The energy producing cells are of two types - primary cells and secondary cells.
1) Primary cells
In a primary cell the reaction occurs only once and it becomes dead after a period of time and hence cannot be used again. Dry cell, mercury cell etc are examples of primary cells.
Dry cell The anode is zinc vessel and the cathode rod. It is surrounded by powdered Mno2. The space between the electrode is filled with a paste of NH4Cl and ZnCl2, the electrolyte. The potential is 1.5 V
Zn ------> Zn2+ + 2 electron (anode)
NH4+ + MnO2 + electron ------> MnO(OH) + NH3 (cathode)
It is of our common experience that dry cells do not have a long life. It is because the acidic ammonium chloride corrodes the zinc container even if the acidic ammonium chloride corrodes the zinc container even if the cell is not in use. In the leak proof cells, the zinc vessel is protected by an outer steel covering. In these cells recharging is not possible.
2) Secondary cells
These are cells which can be recharged and can be used again and again. Eg: Lead storage cell.
Lead storage cell This is a secondary cell in which lead acts as the anode and a grid of lead packed with lead dioxide acts as the cathode. A solution of sulphuric acid (38% by mass and density 1.3 g/ml) is used as the electrolyte.
Anode reaction : Pb + So4 2- -------> PbSO4 + 2 electron
Cathode reaction : PbO2 + 4H+ + SO4 2- + 2 electron ------> PbSO4 + 2H2O
Overall reaction : Pb + PbO2 + 2H2SO4 + 2 electron ------> PbSO4 + 2H2O
When the battery is recharged, the reverse of the reaction takes place .
A fully charged cell has a voltage of 2.2 V
Fuel cell
Fuel cells are galvanic cells in which chemical energy from combustion of fuels are converted to electrical energy. Eg: Hydrogen-oxygen fuel cell
Hydrogen-Oxygen fuel cell The cell consists of three compartments separated by graphite electrodes. H2 is passed through one compartment and O2 is passed through another compartment. Water containing small amount of NaOH is taken in the central compartment.
Anode reaction : [ H2 + 2OH- -----> 2H2O + 2electron] *2
Cathode reaction : O2 + 2H2O + 4electron -----> 4OH-
Net reaction : 2H2 + O2 -----> 2H2O
Fuel cells are very efficient and free from pollution. They produce continuous supply of energy. Fuel cells are used in space ships and in military equipments.
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Mechanism of enzyme catalyzed reactions
The enzymes are acting as catalysts due to the presence of certain specific regions on their surface, called active sites or catalytic sites. Two models have been proposed for enzyme action.
Lock and Key model
The active site of a given enzyme is so shaped that only its specific substrates fit into it. We can compare substrate or reactant molecule to the key and active site to the lock.
Induced-fit model
Modern X-ray crystallographic and spectroscopic methods show that in many cases the enzyme changes shape when the substrate lands at the active site. This induced fit model of enzyme action pictures the substrate inducing the active site to adopt a perfect fit, rather than a rigidly shaped lock and key.
We can explain the mechanism of enzyme action using the transition state theory or the intermediate compound formation theory. It involves the following steps.
1) The reactant molecules bind to a region on the surface of the enzyme called active site which results in the formation of an enzyme-substrate complex.
E + S ------> [ES]
2) The enzyme functions by lowering the activation energy of a particular reaction and forms enzyme product complex. [ES] ------> EP
3) The products are released from the enzyme-product complex and the enzyme is then free to bind a fresh molecule of substrate. EP ------> E + P
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Lock and Key model
The active site of a given enzyme is so shaped that only its specific substrates fit into it. We can compare substrate or reactant molecule to the key and active site to the lock.
Induced-fit model
Modern X-ray crystallographic and spectroscopic methods show that in many cases the enzyme changes shape when the substrate lands at the active site. This induced fit model of enzyme action pictures the substrate inducing the active site to adopt a perfect fit, rather than a rigidly shaped lock and key.
We can explain the mechanism of enzyme action using the transition state theory or the intermediate compound formation theory. It involves the following steps.
1) The reactant molecules bind to a region on the surface of the enzyme called active site which results in the formation of an enzyme-substrate complex.
