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An Introduction To Chemistry
An Introduction To Chemistry The science in which substances are examined to find out what they are made of, how they act under different conditions, and how they are combined or separated to/from other substances. To paraphrase that, Chemistry deals with finding what different substances are made of, what kind of transformations take place, and different chemically related facts about a certain organism or substance. Physical Chemistry is the study of the patterns of chemical behavior in chemical reactions under various conditions, which result from the chemical and physical properties of substances. Much of physical chemistry involves measurement of some kind. It covers the follwoing: Factor-label Method In math you use numbers, in chemistry we use quantities. A quantity is described by a number and a unit. 100 is a number : 100 Kg is a quantity (notice that in chemistry we give meaning to the numbers). In science we solve a lot of the "math" by watching the units of the quantities There are two main rules to solving science problems with the factor-label method: 1. Always carry along your units with any measurement you use. 2. You need to form the appropriate labeled ratios (equalities). Example Problem: How many centimeters in 2 meters? You will see from the metric conversion chart that 1 meter = 100 cm we turn this into a ratio by writing it like this: Once you have the equalities you must pick the one that will cancel out the units leaving the desired units. Then multiply your starting quantity (2 meters) by the equality that will give you your desired units. Practice Problems: 1. How many wheels on 350 Ford pickups (use the equality 1 pickup = 4 tires) -the starting units are pickups, the ending units need to be wheels. 2. How many millimeters in 34 hectometers (use the equality 10,000 mm = 1 hectometer)? Sometimes you will need to multiply by more than one ratio to get to your desired units, you can do this by using linking units. Your setup will look like this: Solids, Liquids, Gases Compared Solids The particles of a solid are always arranged in an orderly manner. They have a constant volume, because the particles are so closely packed together, with very little space between them. Compression of a solid to any large extent is not possible because of this tight pack of particles. Liquids A fluid is any substance that flows, and liquids are examples of fluids. The particles in liquids are allowed to freely move and change their positions. At all times are the particles moving, moving from neighbor to neighbor. This is why we can 'pour' a liquid into another container. A liquids confinement are the borders of its container. This is why when we pour a liquid into another container, there is conformity to the shape of the container. Compression of a liquid to any large extent is not possible. Gases Gases is another example of a fluid, it flows! The particles of gases are however much different than that of solids and liquids. The particles in gases are not neatly arranged, and they don't even touch each other most of the time. There is lots of space in between particles, which is why when put in a container, it is filled with the gas. And when released from a container, the gas is dispersed. The particles in gases are always moving, just like the particles in a liquid. Types of Chemical reactions Combustion A combustion reaction is when all substances in a compound are combined with oxygen, which then produces carbon dioxide and water. Combustion is commonly called burning. It is an exothermic reaction, which means heat is produced and is easily distinguished. Combustion occurs predominantly in automobiles, homes, and in factories. An example of a combustion reaction is as follows: CxHy + O2 --> CO2 + H2O Synthesis A synthesis reaction is when there is a combination of two or more substances and a compound results. An example of a synthesis reaction is as follows: A + B --> AB Decomposition Decomposition is the opposite of synthesis. It is when a compound is broken down into simpler substances, usually through electrolysis. An example of decomposition is as follows: AB --> A + B Dissociation Dissociation is commonly mistaken as decomposition, but there is a difference. When the compound is broken down, it is broken down into ions rather than atoms, so there will be a charge on the product side of the equation. An example of dissociation is as follows: AB --> A+ + B Single Replacement Reactions In a single replacement reaction, there is a rule that is always followed. A metal replaces a metal, or a nonmetal replaces a nonmetal. An example of a single replacement reaction is as follows: A + BC --> AC + B Double Replacement Reactions In a double replacement reaction, this rule is always followed. A metal replaces a metal, and a nonmetal replaces a nonmetal. An example of a double replacement reaction is as follows: AB + XY --> AY + XB |
