# Current Affairs NEET

#### Notes - Arthritis and Cancer

Arthritis and Cancer   (1) Arthritis             (a) Arthritis is any inflammatory condition of the joints characterised by pain and swelling.             (b) Two kinds of arthritis are : rheumatoid arthritis and osteoarthritis.             (c) There is no cure for arthritis; drugs are available which relieve pain.             (d) Rheumatoid arthritis is characterised by inflammation of the synovial membrane.             (e) A kind of rheumatoid arthritis that occurs in younger people is Still?s disease. (f) Osteoarthritis is a disease common among the elderly persons resulting from erosion of articular cartilage.             (g) Paraplegia refer to weakness or paralysis of both legs, often accompanied by loss of sensation. (h) Paraplegia is usually caused by a motor vehicle accident, sports accident, fall or gunshot wounds.   (2) Cancer: Cancer is an abnormal and uncontrolled division of cells, known as cancer cells, that invade and destroy the surrounding tissues. Generally Cancer is defined as uncontrolled proliferation of cells without any differentiation. Cancer cells are different from normal cells in some aspects. They do not remain confined to one part of the body. They penetrate and infiltrate into the adjoining tissues and dislocate their functions. Some of the cancer cells get detached from the main site of origin and travel by blood and lymph to sites distant from the original tumour and form fresh colonies, called metastasis or secondary growth.   Neoplasms or Tumours : A neoplasm (new growth) is a mass of tissue that grows in excess of normal in an uncordinated manner and continues to grow after the initial stimulus has ceased. Tumours are classified as benign or malignant.   Oncology: (G. onkos ? mass, tumour; logos ? study of) is the field of biomedicine devoted to the study and treatment of tumours. (a) Types of Tumours : There are two types of tumours : benign and malignant. (1) Benign Tumour ? (=Nonmalignant Tumour) : It remains confined to the site of its origin and does not spread to  other parts of the body. It causes limited damage to the body. It is non-cancerous. (2) Malignant Tumour (= Cancerous Tumour) : It first grows slowly. No symptoms are noticed. This stage is called the latent stage. The tumor later grows quickly. The cancer cells go beyond adjacent tissue and enter the blood and lymph. Once this happens, they migrate to many other sites in the body where the cancer cells continue to divide. It is metastasis. Only malignant tumours are properly designated as cancer.   Differences between Benign Tumour and Malignant Tumour
Diabetes Mellitus and Cardiovascular Diseases   The main non-communicable diseases are diabetes, inflammatory diseases of joints such as arthritis, gout, cardiovascular diseases and cancer.         (i) Diabetes Mellitus         (a) Diabetes is characterised by chronic hyperglycemia which is excessive concentration of glucose in the blood. (b) Diabetes is primarily a result of relative or complete lack of insulin secretion by the b cells of islets of Langerhans in pancreas.         (c) Diabetes is established by blood and urine sugar levels.                 (ii) Arthritis         (a) Arthritis is any inflammatory condition of the joints characterised by pain and swelling.         (b) Two kinds of arthritis are : rheumatoid arthritis and osteoarthritis.         (c) There is no cure for arthritis; drugs are available which relieve pain.         (d) Rheumatoid arthritis is characterised by inflammation of the synovial membrane.         (e) A kind of rheumatoid arthritis that occurs in younger people is Still?s disease.         (f) Osteoarthritis is a disease common among the elderly persons resulting from erosion of articular cartilage.         (g) Paraplegia refer to weakness or paralysis of both legs, often accompanied by loss of sensation.         (h) Paraplegia is usually caused by a motor vehicle accident, sports accident, fall or gunshot wounds.       (iii) Gout         (a) Gout results from accumulation of uric acid crystals in the synovial joints. (b) Gout is a disease associated with an inborn error of uric acid metabolism that increases production or interferes with the excretion of uric acid.         (iv) Cardiovascular Diseases       (a) Cardiovascular diseases refer to a number of diseases associated with the blood vascular system. (b) Some major cardiovascular diseases are rheumatic heart disease, hypertensive heart disease and coronary heart disease.         (1) Rheumatic heart disease Rheumatic heart disease is an autoimmune disease, most common in children after a severe throat infection by certain strain of Streptococcus bacteria.         An antigen on the surface of these bacteria is very similar to an antigen on the surface of myocardium.         The antibodies against Streptococcus may react with myocardium and cause heart difficulties.         (2) Hypertensive heart disease         Hypertensive heart disease are caused by hypertension, i.e., increased blood pressure.         Serious hypertension is a common cause of chronic heart failure particularly in older people.         (c) Coronary heart diseases Coronary heart diseases are characterised by impaired heart function due to inadequate blood flow to the heart.         Angina pectoris is the chest pain caused most often by myocardial anoxia. Attacks of angina pectoris are often related to exertion, emotional disturbance and exposure to excess cold.         Myocardial infarction is commonly called coronary or heart attack.         Arteriosclerosis is the hardening of arteries due more...