E + S ------> [ES]
2) The enzyme functions by lowering the activation energy of a particular reaction and forms enzyme product complex. [ES] ------> EP
3) The products are released from the enzyme-product complex and the enzyme is then free to bind a fresh molecule of substrate. EP ------> E + P
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Bio polymers and Biodegradable polymers
Bio polymers
Biopolymers are a special class of polymers found in nature, starch, protein and peptides. DNA and RNA are all examples of biopolymers, in which monomer unit, respectively are sugars, amino acids and nucleic acids.
A major difference between biopolymers and polymers can be found in their structures. Bio polymers inherently have a well defined structure. Many biopolymers spontaneously "Fold" in to characteristic shapes, which determine their biological functions and depend in a complicated way on their primary structures. Structural biology is the study of the shapes of biopolymers. In contrast most synthetic structures have much simpler and mere random or statistic structures. Another important difference is the lack of a molecular mass distribution in most biopolymers. As their synthesis is controlled by a template direct process in all bio polymers are alike, they all contain the same sequence and numbers of monomers and thus all have the same mass. This phenomenon is called monodispersity in contrast to the poly dispersity encountered in polymers.
Some biopolymers such as Polyacetic acid, Zein and poly 3-hydroxy butyrate can be used as plastics replacing the need for polystyene or polyethylene based plastics.
Biodegradable polymers
Many synthetic polymers are produced and utilised because they are resistant to chemical and physical degradation. These polymers are resistant to degradation and present disposal problems when their usefulness ceases. Substitution of natural monomers in to synthetic polymers produces polymers that are more easily biodegraded, however such polymers lack properties such as water resistance, that make current polymers so useful. Bio degradable polymers must over come such physical problems as well as lower their costs of production, which currently limit the economic possibility of such biodegradable polymers.
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Biopolymers are a special class of polymers found in nature, starch, protein and peptides. DNA and RNA are all examples of biopolymers, in which monomer unit, respectively are sugars, amino acids and nucleic acids.
A major difference between biopolymers and polymers can be found in their structures. Bio polymers inherently have a well defined structure. Many biopolymers spontaneously "Fold" in to characteristic shapes, which determine their biological functions and depend in a complicated way on their primary structures. Structural biology is the study of the shapes of biopolymers. In contrast most synthetic structures have much simpler and mere random or statistic structures. Another important difference is the lack of a molecular mass distribution in most biopolymers. As their synthesis is controlled by a template direct process in all bio polymers are alike, they all contain the same sequence and numbers of monomers and thus all have the same mass. This phenomenon is called monodispersity in contrast to the poly dispersity encountered in polymers.
Some biopolymers such as Polyacetic acid, Zein and poly 3-hydroxy butyrate can be used as plastics replacing the need for polystyene or polyethylene based plastics.
Biodegradable polymers
Many synthetic polymers are produced and utilised because they are resistant to chemical and physical degradation. These polymers are resistant to degradation and present disposal problems when their usefulness ceases. Substitution of natural monomers in to synthetic polymers produces polymers that are more easily biodegraded, however such polymers lack properties such as water resistance, that make current polymers so useful. Bio degradable polymers must over come such physical problems as well as lower their costs of production, which currently limit the economic possibility of such biodegradable polymers.
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The P - block elements
Elements of group 13 to 18 of the long form of the periodic table from the P-block elements. In the 'P' block elements, the last electron enters into the 'p' orbital of the outermost shell. 'p' block elements are coming under representative elements along with 's' block elements. They are called so, because they exhibit regular trends in properties with in each group and period.
Some general characteristics of 'p' block elements
Atomic and ionic radii of 'p' block elements decrease on moving from left to right and increase from top to bottom in any group. Along a period, there is a progressive increase in the nuclear charge of the atoms while the electrons are added to same outer most shell. This causes a decrease in atomic size from left to right along a period. Down a group the effective nuclear charge decreases due to addition of new shells and this results is an increase of atomic size.Ionization enthalpy of 'p' block elements increases along a period and decreases down the group. Along a period, electron affinity of 'p' block elements increases from left to right due to decrease in atomic size. Down a group electron affinity decreases due to increase in atomic size. Electronegativity of 'p' block elements increases along a period and decreases down the group. Decrease is due to the increase in atomic size. Metallic character decreases along a period and increases down a group. The increase in metallic character down a group is due to increase in atomic size and decrease in ionization enthalpy. Oxidizing properties decrease down the group and increase along a period. 'p' block elements exhibit a variety of positive and negative oxidation states. The maximum oxidation state formed by 'p' block elements is (x-10) where x is the group number. As the atomic number increases, the ns2 electrons fail to participate in chemical bond formation and hence bonds are formed only from the 'p'- electrons. This phenomenon is inert pair effect.
