Prepared by Haq Nawaz MSC (UK), Dip. Applied chemistry (UK) web add: https://haqnawaz123.angelfire.com/firstyear/ Basic Concepts Chemistry: The branch of science which deals with the study of compositions, structures, properties of matter and chemical changes involved in it is called Chemistry. OR
Q. Why Study Chemistry? Chemistry helps us to understand the world around us and how things work. For example, you can answers easily the questions asked by someone related to our daily life such as 'why laundry detergent works better in hot water or how baking soda works or why not all pain relievers work equally well on a headache etc' . If you know some chemistry, you can make educated choices about everyday products that you use. Chemistry has become an interdisciplinary science and today no scientific work can escape chemistry. The study of chemistry is very important because it is used in medicines, industry, agriculture and energy resources etc. The study of chemistry Chemistry is largely an experimental science and can be viewed broadly in three levels
2Na + 2H2O --------> 2NaOH + H2 + Heat
Matter: Anything that occupies space and has mass is called matter.
Mass: The quantity of matter inside a body is called mass.
Weight: The force at which the earth attracts a body toward its centre is called weight Mathematically weight is the product of mass and gravitational acceleration (g) W= mg
Explain that mass and weight are totally different quantities. Mass is a measure of how much matter an object has, while weight is a measure of how strongly gravity pulls on that matter. Thus if you travel to the moon your weight will change because the pull of gravity is weaker there than on Earth but, your mass will stay the same because you are still made up of the same amount of matter. Example; Let suppose your mass is 60Kg then your weight (W=mg) will be approximately 599 Newton on earth surface. If you go to moon your mass remains the same (60Kg) but your weight will become 1/6 times less (as moon gravity is 1/6 times less than earth) and will be approximately 100 Newton. This is one of the reasons that astronauts are able to jump freely on the moon surface despite their bulky suits and equipments. Atoms: The smallest particle of an element which cannot exist independently but take part in a chemical reaction is called Atom Example: Iron, Copper, Zinc etc
Molecules: The smallest particles of element or compound which have independent existence are called molecules Example: H2, O2, CO2, NH3 etc · Molecules are formed by the chemical combination of two or more atoms · Atoms of some elements can exist independently due to which they are called monatomic molecule for example Noble gases ( He, Ne, Ar) Ions: The charged particles formed as a result of gain or lose electrons by substances are called ions. Those substances which gain electron and get negative charge are called anions while those which lose electrons and get positive charge are called cations. A + B ----à A+ + B- Molecular Ions: When molecules gain or lose electrons the resulting charged particles are called molecular ions. Example: CH4+, CO+ etc Isotopes: Atoms of the same element which have same atomic number but different mass number are called isotopes of that element ` OR Atoms of the same elements which have same number of protons but different number of neutrons in nucleus are called isotopes of that element. Example: Hydrogen has got three isotopes: Protium (11H), Deutrium (12H) and Tritium (13H) Isobars: Atoms of the different elements which have different atomic number but same mass number are called isobars Example: 26Fe56 and 27Ni56 Elements: The types of substances which contain only one kind of atoms which have same atomic number are called elements OR The substance in which all atoms have same chemical properties and have same atomic number is called element Example: Hydrogen is an element because all atoms of hydrogen contain one proton and one electron. · Elements cannot be decomposed into simpler substances by any physical and chemical reactions · Elements are further classified into metals, non metals and metalloids Compounds: The types of substances which are formed by the chemical combinations of two or more elements by fixed ratio are called compounds. Example: Water is a compound because formed by the chemical combination of two different elements (hydrogen and oxygen) · Compound always made from the chemical combination of two or more different elements · Compounds can be decomposed into simpler substances by chemical reactions. · The properties of a compound are always different from its constituent's elements. · Compounds are further divided into two main categories organic and inorganic compounds. Atomic Mass Unit (a.m.u): One-twelfth of the mass of one carbon-12 atom is called atomic mass unit. Mathematically 1 amu= mass of one carbon atom (C-12) 12 1amu = 1.66Ã10-24g Atomic mass or Relative atomic mass or average relative atomic mass: The average relative mass of an atom of the element as compared to the mass of carbon atom taken as 12amu is called atomic mass. Example: The atomic mass of hydrogen is 1.008 amu and carbon is 12.01amu. · The word 'average' is used because some atoms have got isotopes and average masses are taken in such cases. Similarly the word 'relative' is used because the mass of one atom is related to another (standard). Relative molecular mass: The sum of all the relative masses of atoms present in the molecule of a compound is called Relative molecular mass. Example: The average relative mass of water can be obtained by adding the atomic masses present in water molecules H2O = 2(1) + 16= 18amu Determination of relative atomic masses of isotopes by mass spectrometry The most accurate and direct method for determining atomic and molecular masses is mass spectrometry. Procedure: 1. A vaporized sample is first passed from the filament where it is bombarded by high energy electrons 2. The high energy electrons convert the sample atoms or molecules into ions by removing electrons 3. The positive charged ions are then passed from accelerated negative charged plates 4. The emerging ions from accelerated plates are then passed from pose of magnet which force ions into circular path.