#### Notes - Diseases Caused by Helminthes

Diseases Caused by Helminthes   Diseases Caused by Helminthes : Helminthes  (flatworms and roundworms) cause many diseases in man. The more common are taeniasis, ascariasis and filariasis (elephantiasis).   (a) Taeniasis : Taeniasis is caused by the pork tapeworm Taenia solium. This tapeworm lives in the human intestine, firmly anchored by hooks and suckers. It lacks mouth and absorbs host?s digested food through its skin (saprozoic nutrition). It is hermaphrodite and undergoes self-fertilization. There is normally a single worm in one host. This worm has enormous power of reproduction.       (1) Life-history : Taenia solium has about 4 metres long, white, flat, ribbon like body comprising a small knob-like scolex, a short neck and a very long strobila of about 850 proglottides. Capsules of the worm pass out in host?s faeces and are ingested by pigs. They release embryos which reach the pig?s striated muscles, encyst and develop into infective larvae. There may be about 3,000 larvae in 500 grams of pork. The infected pork has brownish spots and is called ?measly pork?. Man gets infection by taking raw or undercooked measly pork. Pink or red appearance of the pork, when cut into slices, is an indication of its being undercooked. In the human intestine, the cyst wall breaks down, releasing the larva. The latter grows into an adult worm in 3 to 4 months.       (2) Gravid proglottids : These segments are pregnant uterus, which possess fertilized ova, and all structures disintegrate in it the dropping of gravid proglottids is called apolysis.       (3) Effect on the Host (Pathogenicity) : The tapeworm infection produces little effect on a person with a sound health. Weak person may develop a disease named taeniasis. This disease is characterized by abdominal pain, indigestion, vomiting, constipation, loss of appetite (anorexia) and weight, insomnia, lowered resistance to other diseases and nervous disorder.       (4) Cysticercosis : Tapeworm infection can also occur by taking improperly washed vegetables and water contaminated with the tapeworm capsules. Thus, the vegetarians can also get tapeworm infection. The capsules release embryos which may reach eyes or brain, develop into larvae, called cysticerci, that encyst. The cysts so formed may cause blindness or epilepsy-like symptoms and prove fatal.       (b) Ascariasis : Ascariasis is caused by the roundworm Ascaris lumbricoides. This roundworm  lives in the human small intestine. It lies free, having no organs for attachment. It takes host?s digested food by sucking through the mouth (holozoic nutrition).       (1) Life-history : Man gets infection by taking Ascaris eggs with food and water. Children become infected by ingesting soil. Eggs hatch in the host?s intestine in a few hours, each liberating a tiny (0.2 to 0.3 mm. long) worm called juvenile. The latter grows into an adult worm in 2 to 2.5 months. The adult worm has cylindrical body tapering at each end, 20 to 40 cm. long in female more...