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Related articles Noble gases
Some general characteristics of 'p' block elements
Atomic and ionic radii of 'p' block elements decrease on moving from left to right and increase from top to bottom in any group. Along a period, there is a progressive increase in the nuclear charge of the atoms while the electrons are added to same outer most shell. This causes a decrease in atomic size from left to right along a period. Down a group the effective nuclear charge decreases due to addition of new shells and this results is an increase of atomic size.Ionization enthalpy of 'p' block elements increases along a period and decreases down the group. Along a period, electron affinity of 'p' block elements increases from left to right due to decrease in atomic size. Down a group electron affinity decreases due to increase in atomic size. Electronegativity of 'p' block elements increases along a period and decreases down the group. Decrease is due to the increase in atomic size. Metallic character decreases along a period and increases down a group. The increase in metallic character down a group is due to increase in atomic size and decrease in ionization enthalpy. Oxidizing properties decrease down the group and increase along a period. 'p' block elements exhibit a variety of positive and negative oxidation states. The maximum oxidation state formed by 'p' block elements is (x-10) where x is the group number. As the atomic number increases, the ns2 electrons fail to participate in chemical bond formation and hence bonds are formed only from the 'p'- electrons. This phenomenon is inert pair effect.
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Related articles Noble gases
Isomerism
Compounds with the same molecular formula but with different structures and so different properties are called isomers and the phenomenon is called isomerism.
Different types of isomerism
1) Structural isomerism
If the difference in properties result from the difference in the arrangement of atoms within the molecule, it is called structural isomerism. The various types are:
a) Ionization Isomerism :- This type of isomerism arises when the compound gives different ions in solution. Eg : [CoBr(NH3)5]SO4 and [CoSO4(NH3)5]Br are ionization isomers. The former gives [CoSO4(NH3)5]2+ and (SO4)2- as ions and latter gives [CoSO4(NH3)5]+ and Br- as ions.
b) Co-ordination Isomerism :- Here the cation part and the anion part of the compound are complexed. But the ligands co-ordinated to the metal are different. Eg: [Cu(NH3)4][PtCl4] and [Pt(NH3)4][CuCl4]
c) Linkage Isomerism :- If the ligand has two or more different atoms which can act as donor atoms, it can be linked to the metal differently. This type of ligands are called ambident groups. NO2 can bond to the metal either through N or through Oxygen. Eg: [CoNO2(NH3)5]Cl2 and [Co ONO(NH3)5]Cl2
d) Hydrate Isomerism :- This is similar to ionization isomerism. It is due to the difference in number of water molecules present as ligands and as molecules of hydration (in outside the sphere). Eg: [Cr(H2O)6]Cl3 - violet; [CrCl(H2O)5]Cl2.H2O - blue green; [CrCl2(H2O)4]Cl.2H2O - dark green
2) Stereo isomerism
Stereo isomers contain same atoms and atom-atom bonds but the special arrangements of atoms about the central atom are different. There are two types of stereo isomerism.
a) Geometrical isomerism :- This occurs in square planar and in octahedral complexes. When the ligands occupy the adjacent position it is called the 'cis' isomer and if it is in the opposite side, it is the 'trans' isomer.
b) Optical isomerism :- Substance which can rotate the plane of plane polarised light is called optically active substance. Optical isomers differ in their action towards polarized light. This type of isomerism exists when a molecule and its mirror image are not superimpossible (chiral). The isomer which rotate the plane of polarized light to the right is called dextro rotatory isomer and the one that rotates the plane of polarised light to the left is called laevo rotatory isomer.