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Chemical Compounds A compound is a molecule consisting of two or more elements. It is different than a mixture of different elements or materials. Molecules that are the combination of atoms of the same element are not considered compounds. Compounds are classified according the the number of different elements in the molecule. Questions you may have include:
Compound different than a mixture Compounds are the chemical bonding of two or more different elements into a molecule. They are different than mixtures, which is a combination of two or more different materials that are not in chemical combination. Mixtures can be separated by mechanical means, while compounds can't be separated that way. Another way a compound is different than a mixture is that an individual compound has the same proportion of each element in all of its molecules. For example, the water molecule H2O is a compound that always is made up of two atoms of hydrogen and one atom of oxygen. Examples of other compounds include: Carbon monoxide: CO Carbon dioxide: CO2 Acetone: (CH3)2CO Zinc sulfide: ZnS Magnesium chloride: MgCl2 Molecules that are not compounds There are a number of molecules that are a combination of the same element. Although they can be involved in chemical reactions, they are not considered compounds. Common examples of such molecules include: Oxygen molecule: O2 Ozone: O3 Hydrogen molecule: H2 Nitrogen molecule: N2 Chlorine molecule: Cl2 Types of compounds Compounds can be classified according to the number of different elements in its molecule. The most common are the binary compound, which consists of two elements, and the ternary compound, consisting of three elements. Binary compounds have two elements Examples of binary compounds include: Table salt or sodium chloride: NaCl Iron sulfide: FeS Water: H2O Ternary compounds have three elements Examples of ternary compounds include: Sodium hydroxide: NaOH Perchloric acid: HClO4 Sulfuric acid: H2SO4 Summary A compound consists of two or more elements in a chemically combined as a molecule. This is as opposed to a mixture, which is not a chemical combination. There are molecules that are not considered compounds. Compounds are classified according the the number of different elements in the molecule.
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Chemical Elements A chemical element (usually just called an element) is a class of atoms with a specific number of protons in their nuclei (plural of nucleus in Latin). Each element has its own name and is usually listed according to its atomic number. Isotopes of an element have different numbers of neutrons. Often the average atomic weight of an element is also stated. This number takes into account the percentages of isotopes, the masses of the particles, and nuclear effects. The average atomic weight is approximately the number of protons and neutrons of the most common isotope of the element. Questions you may have include:
Atomic number The elements are listed according to their atomic number. The atomic number is designated by the number of protons in the nucleus. For example, Hydrogen has one proton, Helium has two protons, Oxygen has eight protons, and so on. Since the number of electrons equals the number of protons in an electrically stable atom, the atomic number determines many of the chemical characteristics of the element. This is shown in the Periodic Table. Average atomic weight The atomic weight of an atom was originally defined as a sum of its protons and neutrons. The unit of measurement is the atomic mass unit (amu or u). Mass defect Later, it was found that some mass is lost to binding energy required to hold the nucleus together. This is called the mass defect and is the principle behind nuclear energy, according the to the famous equation E = mc2. Thus the atomic weight of an individual