Chapters 3-4 Stoichiometry: Chemical Calculations Slide 1 of 48
Molecular Masses And Formula Masses Molecular Masses Molecular mass is the sum of the masses of the atoms represented in a molecular formula. Formula Masses Formula mass is the sum of the masses of the atoms or ions present in a formula unit. Slide 2 of 48
The Mole & Avogadro s Number A mole (mol) is an amount of substance that contains as many elementary entities as there are atoms in exactly 12 g of the carbon-12 isotope. 1 mol 12 C = 12.0000000 g/mol Avogadro s Constant (N A ) N A = 6.02214199 x 10 23 mol -1 1 mol = 6.02214199 x 10 23 entities Slide 3 of 48
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Formation of Carbon Dioxide Slide 6 of 48
Molar Mass The molar mass of a substance is the mass of one mole of that substance. The molar mass is numerically equal to the atomic mass, molecular mass, or formula mass. The units of molar mass are grams/mole (g/mol). Examples: 1 mol Na = 22.99 g/mol 1 mol CO 2 = 44.01g/mol Slide 7 of 48
Relating Molecular Formulas to Empirical Formulas The formulas for ionic compounds are empirical formulas. Molecular formulas are simple integer multiples of the empirical formulas. Empirical formula mass x integral factor = molecular mass Integral factor = (molecular mass / empirical formula mass) Slide 8 of 48
Elemental Analysis of C, H, N, S Example: Combustion analysis C 6 H 5 NH 2 + 35/2 O 2 6 CO 2 + 7/2 H 2 O + NO 2 Slide 9 of 48
Chemical Equation Notations Extra notation added to each substance: (g) = gas (l) = liquid (s) = solid (aq) = aqueous (water) solution Extra notation added above the arrow: = heat sometimes the actual temperature is used instead Example: 4 C 6 H 5 NH 2(l) + 35 O 2(g) 24 CO 2 (g) + 14 H 2 O (g) + 4 NO 2 (g) NH 3(aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) Slide 10 of 48
Balancing Chemical Equations If an element is present in just one compound on each side of the equation, try balancing that element first. Balance any reactants or products that exist as the free element last. C 6 H 5 NH 2(l) + 35/4 O 2(g) Stoichiometric coefficients, or mole ratios 6 CO 2 (g) + 7/2H 2 O (g) + NO 2 (g) 4 C 6 H 5 NH 2(l) + 35 O 2(g) 24 CO 2 (g) + 14 H 2 O (g) + 4 NO 2 (g) Slide 11 of 48
Enthalpy Diagrams Reaction path Reaction path H + (aq) + OH - (aq) H 2 O(l) H o = - 55.8 kj N 2 (g) + O 2 (g) 2NO (g) H 0 = - 180 kj Slide 12 of 48
Chemical explosive reaction For a chemical to be an explosive, it must exhibit all of the following: Rapid Expansion (eg. rapid production of gasses or rapid heating of surroundings) Evolution of heat Rapidity of reaction Initiation of reaction Slide 13 of 48
Explosives Chemical explosive reaction C 2 H 4(g) + 3O 2(g) 2CO 2(g) + 2 H 2 O (g) H = -1410.9 kj 4C 3 H 5 N 3 O 9(g) + 5O 2(g) 12CO 2(g) + 10 H 2 O (g) + 6 NO 2(g) 4C 7 H 5 N 3 O 6(g) + 33O 2(g) 28CO 2(g) + 10 H 2 O (g) + 12 NO 2(g) Trinitrotoluene (TNT) Slide 14 of 48
Dynamite is an explosive based on the explosive potential of nitroglycerin using diatomaceous earth (Kieselguhr) as an adsorbent. Slide 15 of 48
Copper in Nitric Acid Oxidation-reduction reaction chemical equation Cu (s) + 2 HNO 3(aq) Cu(NO 3 ) 2(aq) + H 2(g) complete ionic equation Spectator ion Cu (s) + 2 H + (aq) + 2 NO 3 - (aq) Cu 2+ (aq) + H 2(g) + 2 NO 3 - (aq) net ionic equation Cu (s) + 2 H + (aq) Cu 2+ (aq) + H 2(g) Slide 16 of 48
Conduction Illustrated cation anion anode cathode Slide 17 of 48