#### Notes - Diseases Caused by Protozoa

Diseases Caused by Protozoa   Diseases Caused by Protozoans : Protozoans cause many diseases in humans. The major ones in our country are amoebiasis, diarrhoea, ciliary dysentery and malaria. Some diseases are given below:         (a) Amoebiasis (Amoebic Dysentery, Enteritis) : Amoebiasis is widespread in India due to poor sanitary conditions and polluted drinking water. The disease is caused by Entamoeba histolytica all over the world. The parasites live in the large intestine and lower part of the small intestine of humans. Infection occurs by ingesting cysts with food and drinks.         The parasites secrete a proteolytic enzyme, cytolysin, that erodes the mucous membrane of the intestine. This may form bleeding ulcers that produce dysentery. In this disease, the patient passes out blood and mucus with the stools. He also experiences severe gripping pain in the abdomen, fever, nausea, exhaustion and nervousness. In chronic cases, the intestinal will is punctured. This may prove fatal. The parasites that invade the intestinal mucous membrane may be carried by the blood stream to the liver, lungs and brain. In these organs, the parasites, feed on cells and produce severe lesions and abscesses. The latter may cause death.         (b) Diarrohea : Diarrohea is caused by a flagellate protozoan named Giardia intestinalis. Giardia was discovered by Leeuwenhoek in his own stools in 1681. It is the first human parasitic protozoan known. It is found all over the world. It inhabits the upper parts (duodenum and jejunum) of human small intestine all over the world. It lives firmly attached to the intestinal mucous membrane by adhesive disc, each perched on a separate cell. Nutrition is saprozoic, i.e., fluid food is absorbed through the body surface. Reproduction occurs by longitudinal binary fission. At intervals the parasites change into cysts which escape with the host?s faeces. Infection occurs by taking cysts with food and drinks. By covering the mucous membrane of the intestine, the parasites check or reduce the absorption of food, particularly fats. This causes diarrhoea or giardiasis (very loose and frequent stools).         Preventive Measures : Properly washing hands, fruits and vegetables before eating, and protecting the food articles from dust, flies, ants and cockroaches can check human infection.         (iii) Malaria : Malaria has been for thousands of years a very serious disease of the tropical and temperate regions. It was almost eliminated a few years back with the efforts of World Health Organization (WHO) and our National Malaria Eradication Programme (NMEP), but unfortunately, it has appeared again.         Symptoms : The attack of malaria is preceded by yawning, tiredness, headache and muscular pain. During the fever, the patient feels chilly and shivers, and has acute headache, nausea and high temperature. After a few hours, the body perspires freely and the temperature becomes normal. The cycle is repeated if no medicine is taken. Blood smear made during fever shows the malarial parasites. No parasites are seen at more...

#### Notes - Diseases Caused by Bacteria

Diseases Caused by Bacteria   Diseases Caused by Bacteria : The human diseases caused by bacteria include cholera, pneumonia, typhoid, tetanus, diphtheria, whooping cough, tuberculosis, plague, leprosy, syphilis, gonorrhoea, diarrhoea and anthrax. Bacterial diseases and their pathogens
 Disease Causative Bacterium (1)   Cholera (2)   Pneumonia (3)   Typhoid (4)   Tetanus (5)   Diphtheria (6)   Whooping cough (7)   Tuberculosis (8)   Plague (9)   Leprosy (10) Syphilis (11) Gonorrhoea (12) Diarrhoeal Diseases (13) Anthrax Vibrio comma (Vibrio cholerae) Diplococcus pneumoniae Salmonella typhi Clostridium tetani Corynebacterium diphtheriae Bordetella pertussis Mycobacterium tuberculosis Pasteurella pestis Mycobacterium leprae Treponema pallidium Neisseria gonorrhoeae Escherichia coli, Shigella dysenteriae, Campylobacter, Salmonella Bacillus anthracis
(1) Cholera : Cholera is an acute diarrhoeal disease. It is caused by a comma-shaped, motile bacterium called Vibrio comma or Vibrio cholerae. The organisms live in the intestine. Infection occurs with contaminated food and water. Incubation period varies from a few hours to 2-3 days. The symptoms of the disease are sudden onset of severe diarrhoea and vomiting. The stools are watery and give rice-water appearance. If the disease is not checked early, it leads to dehydration, loss of minerals, muscular cramps, suppression of urine and death. Rapid replacement of fluid and electrolytes is needed by oral rehydration therapy. Cholera epidemics are common in out country during fairs and floods and other natural calamities when water supply and sanitation go out of a gear. Preventive measures include proper community sanitation, personal  cleanliness, and taking boiled water and heated food. Cholera vaccine is useful during epidemic and visit to a fair. It, however, provides immunity for a short period, about 6 months. Visits to cholera affected places and families should be avoided. Vibrio cholerae first Isolate by Robert Koch in 1883.   (2) Pneumonia : Pneumonia is a serious disease of the lungs. Lymph and mucus collect in the alveoli and bronchioles. With the more...

