Text of LOGO Course lecturer : Jasmin Šutković 18th March 2015 Chemistry - SPRING 2015 Lecture 3: Chemical...
LOGO Course lecturer : Jasmin utkovi 18th March 2015 Chemistry - SPRING 2015 Lecture 3: Chemical Reactions
Contents International University of Sarajevo 1. What is a Chemical Reaction ? 1.2 The mol and molar masses 2.Determining Empirical and Molecular formulas 3.Chemical equations 4.Mass relationships in chemical equations 5.Types of Chemical equations 6.Earths atmosphere
1. Chemical reactions A process where a substance is converted to one or more other substances with different compositions and properties.
The MOLE To analyze the transformations that occur between individual atoms or molecules in a chemical reaction, it is therefore absolutely essential for chemists to know how many atoms or molecules are contained in a measurable quantity in the laboratory. The unit that provides this link is the mole (mol)!!
The MOLE cont... Abbreviated - mol The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as the amount of any substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 ( 12 C), the isotope of carbon with atomic weight 12. In 12g of C there is 6.02210 23 atoms of carbon, This corresponds to "Avogadro constant" (6.02210 23 ) elementary entities of the substance.Avogadro constant
1.2 Atomic, molecular and Molar masses Atomic mass The mass number is approximately or relativeley equal to the numerical value of the atomic mass (Ar = relative atomic mass )..check PSE ! Exp of atomic mass (Ar) : for H it is 1 or for O it is 16 Molecular mass Molecular mass or relative molecular mass represents the sum of atomic masses in a compuond (the mass of the molecule).
2.1 Molecular mass (Mr) The molecular mass is the mass of a molecule, sometimes shown as Mrmassmolecule It is calculated by adding together the atomic masses of the elements in the substance, each multiplied by its subscript (written or implied) in the molecular formula. Expample 1 : Mr of H2O= 2(1)+ (16) = 18 Mr of a compound is the number that represents how much the mass of this coumpound is higher than the atomic mass unit (amu) =(1/12 mass of C12 isotope) SO the unit for Mr is amu or u.
The procedure for calculating molecular masses is illustrated in Example 2 Find the molecular mass of table sugar (sucrose), which has a molecular formula C 12 H 22 O 11sucrose EXAMPLE 2 molecular mass(Mr) of C 12 H 22 O 11 = 12(12.01) + 22(1.008) + 11(16.00) = 342.30
Empirical formula Unlike molecules, which have covalent bonds, ionic compounds do not have a readily identiable molecular unit. So for ionic compounds we use the formula masses (also called the empirical formula masses) of the compound rather than the molecular mass. The formula mass is the sum of the atomic masses of all the elements in the empirical formula, each multiplied by its subscript (written or implied). It is directly analogous to the molecular mass of a covalent compound.
Molar Mass Molar mass is measured in grams/mole, and is the mass of one mole of any substance. In each case, the number of grams in 1 mol is the same as the number of atomic mass units that describe the atomic mass, the molecular mass, or the formula mass, respectively. Exp: Relative atomic mass if Natrium is 23 (Ar) and Molar mass of Natrium is 23 g/mol
Example... The molar mass of ethanol is the mass of ethanol (C2H5OH) that contains 6.022 x 10 23 ethanol molecules. Using the procesure in Example 1 and 2, the molecular mass (M) of ethanol would be 46.069 g/mol. How..? Because 1 mol of ethanol contains 2 mol of carbon atoms (2 12.011 g), 6 mol of hydrogen atoms (6 1.0079 g), and 1 mol of oxygen atoms (1 15.9994 g), its molar mass is 46.069 g/mol.
Flowchart for converting between units.
2. Determining molecular and empirical formulas When a new chemical compound, such as a potential new pharmaceutical, is synthesized in the laboratory or isolated from a natural source, chemists determine its elemental composition, its empirical formula, and its structure to understand its properties. In this section, we focus on how to determine the empirical formula of a compound.