Applications of Mass Spectroscopy:
Symbol: The short hand representation of single element is called symbol OR An abbreviation or short form of the chemical name of an element is called symbol Example: Symbol of hydrogen is H, Oxygen is O and iron is Fe etc Formula: The symbolic representation of the molecules of substances is called formula OR The representation of the molecules of substances in the form of symbols is called formula. Example: The formula of water is H2O, Sulphuric acid H2SO4 etc Types of formula: Empirical formula: The chemical formula which represents simple ratio of atoms present in the molecule of a compound is called empirical formula.
Example: The empirical formula of acetylene and benzene is 'CH' Molecular formula: The chemical formula which represents the actual ratio of atoms present in the molecule of a compound is called molecular formula.
Molecular formula = (Empirical formula)n where n is obtained by dividing molecular weight on empirical formula weight (n= M.W/E.W) Example: The molecular formula of acetylene is C2H2 and benzene is C6H6 Percent composition by mass: The percent by mass of each element presents in a compound is called percent composition by mass Mathematically: Percent composition = Mass of each element in a compound à 100 Molar mass of compound (M.W) Note:
Mole: The atomic weight or molecular weight expressed in grams is called mole OR Gram atomic mass or gram molecular mass or gram formula mass of a substance is called mole Mole is denoted by 'n' and S.I unit of substances Mathematically: Mole = Given mass of substance in grams Atomic or molecular or formula weight
Mole = Given numbers of atoms/ion/molecules Avogadro number
Example: The molecular weight of water is 18 amu so 18 gram water is equal to one mole Molar Mass: The mass of one mole of a substance is called molar mass Example: The molar mass of water is 18g per mole Avogadro's number: The number of particles (atoms, ions or molecules) present in one mole of a substance is called Avogadro's number.
Example: One mole of sodium = 6.023Ã1023 sodium atoms One mole of H2O = 6.023Ã1023 water molecules Chemical reaction: The process in which substances react and formed new substances is called chemical reaction OR Chemical change is called chemical reaction Example: H2 + Cl2---Ã 2HCl Chemical equations: The symbolic representation of chemical reaction is called chemical equation OR The representation of chemical reactions in the form of symbols and formulas is called chemical equation. Parts of chemical equations: chemical equations always contain two parts 1) Reactant: The substances which react and undergo a chemical change are called reactants. They are always present on the left hand side 2) Products: The substances which produced as a result of chemical change are called products. They are always present of the right hand side
Example: H2 + Cl2---à 2HCl Reactants à product Writing chemical equation:
Example: 2P(s) + 3Cl2(g) Ã 2PCl3 (l) KBr(aq) + AgNO3(aq) Ã KNO3(aq) + AgBr(s) Law of conservation of mass: Mass can neither be created nor destroyed during chemical reaction but can only change from one form to another. OR During a chemical reaction total mass of the reactants is always equal to the total mass of the products. Example: 2H2 + O2Ã 2H2O Reactants Mass= 2(2) + 2(16) = 36 Products mass= 2(2+16) = 36 Law of the definite proportions or constant compositions: A pure chemical compound always contains same elements combined together in same proportion by mass. Example: Pure water obtained from any source like river, spring, sea, well etc will always contains hydrogen and oxygen combined together in the ratio of 1:8 by mass Stoichiometry: The branch of chemistry which studies the amount of reactants and products in a balanced chemical equation is called stoichiometry. Stoichiometric calculations based on two important assumptions which are given below 1) Reactants are completely converted into products 2) No side reaction occur There are three relationships involved in stoichiometric calculations 1. Mass - Mass Relationship 2. Mass - Volume Relationship 3. Volume - Volume Relationship
Limiting Reactant or Reagent: The reactant which is completely consumed in chemical reaction and gives less amount of product is called limiting reactant. Excess Reactant: The reactant which is not consumed completely in chemical reaction is called excess reactant. Example: Let suppose we have two hydrogen molecules and two oxygen molecules which chemically react and form water. Now one of them must be consumed completely.