atom is slightly different than the number of protons and neutrons. Isotopes An element has several different number of neutrons in its nucleus. Each is called an isotope of that element. For example, Oxygen typically has 8 protons and 8 neutrons in its nucleus, with an atomic weight of about 16 u. But there is a very small percentage of Oxygen atoms with 9 neutrons in their nuclei and atomic weight of approximately 17 u. There are even some atoms with 10 neutrons. Thus for the element Oxygen, taking into account for the mass defect and averaging the atomic weight for all its isotopes, you get an average atomic weight of 15.9994 u for Oxygen. Finding number of neutrons Looking on the list of elements below, you will see that the Average Atomic Weight is not integer. You can find the number of neutrons in the most common and stable nucleus of an element by simply rounding off the atomic weight and subtracting the atomic number (number of protons). For example, Magnesium (Mg) is number 12 and has an average atomic weight of 24.3050 u. This rounds off to 24. Thus the number of protons in the most common isotope of Magnesium is 24 - 12 = 12 neutrons. List of elements Following is a list of all the elements, according to atomic number. Elements with the weight in [brackets] are so unstable that scientists have not been able to accurately measure the weight. All of the elements after Uranium (number 92) are artificial and unstable. An artificial element is one that is so unstable that it does not occur in nature. High energy atomic collisions can manufacture such an element. It immediately decays into a stable element. Atomic Number Symbol Name Average Atomic Weight(u) 1 H Hydrogen 1.00794 2 He Helium 4.002602 3 Li Lithium 6.941 4 Be Beryllium 9.012182 5 B Boron 10.811 6 C Carbon 12.0107 7 N Nitrogen 14.0067 8 O Oxygen 15.9994 9 F Fluorine 18.9984032 10 Ne Neon 20.1797 11 Na Sodium 22.989770 12 Mg Magnesium 24.3050 13 Al Aluminium 26.981538 14 Si Silicon 28.0855 15 P Phosphorus 30.973761 16 S Sulfur 32.065 17 Cl Chlorine 35.453 18 Ar Argon 39.948 19 K Potassium 39.0983 20 Ca Calcium 40.078 21 Sc Scandium 44.955910 22 Ti Titanium 47.867 23 V Vanadium 50.9415 24 Cr Chromium 51.9961 25 Mn Manganese 54.938049 26 Fe Iron 55.845 27 Co Cobalt 58.933200 28 Ni Nickel 58.6934 29 Cu Copper 63.546 30 Zn Zinc 65.39 31 Ga Gallium 69.723 32 Ge Germanium 72.64 33 As Arsenic 74.92160 34 Se Selenium 78.96 35 Br Bromine 79.904 36 Kr Krypton 83.80 37 Rb Rubidium 85.4678 38 Sr Strontium 87.62 39 Y Yttrium 88.90585 40 Zr Zirconium 91.224 41 Nb Niobium 92.90638 42 Mo Molybdenum 95.94 43 Tc Technetium [98] 44 Ru Ruthenium 101.07 45 Rh Rhodium 102.90550 46 Pd Palladium 106.42 47 Ag Silver 107.8682 48 Cd Cadmium 112.411 49 In Indium 114.818 50 Sn Tin 118.710 51 Sb Antimony 121.760 52 Te Tellurium 127.60 53 I Iodine 126.90447 54 Xe Xenon 131.293 55 Cs Caesium 132.90545 56 Ba Barium 137.327 57 La Lanthanum 138.9055 58 Ce Cerium 140.116 59 Pr Praseodymium 140.90765 60 Nd Neodymium 144.24 61 Pm Promethium [145] 62 Sm Samarium 150.36 63 Eu Europium 151.964 64 Gd Gadolinium 157.25 65 Tb Terbium 158.92534 66 Dy Dysprosium 162.50 67 Ho Holmium 164.93032 68 Er Erbium 167.259 69 Tm Thulium 168.93421 70 Yb Ytterbium 173.04 71 Lu Lutetium 174.967 72 Hf Hafnium 178.49 73 Ta Tantalum 180.9479 74 W Tungsten 183.84 75 Re Rhenium 186.207 76 Os Osmium 190.23 77 Ir Iridium 192.217 78 Pt Platinum 195.078 79 Au Gold 196.96655 80 Hg Mercury 200.59 81 Tl Thallium 204.3833 82 Pb Lead 207.2 83 Bi Bismuth 208.98038 84 Po Polonium [209] 85 At Astatine [210] 86 Rn Radon [222] 87 Fr Francium [223] 88 Ra Radium [226] 89 Ac Actinium [227] 90 Th Thorium 232.0381 91 Pa Protactinium 231.03588 92 U Uranium 238.02891 93 Np Neptunium [237] 94 Pu Plutonium [244] 95 Am Americium [243] 96 Cm Curium [247] 97 Bk Berkelium [247] 98 Cf Californium [251] 99 Es Einsteinium [252] 100 Fm Fermium [257] 101 Md Mendelevium [258] 102 No Nobelium [259] 103 Lr Lawrencium [262] 104 Rf Rutherfordium [261] 105 Db Dubnium [262] 106 Sg Seaborgium [266] 107 Bh Bohrium [264] 108 Hs Hassium [277] 109 Mt Meitnerium [268] 110 Uun Ununnilium [281] 111 Uuu Unununium [272] 112 Uub Ununbium [285] 114 Uuq Ununquadium [289] 116 Uuh Ununhexium unknown 118 Uuo Ununoctium unknown Summary An element is a basic chemical unit. Elements have an atomic number and atomic weight assigned to them. There are 92 natural elements, plus some that have been artificially created. Artificial elements are highly unstable and usually exist for only a fraction of a second. regards faryal shah