#### Diseases caused by Viruses

Diseases caused by Viruses   (a) Important Diseases caused by Viruses: The human diseases caused by viruses include influenza, chickenpox, smallpox measles, rabies, mumps, polio, trachoma, hepatitis and AIDS.       (1) Influenza: Influenza, commonly called flu, is a highly infectious disease, which has still not been conquered. It is caused by many kinds of viruses, such as myxovirus. The latter affect the mucous membrane of nose, throat and upper respiratory tract. The common symptoms are discharge from the nose, sneezing, fever, body aches, coughing and general weakness. The infection spreads by discharges from the nose and throat. The incubation period is just from 24-72 hours. Influenza generally lasts for 4 or 5 days. Rest quickens the recovery. If neglected, complications like pneumonia, bronchitis and ear infection may develop. There is no vaccine for influenza.         Influenza tends to occur in epidemic or pandemic form with varying virulence.       (2) Chickenpox : It is a common, relatively mild, highly contagious disease of children, generally under 10 years of age. It is caused by a virus called chickenpox virus (varicella zoster). Fever, aches and general discomfort are the symptoms. Dewdrop-like sores appear in successive crops, first on the trunk. The sores open and a fluid seeps out a short time later. The disease spreads by direct contact with skin sores or with clothes and other articles soiled with discharges from sores. Incubation period is 2-5 weeks. The sores heal without leaving scars. Preventive measure is isolation of the patient till all crusts fall off. One attack of chickenpox ordinarily gives permanent immunity to the disease. There is no vaccine against chickenpox. Chickenpox is rarely fatal, but in adults attack could be severe.       (3) Smallpox : Smallpox is an acute, highly communicable disease. It is caused by a virus named variola virus. It starts as a sudden onset of high fever accompanied by headache, backache, and pains all over the body. Rash appears on the 3rd or 4th day of illness. The rash gradually changes into pustules (pimples) containing clear fluid. The pustules finally form scabs which fall off by the 3rd week. The scabs leave behind permanent pitted scars, the pockmarks, on the skin. The disease may lead to blindness.         Smallpox spreads by exudate from pustules on the skin of the infected persons. It also spreads by oral and nasal discharges during coughing and sneezing, and by contact with the clothes of the patient soiled with discharges. Its incubation period is about 12 days. It is very serious, disfiguring and highly fatal disease. It has now been largely controlled through vaccination. Smallpox vaccine was first prepared by Edward Jenner in 1798.       (4) Measles : Measles is one of the most prevalent and serious diseases of children, generally 3-5years old. It is caused  by a virus named rubeola virus. It is characterized by fever, inflammation of nasal mucous membrane, red watery eyes more...

#### Notes - Free Energy and Work Function

Free Energy and Work Function     Gibb's free energy (G) is a state function and is a measure of maximum work done or useful work done from a reversible reaction at constant temperature and pressure.             (1) Characteristics of free energy (i) The free energy of a system is the enthalpy of the system minus the product of absolute temperature and entropy i.e., $G=H-TS$ (ii) Like other state functions E, H and S, it is also expressed as $\Delta G$. Also $\Delta G=\Delta H-T\Delta {{S}_{system}}$where $\Delta S$ is entropy change for system only. This is Gibb's Helmholtz equation.       (iii)    At equilibrium   $\Delta G=0$       (iv) For a spontaneous process decrease in free energy is noticed i.e., $\Delta G=-ve$. (v) At absolute zero, $T\Delta S$is zero. Therefore if $\Delta G$is ? ve, $\Delta H$should be ? ve or only exothermic reactions proceed spontaneously at absolute zero.       (vi) $\Delta {{G}_{system}}=T\Delta {{S}_{universe}}$       (vii) The standard free energy change,  $\Delta {{G}^{o}}=-2.303RT{{\log }_{10}}\,K,$ where K is equilibrium constant. (a) Thus if $K>1,$then $\Delta {{G}^{o}}=-ve$thus reactions with equilibrium constant K>1 are thermodynamically spontaneous.          (b) If K<1, then $\Delta {{G}^{o}}=+ve$ and thus reactions with equilibrium constant K<1 are thermodynamically spontaneous in reverse direction. (2) Criteria for spontaneity of reaction: For a spontaneous change $\Delta G=-ve$ and therefore use of $\Delta G=\Delta H-T\Delta S,$provides the following conditions for a change to be spontaneous.   more...