Laws and order in Chemical binding Dalton's Atomic Theory - Dalton Law of Conservation of Mass - Lavosier Law of Definite Proportion J.Proust Law of Multiple Proportion - Dalton's Law http://sciencepark.etacude.com/chemistry/l aw.php
Calculating mass percanteges The law of denite proportions states that a chemical compound always contains the same proportion of elements by mass; that is, the percent composition the percentage of each element present in a pure substance is constant (although we now know there are exceptions to this law).
Calculating mass percanteges cont... For example, today we know that sucrose (cane sugar) is 42.11% carbon, 6.48%hydrogen, and 51.41% oxygen by mass. This means that 100.00 g of sucrose always contains 42.11 g of carbon, 6.48 g of hydrogen, and 51.41 g of oxygen.
How to prove this ? First we will use the molecular formula of sucrose (C12H22O11) to calculate the mass percentage of the component elements; then we will show how mass percentages can be used to determine an empirical formula. According to its molecular formula, each molecule of sucrose contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms. We can then use these masses to calculate the percent composition of sucrose. To three decimal places, the calculations are the following :
Sucrose example cont... Sum = 342.297g = 342.297 g/mol = Molar Mass (M) The mass percentage of each element in sucrose is the mass of the element present in 1 mol of sucrose divided by the molar mass of sucrose, multiplied by 100 to give a percentage. The result is shown to two decimal places:
Sucrose example cont... You can check your work by verifying that the sum of the percentages of all the elements in the compound is 100%: 42.12% + 6.48% + 51.41% = 100.01% If the sum is not 100%, you have made an error in your calculations. (Rounding to the correct number of decimal places can, however, cause the total to be slightly di erent from 100%.) Thus 100.00g of sucrose contains 42.12 g of carbon, 6.48g of hydrogen, and 51.41 g of oxygen; to two decimal places, the percent composition of sucrose is indeed 42.12% carbon, 6.48% hydrogen, and 51.41% oxygen.
Percentage = Compositions
Empirical formula of Penicilin Antibiotics are chemical compounds that selectively kill microorganisms, many of which cause diseases. Although we may take antibiotics for granted today, penicillin was discovered only about 80 years ago. The discovery of penicillin is a historical detective story in which the use of mass percentages to determine empirical formulas played a key role. In 1928, Alexander Fleming discovered penicilin
Story of Penicilin . It was Alexander Fleming at St Marys Hospital Medical School who noticed a mould growing on a culture plate of bacteria one day in September 1928. Around the Penicillium notatum mould was a clear area where the colonies of bacteria appeared to have been dissolved. Fleming set about establishing the identity of the substance that was killing the bacteria, giving it the name penicillin
Penicilin empirical formula cont... Although Fleming was unable to isolate penicillin in pure form, the medical importance of his discovery stimulated researchers in other laboratories. Finally, in 1940, two chemists at Oxford University, Howard Florey and Ernst Chain isolated penicilin and its use greatly increased the survival rate of wounded soldiers in World War II. As a result of their work, Fleming, Florey, and Chain shared the Nobel Prize in Medicine in 1945
They discovered that a typical sample of penicillin G contains 53.9% carbon, 4.8% hydrogen, 7.9% nitrogen, 9.0% sulfur, and 6.5% sodium by mass. The sum of these numbers is only 82.1%, rather than 100.0%, which implies that there must be one or more additional elements. A reasonable candidate is oxygen, which is a common component of compounds that contain carbon and hydrogen! Hovewre for techniqual reasons it is difficult to analyze the oxygen directly. If we assume that all the missing mass is due to oxygen, then penicillin G contains (100.0% - 82.1%) = 17.9% oxygen. From these mass percentages, the empirical formula and eventually the molecular formula of the compound can be determined. Penicilin empirical formula cont...
The Way... In order to determine the empirical formula we need to convert the mass percentages to relative number of atoms. We assume that we deal with 100g of compounds... 100-82.1 = 17,9
The way...cont.... But..moles are not whole number ration that we need for empirical formula... The empirical formula expresses the relativ