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Different types of isomerism
1) Structural isomerism
If the difference in properties result from the difference in the arrangement of atoms within the molecule, it is called structural isomerism. The various types are:
a) Ionization Isomerism :- This type of isomerism arises when the compound gives different ions in solution. Eg : [CoBr(NH3)5]SO4 and [CoSO4(NH3)5]Br are ionization isomers. The former gives [CoSO4(NH3)5]2+ and (SO4)2- as ions and latter gives [CoSO4(NH3)5]+ and Br- as ions.
b) Co-ordination Isomerism :- Here the cation part and the anion part of the compound are complexed. But the ligands co-ordinated to the metal are different. Eg: [Cu(NH3)4][PtCl4] and [Pt(NH3)4][CuCl4]
c) Linkage Isomerism :- If the ligand has two or more different atoms which can act as donor atoms, it can be linked to the metal differently. This type of ligands are called ambident groups. NO2 can bond to the metal either through N or through Oxygen. Eg: [CoNO2(NH3)5]Cl2 and [Co ONO(NH3)5]Cl2
d) Hydrate Isomerism :- This is similar to ionization isomerism. It is due to the difference in number of water molecules present as ligands and as molecules of hydration (in outside the sphere). Eg: [Cr(H2O)6]Cl3 - violet; [CrCl(H2O)5]Cl2.H2O - blue green; [CrCl2(H2O)4]Cl.2H2O - dark green
2) Stereo isomerism
Stereo isomers contain same atoms and atom-atom bonds but the special arrangements of atoms about the central atom are different. There are two types of stereo isomerism.
a) Geometrical isomerism :- This occurs in square planar and in octahedral complexes. When the ligands occupy the adjacent position it is called the 'cis' isomer and if it is in the opposite side, it is the 'trans' isomer.
b) Optical isomerism :- Substance which can rotate the plane of plane polarised light is called optically active substance. Optical isomers differ in their action towards polarized light. This type of isomerism exists when a molecule and its mirror image are not superimpossible (chiral). The isomer which rotate the plane of polarized light to the right is called dextro rotatory isomer and the one that rotates the plane of polarised light to the left is called laevo rotatory isomer.
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Carbon fiber
Carbon fiber are high performance advanced materials which are stronger than steel, stiffer than titanium and lighter than aluminium. Carbon fibers are generally produced by thermal degradation of materials such as viscose rayon, poly acrylonitrile, pitch, resins, methane, benzene etc. Their properties are influenced by the manufacturing techniques employed.
Carbon fibers are generally reinforced in two ways. Carbon fibers reinforced in a light weight matrix such as epoxy resin, polyester resin or polyamide are called carbon fiber reinforced plastics (CFRP). When the carbon fibers are reinforced in a carbon matrix, they are known as carbon fiber reinforced carbons (CFRC).These are otherwise known as carbon-carbon composites.
The application of reinforced carbon fibers can be broadly classified into three:
1) High technology sector including, aerospace, military and nuclear fields
2) General engineering sector including sports, transportation and chemical fields
3) Biomedical sector
In aerospace sector, reinforced carbon fibers are used to make aircraft wings, tail parts and helicopter rotor blades. Due to high thermal conductivity of carbon fibers, they are used as wall material of nuclear fission reactor and to make automobile parts. Due to very high strength and stiffness, carbon fibers are used to make sports goods and racing vehicle bodies. In biomedical sector, carbon fiber is used to make artificial human body parts such as bone plates, ligaments etc. Activated carbon fibers are used for water treatment. In defence sector, carbon fibers are used to make nose tips and head shields of missiles and rockets.
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Carbon fibers are generally reinforced in two ways. Carbon fibers reinforced in a light weight matrix such as epoxy resin, polyester resin or polyamide are called carbon fiber reinforced plastics (CFRP). When the carbon fibers are reinforced in a carbon matrix, they are known as carbon fiber reinforced carbons (CFRC).These are otherwise known as carbon-carbon composites.
The application of reinforced carbon fibers can be broadly classified into three:
1) High technology sector including, aerospace, military and nuclear fields
2) General engineering sector including sports, transportation and chemical fields
3) Biomedical sector
In aerospace sector, reinforced carbon fibers are used to make aircraft wings, tail parts and helicopter rotor blades. Due to high thermal conductivity of carbon fibers, they are used as wall material of nuclear fission reactor and to make automobile parts. Due to very high strength and stiffness, carbon fibers are used to make sports goods and racing vehicle bodies. In biomedical sector, carbon fiber is used to make artificial human body parts such as bone plates, ligaments etc. Activated carbon fibers are used for water treatment. In defence sector, carbon fibers are used to make nose tips and head shields of missiles and rockets.
