Lecture Notes Chemistry E-1. Chapter 3

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1 Lecture Notes Chemistry E-1 Chapter 3

2 The Mole A mole is a certain number of particles of any chemical substance. (The exact number is called Avogadro's Number, and it is very large: or: 602,000,000,000,000,000,000,000) Let's look at a mole of some common substances, and find the mass of each: A mole of lead (Pb): How many atoms of lead does this sample contain? What is the mass of this sample? A mole of mercury (Hg): How many atoms of mercury does this sample contain? What is the mass of this sample? A mole of water (H 2 O): How many molecules of water does this sample contain? How many atoms of oxygen? How many atoms of hydrogen? What is the mass of this sample? Is there another, more convenient, definition of a mole?

3 Using Moles in Dimensional Analysis What "conversion factors" can you derive from the formula for carbon dioxide, CO 2? Knowing that Avogadro's Number is , are there any additional conversion factors you can come up with?

4 Using Moles in Dimensional Analysis Now calculate the molar mass of CO 2. Are there still more conversion factors you can derive?

5 Molar Mass Self-Quiz 1. Using a periodic table, calculate the molar mass of propane, C 3 H If I have 25.0 grams of C 3 H 8, how many moles of propane do I have? 3. If I have 25.0 grams of C 3 H 8, how many molecules of propane do I have? How many atoms of carbon? How many atoms of hydrogen?

6 Chemical Formulas and Percent Composi4on The empirical formula represents the smallest whole-number ratio of the various types of atoms in a compound. The molecular formula is the exact formula of the molecule involved. Percent composition Percent composition (by mass) = Mass of element Molar mass! 100% Bees release the pheromone isopentyl acetate (C 7 H 14 O 2 ) when they sting in order to attract other bees to join the attack. An analysis of a pure substance extracted from a banana found that it contained 64.6% C, 10.9% H and 24.6 % O. Is this substance from the banana the same one released by the bees?

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8 Empirical and Molecular Formulas An organic compound isolated from a plant was found to contain 40.92% C, 4.58% H, and 54.50% O by mass and have a molar mass of Is this substance Shikimic Acid (C 7 H 16 O 5 ) Determine the empirical formula and the molecular formula of the compound to see if it is Shikimic Acid?

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10 Combustion Analysis When any compound containing carbon and hydrogen is completely combusted, carbon dioxide and water are produced as products. (see propane from before) If the mass of CO 2 and H 2 O produced are measured, the empirical formula can be determined. Compound X contains only carbon, hydrogen, and oxygen. When a gram sample of Compound X is subjected to combustion analysis, grams of CO 2 and grams of H 2 O are produced. Determine the empirical formula of Compound X.

11 Combustion Analysis

12 Combustion Analysis Compound X has a molar mass of approximately 475 g/mol. Determine the molecular formula of Compound X. Mass Spectrometer is used to determine molar mass

13 "Stoichiometry": The Quantitative Study of Chemical Reactions In the lab this week, you will study the combination reaction of copper and oxygen to form copper (II) oxide. Write a complete, balanced chemical equation for this reaction. If you start with grams of copper metal, and all of it is able to react with excess oxygen, what mass of copper (II) oxide will be produced? What mass of oxygen was consumed in this reaction? Note the general pattern: mass of reactant moles of reactant coefficients from balanced equation moles of product mass of product

14 Percent Yield Often, chemical reactions will not give as much product as is theoretically possible. You will find this to be the case in the lab when you react copper with oxygen. Suppose you heat grams of copper in oxygen but rather than producing grams of CuO you actually make g of copper (II) oxide. What is the percent yield of this reaction? What mass of copper remains unreacted?

15 Percent Yield Given that there is still unreacted copper metal in the reaction mixture, what will be the total mass of the reaction mixture after 13% of the copper has reacted with oxygen?

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17 Limiting Reactants: Analogy Let's consider a reaction between a person (Pe) and two gloves (Gl): Pe + 2 Gl PeGl 2 (a "gloved person") Suppose we have 4 people and 6 gloves. How many "gloved people" can we produce? Concept of a "limiting reactant" Easiest way to calculate: 4 people reacting with an unlimited number of gloves: An unlimited number of people reacting with 6 gloves: Which result gives the correct answer?

18 Limiting Reactants: Application Nitrogen monoxide will react with oxygen to produce nitrogen dioxide. Write and balance the equation for this process. (All species are gases.) If we have 100 grams of nitrogen monoxide and 50 grams of oxygen, what is the maximum mass of nitrogen dioxide we can produce?

19 Stoichiometry of Mixtures You have a mixture of Cu and CuO which weighs a total of grams. You reduce this mixture to obtain pure Cu; the mass of pure Cu is grams. What was the mass of Cu and the mass of CuO in the original mixture?

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21 Chemical Reactions How can we describe the reaction between sodium and chlorine to form sodium chloride? Describe in words: Describe using chemical symbols: Balanced chemical equation: This is an example of a combination reaction. Why does this name make sense Now observe the decomposition of nitrogen triiodide to form nitrogen and iodine. What do you observe in this decomposition reaction? Does this name make sense? How can we describe the decomposition of nitrogen triiodide to form nitrogen and iodine? Describe in words: Describe using chemical symbols: Balanced chemical equation:

22 Combustion Reactions Combustion reactions usually involve reaction with oxygen in the air. Let's examine the combustion of magnesium, Mg: Write an equation for the reaction of magnesium to form magnesium oxide. Then balance the equation. Watch the reaction take place. What evidence is there that a chemical reaction has occurred? Now let's examine the combustion of propane, C 3 H 8 : Propane is an example of a hydrocarbon, that is, a compound which contains hydrogen and carbon. When a hydrocarbon is burned completely in air, the products are carbon dioxide and water. Write and balance the equation for the complete combustion of propane. Watch the reaction take place. What evidence is there that a chemical reaction has occurred?

23 Displacement Reactions Sodium metal will react with water to produce sodium hydroxide and hydrogen gas. Write an equation for this reaction. Make sure that the equation is balanced. Can you predict anything about what might happen when this reaction is carried out This is an example of a "single displacement" or "single replacement" reaction. Why is this an apt description of this type of reaction? Silver nitrate reacts with sodium chloride to give the solid, silver chloride, and soluble sodium nitrate.

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