds
Reducing bleaches convert double bonds to single bonds
How does colour come about in organic compounds? The normal electronic configuration of a molecule is known as the ground state (which is the most stable state of an electron). When a molecule absorbs light of a particular wavelength, it can be promoted from its highest occupied molecular orbital () to its lowest unoccupied molecular orbital (LUMO), i.e. from its original orbital in the ground state to a higher orbital. Such a transition is called an electronic transition. The molecule is then in an excited state. E = hc/l Where E is energy of radiation
h is Plancks constant
c is the speed of light
lis the wavelength of radiation Ultraviolet light and visible light have sufficient energy to cause only two types of electronic transitions. The transition, which requires lower energy, is the excitation of a nonbonding electron (n) into a p antibonding molecular orbital. This is called an n p * transition, caused by visible light that has a longer wavelength, hence lesser energy. Similarly the other electronic transition with a higher energy, is excitation of an electron from a p bonding molecular orbital into a p antibonding molecular orbital. This is called a p p * transition, caused by UV light that has a shorter wavelength, hence more energy. Double bonds that are separated by just one single bond are said to be conjugated double bonds. On the other hand, isolated double bonds occur when 2 or more single bonds are found between the ends of the double bonds. conjugated double bonds
(more stable than isolated double bonds)
(isolated double bond) Conjugated double bonds interact with each other and systems containing conjugated double bonds tend to be more stable than similar systems containing isolated double bonds. Because conjugation raises the energy of the and lowers the energy of the LUMO, electronic transitions are easier for conjugated systems than non-conjugated systems. The more conjugated double bonds there are in a compound, the less energy is required for the electronic transition and therefore the longer the wavelength at which the transition takes place. This corresponds to the region of the visible spectrum of light. Hence making such conjugated system highly coloured. If a compound has enough conjugated double bonds, it will absorb visible light. White light is a mixture of all possible wavelengths of visible light. If any colour is removed from white light, the remaining light appears coloured. So, if a compound absorbs any visible light, it will appear coloured. Its colour depends on the color transmitted to the eye. In other words, it depends on the colour produced from the wavelengths of light that are not absorbed. b-Carotene, a precursor of vitamin A, is an orange substance found in carrots, apricots and sweet potatoes. Lycopene is red and is found in tomatoes, watermelon and pink grapefruit.
Lycopene Examples of Bleaching Agents
There are a few common examples of chemical agents used as bleaches.
They are (click for more detail): 1) CHLORINE- Containing bleaches
The green coloured spheres are the individual chlorine atoms.Manufacture: Nearly dry slaked lime readily absorbs chlorine to form bleaching powder.
2CaO+Cl2 2CaClO In commercial production, electrolytic chlorine, diluted with air, passes up a rotating inclined cylinder down which slaked lime falls. The rotating inclined cylinder ensures that the slacked lime is constantly being stirred to expose a fresh uncombed surface. Water-cooling regulates the temperature of the system. The product, bleaching powder or chloride of lime, is an off-white solid, which smells of chlorine. Chemical Aspects of chlorine Fully chlorinated bleaching powder is a complex mixture of a probably basic calcium hypochlorite and a non-deliquescent basic calcium chloride: 3Ca(ClO)2.2Ca(OH)2.2H2O
Which is usually written as:Ca(ClO)2 + CaCl2.Ca(OH)2.H2O
In addition, unchanged slaked lime, hydrated calcium chloride CaCl2.4H2O, and calcium chlorate(I) are present.Bleaching powder is partially soluble in water, the hypochlorite dissolving and being responsible for oxidising and bleaching powers. On boiling the aqueous solution, calcium chlorate(I) and calcium chloride are formed. One method of assaying bleaching powder involves the measurement of the volume of oxygen liberated from a known mass of bleaching powder acting in a suspension of approximately 5-volume hydrogen peroxide: OCl- +H2O2 ----> Cl- + H2O+ O2
Available chlorine is a measure of the oxidising power of the hypochlorite ion, determined by titrimetric analysis. Acidification of bleaching powder leads to the following changes:
Ca(ClO)2 + CaCl2.Ca(OH)2.H2O + 6H+
Hypochlorous acid or chlorate (I) acid [HClO] is a mild oxidising agent and bleach. Chlorine in water gives HCl and HClO:
Cl2 + H2O -> HCl + HClO
If a coloured, oxidisable material is present, HOCl releases its oxygen to oxidise the material to a colourless compound. This process is known as bleaching.
HClO + dye stuff HCl + bleached dye stuff
Liquid bleach is usually aqueous sodium hypochorite (NaClO), a sodium salt of HCl acid.
The Magic Of Chlorine Gas
Taken from "Qualitative AnalysisHandbook for O-level Chemistry" by Peter S.P. Lim & B.H. Yeap
Picture (a) shows moist red and blue litmus paper. Test tube contains Clorox, a solution that contains dissolved chlorine gas Cl2 .
Picture (b) shows both litmus papers are decolourised (bleached) when held at the mouth of the test tube containing heated Clorox solution.
The blue litmus paper turns red. The red colour is then bleached to white. The red litmus paper is also bleached white.
The observation that confirms that the gas produced is chlorine is the bleaching of the litmus paper. Hence chlorine has a bleaching property.
The process of bleaching by bromine is similar to that of chlorine. It is less oxidising but more stable than chlorine water.
Iodine is only very slightly soluble in water and its oxidising and bleaching powers are feeble.
The yellow coloured sphere is the sulphur atom whereas the red atoms are the oxygen atoms.
The sulpur dioxide molecule has a bent shape.
In moist conditions, sulphur dioxide is a bleaching agent. In the reducing action of SO2, oxygen is removed from coloured dye. Sulphur dioxide is used to bleach wool and silk. Salts of sulphur dioxide are also used in bleaching. Calcium hydrogen sulphite Ca(HSO3)2 is used to whiten wood pulp in the manufacturing of paper.
Other than SO2, sodium hydrosulphite (Na2S2O4) is also used to bleach wool. However, bleaching using SO2 is not permanent. On exposure to air and light, the original colour slowly returns as oxygen in air oxidises the bleached substance.3)Potassium Manganate(VII), KMnO4 Potassium Manganate(VII) is used to remove colour from fabrics that are able to tolerate strong oxidation due to its powerful oxidising property. 4)Hydrogen Peroxide, H2O2
70% of all H2O2 manufactured is used to bleach fibres and paper. However H2O2 is most widely used for bleaching cotton cloth due to the strong oxidising properties of H2O2. On the other hand, H2O2 can also be used as an antiseptic and a bleaching agent for hair. It is sold as a solution labeled 20 volumes (as an antiseptic) or 30 volumes (for bleaching hair). One advantage of using hydrogen peroxide for bleaching is that the only waste product is water.5)Sodium Peroxide, Na2O2
Sodium peroxide, a compound of H2O2 is a pale yellow solid with oxidising and bleaching properties. This type of bleach is called the oxygen-releasing bleach. They are not as active as chlorine bleach but they are better for synthetic fibers. More bleach, hotter water and a more alkaline solution is required than using a chlorine bleach but they work just as well when used properly.6)Borax Na2B4O7
Borax, also known as sodium pyroborate, Na2B4O7, is often added to oxygen-releasing bleaches because of their ability to clean. Its pH is about 9.5, so it produces a basic solution, which makes the bleach work better.7) Sodium perborate, NaBO3 It is the most common bleaching chemical added to powdered detergents. It works by converting some water molecules into H2O2, an oxidising agent in warm water. Activators are added to make perborates work better at lower temperatures. They are less harmful than chlorine-containing bleaches.
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Last modified: 4/7/99