Electrolytic Properties of Water-Soluble Solutes With a few exceptions, ionic compounds are strong electrolytes. Only a few molecular compounds are strong electrolytes. Most molecular compounds are either nonelectrolytes or weak electrolytes. Most organic compounds are molecular and nonelectrolytes; although, carboxylic acids and amines are weak electrolytes. Slide 18 of 48
Electrolytic Properties of Aqueous Solutions Slide 19 of 48
Solubility Rules soluble mostly soluble NO 3-, CH 3 COO -, ClO 4 - Li +, Na +, K +, Rb +, Cs +,NH 4 + Cl -, Br -, I -, SO 4 2- mostly insoluble CO 3 2-, OH -, PO 4 3-,S 2- Slide 20 of 48
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Calculating Ion Concentrations in Solution Bracket symbols, [ ], are used to represent molar concentrations of ions or molecules in solution. Given 0.010 M Na 2 SO 4 : [ Na 1+ ] = 0.020 M [ SO 4 2- ] = 0.010 M Only one concentration is reported for an ion in a solution, even if the ion has two or more sources. Slide 22 of 48
Strong and Weak Acids Strong acids are acids that are completely ionized in water and are good conductors of electricity. Weak acids are acids in which only some of the molecules are ionized in water; the rest remain as intact molecules. Some acids have more than one ionizable hydrogen atom. These acids are usually strong acids for the first ionization and weak acids for the rest of the ionizations. Slide 23 of 48
Strong and Weak Bases Strong bases are ionic hydroxides that completely ionize in water and are good conductors of electricity. Weak bases are substances that act as bases but remain mostly molecular at equilibrium in water. Ammonia (NH 3 ) is an example of a weak base. Slide 24 of 48
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Oxidation-Reduction Reaction Mg (s) + Cu 2+ (aq) Mg 2+ (aq) + Cu (s) Slide 26 of 48
Rules for Assigning Oxidation Numbers For the atoms in a neutral species, the total of all the oxidation numbers is zero. In their compounds, the Group 1A metals all have an oxidation number of +1. In its compounds, the oxidation number of fluorine is -1. In most of its compounds, hydrogen has an oxidation number of +1. Exception: NaH, MgH 2, (H -1 ) In most of its compounds, oxygen has an oxidation number of -2. Exception: H 2 O 2 (O -1 ), CsO 2 (O -1/2 ) Slide 27 of 48
Writing and Balancing Oxidation-Reduction Equations Must balance both mass and electric charge. In disproportionation reactions, a portion of the reactant is oxidized and a portion of that same reactant is reduced. H 2 O 2 (aq) H 2 O (aq) + 1/2 O 2 (g) reduction oxidation Slide 28 of 48
Agents of Oxidation-Reduction An oxidizing agent causes another substance to be oxidized while itself is reduced. A reducing agent causes another substance to be reduced while itself is oxidized. Rocket fuels utilize this process to achieve the lift in the rocket. reducing agent H 2 (g) + 1/2O 2 (g) H 2 O (g) oxidizing agent Slide 29 of 48
Limiting Reactants Many reactions are carried out with a limited amount of one reactant and a plentiful amount of the others. The reactant that is completely consumed in the reaction limits the amounts of products and is called the limiting reactant, or limiting reagent. Keep in mind that the limiting reactant is not necessarily the one present in the smallest quantity. Slide 30 of 48