#### Notes - Bond Energy

Bond Energy or Bond Enthalpies   When a bond is formed between atoms, energy is released. Obviously same amount of energy will be required to break the bond. The energy required to break the bond is termed bond dissociation energy. The more precise definition is,         ?The amount of energy required to break one mole of bond of a particular type between the atoms in the gaseous state, i.e., to separate the atoms in the gaseous state under 1 atmospheric pressure and the specified temperature is called bond dissociation energy.?       For example,   $H-H(g)\to 2H(g);$         $\Delta H=+\,433\,kJ\,mo{{l}^{-1}}$                            $Cl-Cl(g)\to 2Cl\,(g);$      $\Delta H=+\,242.5\,kJ\,mo{{l}^{-1}}$                            $H-Cl(g)\,\to H(g)+Cl(g);$$\Delta H=+\,431\,kJ\,mo{{l}^{-1}}$                            $I-I(g)\to 2I(g);$              $\Delta H=+\,15.1\,kJ\,mo{{l}^{-1}}$                            $H-I(g)\to H(g)+I(g);$     $\Delta H=+\,299\,kJ\,mo{{l}^{-1}}$ The bond dissociation energy of a diatomic molecule is also called bond energy. However, the bond dissociation energy depends upon the nature of bond and also the molecule in which the bond is present. When a molecule of a compound contains more than one bond of the same kind, the average value of the dissociation energies of a given bond is taken. This average bond dissociation energy required to break each bond in a compound is called bond energy.   Bond energy is also called, the heat of formation of the bond from gaseous atoms constituting the bond with reverse sign.                            $H(g)+Cl(g)\to H-Cl\,(g);$$\Delta H=-\,431\,kJ\,mo{{l}^{-1}}$       Bond energy of $H-Cl=-$ (enthalpy of formation) $=-(-431)=+\,431\,kJ\,mo{{l}^{-1}}$          Consider the dissociation of water molecule which consists of two $O-H$ bonds. The dissociation occurs in two stages.                            ${{H}_{2}}O(g)\to H(g)+OH(g);\,$$\Delta H=497.89\,kJ\,mo{{l}^{-1}}$                            $OH(g)\to H(g)+O(g);$   $\Delta H=428.5\,kJ\,mo{{l}^{-1}}$       The average of these two bond dissociation energies gives the value of bond energy of $O-H.$       Bond energy of $O-H$ bond $=\frac{497.8+428.5}{2}=463.15\,kJ\,mo{{l}^{-1}}$ Similarly, the bond energy of $N-H$ bond in $N{{H}_{3}}$ is equal to one ? third of the energy of dissociation of $N{{H}_{3}}$ and those of $C-H$ bond in $C{{H}_{4}}$ is equal to one ? fourth of the energy of dissociation of $C{{H}_{4}}.$       Bond energy of $C-H=\frac{1664}{4}=416\,kJ\,mo{{l}^{-1}}$                            $[C{{H}_{4}}(g)\to C(g)\,+4H(g);$$\Delta H=1664\,kJ\,mo{{l}^{-1}}]$             Applications of bond energy         (1) Heat of a reaction $=\Sigma$Bond energy of reactants ? $\Sigma$ Bond energy of products.             Note : In case of atomic species, bond energy is replaced by heat of atomization.                 Order of bond energy in halogen $Cl>Br>{{F}_{2}}>{{I}_{2}}$ (2) Determination of resonance energy : When a compound shows resonance, there is considerable difference between the heat of formation as calculated from bond energies and that determined experimentally.       Resonance energy = Experimental or actual heat of formation ~ Calculated heat of formation.