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Chemistry of Photography
The principle of photography is based on the photochemical reaction of light on certain silver halides. The photographic technique involves the following steps:
1) Preparation of sensitive plate or film :- It is an emulsion of AgBr in gelatin uniformly coated on a glass plate or a celluloid sheet (film) in a dark room.The process is done in dark because AgBr is highly light sensitive.
2) Exposure :- The sensitive plate or film is mounted on a camera and exposed for a few seconds to the image of a properly focused object. An invisible change occurs in that part of the emulsion on which light falls.The chemical reaction is that light reduces AgBr to Ag.
Br- + photon -------> electron + Br
Ag+ + electron -------> Ag
An image of the object is formed on the film which is not visible and therefore called latent image.
3) Development of the latent image :- To make the latent image visible, the film is passed through a reducing bath containing hydroquinone (an organic reducing agent). More AgBr is reduced and the rate of reduction depends on the intensity of illumination during exposure. Thus, parts of the film which were most strongly illuminated become the darkest.
2AgBr + C6H4(OH)2 --------> 2Ag + 2HBr + C6H4O2
This process is carried out in dark to prevent general darkening of the picture. Here the image becomes visible, but the shade is negative in relation to that of the object, ie, the brighten parts will be darker and vice-versa. Hence, it is called negative plate or film. But at this stage we cannot expose it to daylight.
4) fixing :- To make the negative film to be handled in daylight, the film is dipped in a solution of sodiumthiosulphate (hypo). It dissolves away the unreacted AgBr as a complex but leaves the metallic silver unchanged.
AgBr + 2 Na2S2O3 -------> Na3[Ag(S2O3)2] + NaBr
Since no AgBr remains in the film it can be handled in daylight.
5) Printing :- Here we get a positive print of the image. The negative film is placed over a piece of paper that has been coated with photographic emulsion. This paper is then exposed to light for a few seconds. The image gets reversed and we get a positive print. This is then developed as before and fixed using hypo.
6) Toning :- This is done to improve the appearance of the photograph. For this the print is dipped in gold chloride (AuCl3) or potassium chloro platinate (K2PtCl6) which respectively gives a purple or steel grey shade to the photograph. Here the Ag particles of the picture are partially replaced by gold or platinum.
AuCl3 + 3Ag -------> 3AgCl + Au
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1) Preparation of sensitive plate or film :- It is an emulsion of AgBr in gelatin uniformly coated on a glass plate or a celluloid sheet (film) in a dark room.The process is done in dark because AgBr is highly light sensitive.
2) Exposure :- The sensitive plate or film is mounted on a camera and exposed for a few seconds to the image of a properly focused object. An invisible change occurs in that part of the emulsion on which light falls.The chemical reaction is that light reduces AgBr to Ag.
Br- + photon -------> electron + Br
Ag+ + electron -------> Ag
An image of the object is formed on the film which is not visible and therefore called latent image.
3) Development of the latent image :- To make the latent image visible, the film is passed through a reducing bath containing hydroquinone (an organic reducing agent). More AgBr is reduced and the rate of reduction depends on the intensity of illumination during exposure. Thus, parts of the film which were most strongly illuminated become the darkest.
2AgBr + C6H4(OH)2 --------> 2Ag + 2HBr + C6H4O2
This process is carried out in dark to prevent general darkening of the picture. Here the image becomes visible, but the shade is negative in relation to that of the object, ie, the brighten parts will be darker and vice-versa. Hence, it is called negative plate or film. But at this stage we cannot expose it to daylight.
4) fixing :- To make the negative film to be handled in daylight, the film is dipped in a solution of sodiumthiosulphate (hypo). It dissolves away the unreacted AgBr as a complex but leaves the metallic silver unchanged.
AgBr + 2 Na2S2O3 -------> Na3[Ag(S2O3)2] + NaBr
Since no AgBr remains in the film it can be handled in daylight.
5) Printing :- Here we get a positive print of the image. The negative film is placed over a piece of paper that has been coated with photographic emulsion. This paper is then exposed to light for a few seconds. The image gets reversed and we get a positive print. This is then developed as before and fixed using hypo.
6) Toning :- This is done to improve the appearance of the photograph. For this the print is dipped in gold chloride (AuCl3) or potassium chloro platinate (K2PtCl6) which respectively gives a purple or steel grey shade to the photograph. Here the Ag particles of the picture are partially replaced by gold or platinum.
AuCl3 + 3Ag -------> 3AgCl + Au
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