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Chemical Equations chemical equation describes the amounts of chemical materials needed to form new substances. This type of equation is important is defining how many units of each substance must be mixed to get the desired result. It is similar to a cookbook recipe. The chemical equation also shows how many units there will be of each resulting substance. There is a parallel between chemical equations and algebraic equations. Questions you may have include:
Chemical cookbook A chemical equation is similar to a cookbook recipe in that it shows how many units of each substance is required to give the desired result. It shows the combination of various elements and/or molecules and then the resulting elements and/or molecules. Just like with an algebraic equation, the number of atoms on the left must equal the number of atoms on the right. An example of a chemical recipe or equation is combining 2 units of Sodium (Na) with one molecule of Chlorine gas (Cl2) to form 2 units of table salt: 2Na + Cl2 → 2NaCl As you recall in Chemical Formulas, the full-sized number in front of an element or molecule is how many units there are of that item. The small sub-number behind an element indicates how many atoms of that element there are in the molecule. Also note that Chlorine gas is never a single atom. It is always a molecule (Cl2). This is also true for Hydrogen gas (H2) and Oxygen (O2). Yields symbol The yields symbol ( → ) is used instead of the equal sign ( = ). The equation above is read, Sodium plus Chlorine yields Sodium Chloride. It means that this chemical reaction goes in one direction. ←→ symbol There are chemical reactions where molecules may go back and forth or combine and separate. In those special cases, the ( ←→ ) symbol is used. One example is when you mix salt in water, resulting in salty water, which is water containing Sodium and Chlorine ions. This chemcial reaction goes both ways. NaCl + H2O ←→ H2O + Na+1 + Cl-1 Note that ions have a small superscript number indicating their excess charges. Na+1 means the Sodium ion is missing an electron, thus its (+) charge. Also note that ions are individual atoms, so when the solution is formed, an element like Cl does not need to be a molecule. It is only Cl2 when existing as a gas. Depending on the mixture and temperature, the water can be salty or the salt can precipitate out and collect on the bottom of the container. Complex equations Just as a cookbook recipe usually has a number of ingredients, so can chemical equations by complex. In some highly complex chemical reactions, you may even have a series of equations for chemical reactions that must be done in a particular order. An example of a single-step chemical reaction involving several compounds is a method to create Chlorine gas by heating Manganese Dioxide mixed with Sodium Chloride and Sulfuric acid is seen in the following equation: 2NaCl + 2H2SO4 + MnO2 → Na2SO4 + MnSO4 + 2H2O + Cl2 You can see the importance of balancing such an equation. Balancing equations Sometimes you will see a chemical equation that must be balanced. For example, suppose you were going to burn some Propane gas (C3H8). Combining Propane with Oxygen results in Carbon Dioxide and water. Does C3H8 + O2 → CO2 + H2O ?? You can see that the number of Carbon (C), Oxygen (O) and Hydrogen (H) atoms on the left of the equation does not equal the number on the right side. There are 3 C, 8 H, and 2 O on the left and 1 C, 3 O, and 2 H on the right. Use trial-and-error So, to balance the equation, you must do some clever trial-and-error guesses. Sometimes the unbalanced equation is written with unknowns, similar to what you would do in Algebra: wC3H8 + xO2 → yCO2 + zH2O where w, x, y and z are the unknown numbers from of each molecule in the equation. Logical approach One logical, trial-and-error approach to balancing this chemical equation is as follows: Since there are 8 H on the left, perhaps there are 4 H2O on the right. Since there are 3 C on the left, perhaps there are 3 CO2 on the right. The resulting equation is then: C3H8 + O2 → 3CO2 + 4H2O ?? The C's and H's balance, but there are 10 O on the right and only 2 on the left. So, let's try 5 O2 on the left. Now the equation balances out. C3H8 + 5O2 → 3CO2 + 4H2O Count the number of Carbon atoms, Hydrogen atoms, and Oxygen atoms on the left and compare with the number on the right side of the equation. Summary Chemical equations are similar to algebraic equations, in that the total number of atoms of each element on the left side must equal the number for that element on the right side. You can have complex equations and series of equations for some chemical reactions. You usually can use a logical trial-and-error method to balance a chemical equation. regards faryal shah