#### Notes - Heat of Reaction

Heat of Reaction     Heat of reaction is defined as the amount of heat evolved or absorbed when quantities of the substances indicated by the chemical equation have completely reacted. The heat of reaction (or enthalpy of reaction) is actually the difference between the enthalpies of the products and the reactants when the quantities of the reactants indicated by the chemical equation have completely reacted. Mathematically, Enthalpy of reaction (heat of reaction) $=\Delta H=\Sigma {{H}_{P}}-\Sigma {{H}_{R}}$   (1) Factors which influence the heat of reaction : There are a number of factors which affect the magnitude of heat of reaction. (i) Physical state of reactants and products : Heat energy is involved for changing the physical state of a chemical substance. For example in the conversion of water into steam, heat is absorbed and heat is evolved when steam is condensed.  Considering the following two reactions      ${{H}_{2}}(g)+\frac{1}{2}{{O}_{2}}(g)={{H}_{2}}O(g);$ $\Delta H=-\,57.8\,kcal$      ${{H}_{2}}(g)+\frac{1}{2}{{O}_{2}}(g)={{H}_{2}}O(l);$ $\Delta H=-\,68.32\,kcal$ It is observed that there is difference in the value of $\Delta H$ if water is obtained in gaseous or liquid state. $\Delta H$ value in second case is higher because heat is evolved when steam condenses. Hence, physical sate always affects the heat of reaction. (ii) Allotropic forms of the element : Heat energy is also involved when one allotropic form of an element is converted into another. Thus, the value of $\Delta H$ depends on the allotropic form used in the reaction. For example, the value of $\Delta H$ is different when carbon in the form of diamond or in amorphous form is used. C (diamond) $+{{O}_{2}}(g)\,\to C{{O}_{2}}(g);$  $\Delta H=-\,94.3\,kcal$                 C (amorphous) $+{{O}_{2}}(g)\to C{{O}_{2}}(g);$ $\Delta H=-\,97.6\,kcal$ The difference between the two values is equal to the heat absorbed when 12g of diamond is converted into 12g of amorphous carbon. This is termed as heat of transition. (iii) Temperature : Heat of reaction has been found to depend upon the temperature at which reaction is occurring. The variation of the heat of reaction with temperature can be ascertained by using Kirchhoff's equation.                $\frac{\Delta {{H}_{{{T}_{2}}}}-\Delta {{H}_{{{T}_{1}}}}}{{{T}_{2}}-{{T}_{1}}}=\Delta {{C}_{P}}$ Kirchhoff's equation at constant volume may be given as,                               $\frac{\Delta {{E}_{{{T}_{2}}}}-\Delta {{E}_{{{T}_{1}}}}}{{{T}_{2}}-{{T}_{1}}}=\Delta {{C}_{\nu }}$ (iv) Reaction carried out at constant pressure or constant volume : When a chemical reaction occurs at constant volume, the heat change is called the internal energy of reaction at constant volume. However, most of the reactions are carried out at constant pressure; the enthalpy change is then termed as the enthalpy of reaction at constant pressure. The more...

#### Notes - 2nd and 3rd Law of Thermodynamics

Second Law of Thermodynamics   All the limitations of the first law of thermodynamics can be remove by the second law of thermodynamics. This law is generalisation of certain experiences about heat engines and refrigerators. It has been stated in a number of ways, but all the statements are logically equivalent to one another.          (1) Statements of the law (i) Kelvin statement : ?It is impossible to derive a continuous supply of work by cooling a body to a temperature lower than that of the coldest of its surroundings.? (ii) Clausius statement : ?It is impossible for a self acting machine, unaided by any external agency, to convert heat from one body to another at a higher temperature or Heat cannot itself pass from a colder body to a hotter body, but tends invariably towards a lower thermal level.? (iii) Ostwald statement : ?It is impossible to construct a machine functioning in cycle which can convert heat completely into equivalent amount of work without producing changes elsewhere, i.e., perpetual motions are not allowed.? (iv) Carnot statement: ?It is impossible to take heat from a hot reservoir and convert it completely into work by a cyclic process without transferring a part of it to a cold reservoir.?   (2) Proof of the law : No rigorous proof is available for the second law. The formulation of the second law is based upon the observations and has yet to be disproved. No deviations of this law have so far been reported. However, the law is applicable to cyclic processes only.   Third Law of thermodynamics   This law was first formulated by German chemist Walther Nernst in 1906. According to this law, ?The entropy of all perfectly crystalline solids is zero at the absolute zero temperature. Since entropy is a measure of disorder, it can be interpreted that at absolute zero, a perfectly crystalline solid has a perfect order of its constituent particles.? The most important application of the third law of thermodynamics is that it helps in the calculation of absolute entropies of the substance at any temperature T. $S=2.303{{C}_{p}}\log T$ Where $Cp$the heat capacity of the substance at constant is pressure and is supposed to remain constant in the range of 0 to T.            Limitations of the law          (1) Glassy solids even at 0K has entropy greater than zero. (2) Solids having mixtures of isotopes do not have zero entropy at 0K. For example, entropy of solid chlorine is not zero at 0K. (3) Crystals of Co, N2O, NO, H2O, etc. do not have perfect order even at 0K thus their entropy is not equal to zero.   Entropy and Entropy change   (1) Definition : Entropy is a thermodynamic state quantity which is a measure of randomness or disorder of the molecules of the system. Entropy is represented by more...

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 $\Delta H$ $\Delta S$ $\Delta G$ Reaction characteristics Example ? + Always negative Reaction is spontaneous at all temperatures $2{{O}_{3(g)}}\to 3{{O}_{2(g)}}$ + ? Always positive Reaction is non spontaneous at all temperatures $3{{O}_{2(g)}}\to 2{{O}_{3(g)}}$