+ â H2 molecules O2 molecules H2O molecules O2 molecule In the above example hydrogen is limiting reactant because consumed completely while oxygen is excess reactant because not consumed completely. Yield: The amount of product obtained during chemical reaction is called yield. Yields are two types 1) Theoretical yield: The amount of product predicted by balanced chemical equations is called theoretical yield. 2) Actual yield: The amount of product practically obtained in the laboratory is called actual yield. Actual yield is always less than theoretical yield because
Percent Yield: The ratio between actual yield and theoretical yield multiplied by 100 is called percent yield. Mathematically: % yield = actual yield____Ã100 Theoretical yield
Precisions: The closeness of agreement between independent tests results is called precision OR The closeness of individuals measurement with one another is called precisions.
Accuracy: The closeness of agreement between the test result and the accepted reference value is called accuracy OR The closeness of individual measurements to the accepted value is called accuracy.
Significant figures: The numbers of digits in measurement which are known with certainty are called significant figures. Mathematically: Certain digits + one doubtful digit Example: 3.62 contains three significant figure (certain digits 3 and 6 while doubtful digit is 2).
Rules for determination of significant figures. (1) All the non-zero digits are significant figures. Exponential Notations: The expression of number in the power of 10 is called exponential notation. Example 230000000000 = 23Ã1010 Rounding off Data: The conversion of larger numbers to smaller numbers by dropping non significant digits is called rounding off data Rules for rounding off data: 1. If the digit to be dropped is greater than 5 then the last remaining digits is increased by one. For example in 2.689 if 9 to be dropped then increased 8 by one so the answer will be 2.69 2. If the digit to be dropped is less than 5 then the last remaining digits is unchanged. For example in 3.674 if 4 to be dropped then 7 remains unchage and the answer will be 3.67 3. If the digit to be dropped is exactly five then the last remaining digits is increased by one if odd and remained unchanged if even. For example in 2.685 if 5 to be dropped then 8 remained unchanged because even. In 3.675 if 5 to be dropped then 7 is increased by one because odd so answer will be 3. Q. Why on the surface of the moon an apple would weight only one sixth of its weight on the earth Answer. Weight is the force of gravity exerted on objects and moon gravity is one sixth (1/6) of earth so apple weight would be one sixth less on moon surface. Q. Why the atomic mass of carbon is 12.01 amu not 12.00 amu Answer: The carbon has got isotopes so then average mass are taking when atom has more than one isotopes due to which its mass is 12.01amu. This is also one of the reason that when we say 12.01amu we are referring to the average value. Q. Why it is important to mention the physical state of substances in chemical equation Answer. The physical states of substances are also important condition for reaction to take place. For example aqueous KBr and Aqueous AgNO3 react to form aqueous KNO3 and solid AgBr but the reaction will not take place when solid KBr and solid AgNO3 are mixed. Q. A chemical equation is not a complete description of what actually happens during chemical reaction why? Answer. Chemical equation does not explain how the products are formed and how long it will take for the reaction to occur. For example, rusting of iron to form iron oxide will take weeks or months but burning iron to form iron oxide will take seconds. However both reactions are represented by same chemical equation. Q. In stoichiometric studies the reaction must be reprsented by balanced chemical equations why? Answer. Stiochiometric studies is based on law of conservation of mass and if chemical equation is not balanced it will not obey the law of conservation of mass. In balanced chemical equations the mass of reactants will be equal to the mass of the products. Q. Why in stoichiometric calculations we generally use molecular equations. Answer: In stoichimetric calculations we are interested in the masses of whole units not just the masses of separate cations and anions. Q. Why chemists usually choose the more expensive chemical as the limiting reagent? Answer. They choose more expensive chemical as limiting reactant to make sure that all are consumed and to save money.
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