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Farhan Darya Khan (Wednesday, October 20, 2010) |
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Chemical Formulas Chemical formulas such as HClO4 can be divided into empirical formula, molecular formula, and structural formula. Chemical symbols of elements in the chemical formula represent the elements present, and subscript numbers represent mole proportions of the proceeding elements. Note that no subscript number means a subscript of 1. From a chemical point of view, an element contained in the substance is a fundamental question, and we represent the elemental composition by a chemical formula, such as H2O for water. This formula implies that the water molecules consist of 2 hydrogen, and 1 oxygen atoms. The formula H2O is also the molecular formula of water. For non-molecular substances such as table salt, we represent the composition with an empirical formula. Sodium chloride is represented by NaCl, meaning that sodium and chlorine ratio in sodium chloride is 1 to 1. Again, the subscript 1 is omitted. Since table salt is an ionic compound, the formula implies that numbers of Na+ ions, and Cl- ions are the same in the solid. The subscript numbers in an empirical formula should have no common divisor. A structural formula reflects the bonding of atoms in a molecule or ion. For example, ethanol can be represented by CH3CH2OH. This is a simple way of representing a more elaborated structure shown on your left. Molecular structures are often beautiful, but the representation is an artwork. For example, a 3-dimensional structure of cyclohexane is shown on the right. This is a chair form, and another structure has a boat form. You will learn more about it in organic chemistry. The molecular formula of benzene is C6H6, and its empirical formula is CH. You may refer to a substance by its name, and recognize it by its properties. Properties are related to the structure and the composition of the molecules. Knowing the chemical formula is a giant step towards understanding a substance. Formula Weights, Molecular Weights and Molar Masses The formula weight is the sum of all the atomic weights in a formula. The evaluation of formula weight is illustrated in this example. Example 1 What is the formula weight of sufuric acid H2SO4? Solution: The formula also indicates a mass as the sum of masses calculate this way 2*1.008 + 32.0 + 4*16.0 = 98.0 where 1.008, 32.0 and 16.0 are the atomic weights of H, S, and O respectively. Discussion: If the formula is a molecular formula, the mass associated with it is called molecular mass or molecular weight. As an exercise, work out the following problem. What is the molecular weight of caffeine, C8H10N4O2? The diagram shown here is a model of the caffeine molecule. With the aid of a table of atomic weights, a formula indirectly represents the formula weight. If the formula is a molecular formula, it indirectly represents the molecular weight. For simplicity, we may call these weights molar masses, which can be formula weights or molecular weights. A chemical formula not only represents what a substance is made of, it provides a great deal of information about the substance. Do you know that chemical formulas are used all over the world, regardless of the language? Chinese, Russian, Japanese, African, and South Americans use the same notations we do. Thus, H2S is recognized as a smelly gas all over the world. Chemical formula is an international or universal language. Weight percentage and mole percentage A chemical formula not only gives the formula weight, it accurately represents the percentages of elements in a compound. On the other hand, if you know the percentage of a compound, you may figure out its formula. Percentage based on weights is called weight percentage, and percentage based on the numbers of atoms or moles is called mole percentage. Example 2 What are the weight and mole percentages of S in sufuric acid? Solution: From example 1, we know that there are 32.0 g or S in 98.0 g or sulfuric acid. Thus the weight percentage is Weight percentage = 32/98 = 32.7% From the formula, there is one S atom among 7 atoms in H2SO4 Mole percentage = 1/7 = 14.3% Discussion: You have learned what weight and mole percentages are and how to evaluate them in this example. As an exercise, work out the the following problem: What are the weight and mole percentages of C, H, N, and O for caffeine, C8H10N4O2? Determination of Chemical Formulas How would you find the chemical formula of a substance? If you know the substance, its formula and other information is usually listed in a handbook. Handbooks such as the CRC Handbook of Chemistry and Physics contain information on millions of substances. If you are a researcher and you made a new compound that no one has ever made it before, then you need to determine its empirical or molecular formula. For an organic compound, you burn it completely to convert all carbon (C) to CO2, and all hydrogen (H) to H2O. CxH2y =(burned in O2)=> x CO2 + y H2O Thus, from the weight of CO2 and H2O produced by burning a definite amount of the substance, you can figure out the percent of C and H in the compound. Nitrogen is determined by converting it to NH3. The amount of NH3 can be determined by titration, and the percentage can also be determined. Percentage of O is usually obtained by subtracting all percentages of C, H, and N, if the compound does not contain any other element. Example 3 A compound containing 92.3 weight percent of carbon and 7.7 weight percent of H. What is the empirical formula? Solution: Assume that you have 100 g of the compound, then you have 92.3 g of carbon and 7.7 g of hydrogen. Thus the mole ratio of C to H should be 92.3 7.7 ---- : ----- = 7.7 : 7.7 = 1 : 1 12 1.008 Thus, the empirical formula is CH. Discussion: You have learned how to determine a chemical formula if the percentages of various elements present in the compound are known in this example. To test your skill, you may be asked to work out the empirical formula of any compound. Try this problem: Aspartic acid contains 36.09% C, 5.30% H, 10.52% N, and 48.08 O by weight. What is the empirical formula for aspartic acid? Aspartic acid is one of the non-essential aminoacids, usually present in young plants. It is obtained by hroolysis of asparagine, which is abundant in asparagus. Example 4 A compound with an empirical formula of CH has a molecular weight of 78 g/mol. What is the molecular formula? Solution: The formula weight of CH is 13.0. Since 78/13 = 6, the molecular formula is C6H6, the formula for benzene. Discussion: This example illustrates the difference between empirical and molecular formula, for which, the molecular weight must be known. Example 5 When 1.00 g of benzene is burned, how much CO2 and H2O should be produced? Hint: 1 mol C 1 mol CO2 44.0 g CO2 1 g CH --------- ----------- ------------ = 3.38 g CO2 13 g CH 1 mol C 1 mol CO2 Use the same method to calculate the amount of H2O produced (Ans. 0.692 g). Example 6 When 1.00 g of a compound containing only carbon and hydrogen is burned completely, 3.14 g of CO2 and 1.29 g of H2O is produced. What is the empirical formula? Hint: Amounts of carbon and H in 3.14 g of carbon dioxide, 12 g C 1 mol C 3.14 g CO2 -------- --------- = 0.0714 mol C 44 g CO2 12 g C 2 g H 1 mol H 1.29 g H2O -------- --------- = 0.143 mol H 18 g H2O 1 g H Thus, mole ratio of C : H is 0.0714 : 0.143 = 1 : 2. Therefore, the empirical formula is CH2 Discussion: The molecular formula for ethylene is C2H4 and cyclohexane is C6H12. What are their molecular weights? Example 7 Chloroform is a common solvent used in chemical labs. It has a molecular formula of CHCl3. What is the weight percentage of chlorine (Cl)? (Atomic weight, Cl, 35.453; H, 1.00794; C, 12.0110) Hint: You should understand the reason for using this formula to calculate it: 3*35.453 ---------------------------- = 89.094% (weight percentage) 3*35.453 + 12.011 + 1.00794 Discussion: What is the weight percentage of C in CCl4? regards faryal shah
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Farhan Darya Khan (Wednesday, October 20, 2010), hamarapakistan (Monday, December 07, 2009) |
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Have you appeared and cleared in Chemistry paper?
if yes , please guide me about the books regarding chemistry paper 1 and 2 |
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