Yields of Chemical Reactions The theoretical yield of a chemical reaction is the calculated quantity of product in the reaction. The actual yield is the measured mass of product obtained from a chemical reaction. There are many factors involved in why the actual yield will be less than the theoretical yield. The percent yield is the ratio of the actual yield to the theoretical yield times 100%. percent yield = (actual yield / theoretical yield ) x 100% Slide 31 of 48
Green Chemistry: Atomic Economy Percentage atomic economy (% AE) =(formula mass of desired final product / sum of formula masses of all the reactants) x 100% Example: production of ethylene oxide, CH 2 CH 2 O (i) Chlorohydrin route C 2 H 4 + Cl 2 + Ca(OH) 2 C 2 H 4 O + CaCl 2 + H 2 O % AE = ( formula mass of C 2 H 4 O / sum of formula masses of C 2 H 4 + Cl 2 + Ca(OH) 2 ) x 100% = [44.05/ (28.05 + 70.91 + 74.09)] x 100% = 25.46% Slide 32 of 48
Green Chemistry: Atomic Economy Example: production of ethylene oxide, CH 2 CH 2 O (ii) Ag catalysis route C 2 H 4 + 1/2 O 2 C 2 H 4 O % AE = ( formula mass of C 2 H 4 O / sum of formula masses of C 2 H 4 + 1/2 O 2 ) x 100% = [44.05/ (28.05 +16.00)] x 100% = 100% Route (ii) is GREENER than route (i). Slide 33 of 48
Fossil energy resources Fossil energy resources: reserves (1999) consumption (1998) years Petroleum 1033.2 26.88 39 (billion barrels) Natural gas 5141.6 82.19 63 (trillion cubic feet) Coal 1087.19 5.0427 215 ( billion short tones) Limitation of fossil energy & environmental pressures Slide 34 of 48
Petroleum Distillation Slide 35 of 48
Gasoline- C 5 ~ C 12 hydrocarbons Engine knock- the noise due to gasoline components burned unevenly Octane number- the engine performance on burning the gasoline as burning a mixture of isooctane and heptane in different ratios. set standard isooctane: 100 n-heptane: 0 CH 3 CH 3 CH 3 C CH 2 C CH 3 CH 3 H CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 Slide 36 of 48
Fuel Additives- Octane-booster (methyl-tert-butyl ether) RFG- reformulated gasoline: >2 wt% O Oxygenated fuel Slide 37 of 48
Fuel cells General advantages of fuel cells: high energy conversion efficiency (> 40%, 70%, ~100%) extremely low emission of pollutants extremely low noise or acoustical pollution effective reduction of greenhouse gas process simplicity for conversion of chemical to electrical energy Other advantages depend on fuel cell type: fuel flexibility; co-generation capacity; modular design for mass production; relative rapid load response Slide 38 of 48
Fuel cells Fuel cells Phosphoric acid fuel cell (PAFC) or alkaline fuel cells (AFC) Slide 39 of 48
Fuel cells Proton-exchange membrane fuel cell PEMFC Slide 40 of 48
Is H 2 a clean fuel? Generation of H 2 in industrysteam reforming of fossil fuels Coal C + H 2 O CO + H 2 Natural gas CH 4 + H 2 O CO + 3H 2 Synthetic gas (syn gas) Gasoline C 8 H 18 + 8H 2 O 8CO + 17H 2 Generation of more H 2, CO + H 2 O CO 2 + H 2 water-gas shift reaction Utilization by Renewable Energy (e.g. solar) in the future H 2 O 1/2 O 2 + H 2 Slide 41 of 48
Direct-methanol fuel cell (DMFC) Anode: CH 3 OH + H 2 O CO 2 + 6H + + 6e - Cathode: 1.5O 2 + 6H + + 6e - 3H 2 O Net reaction: CH 3 OH + 1.5O 2 CO 2 + 2H 2 O Slide 42 of 48
Solutions and Solution Stoichiometry Solute: the substance being dissolved Solvent: the substance doing the dissolving Water is often called the universal solvent. The concentration of a solution refers to the quantity of a solute in a given quantity of solvent. A concentrated solution contains a relatively large amount of solute as compared to the solvent. Adilute solution contains a relatively small concentration of solute as compared to the solvent. Slide 43 of 48
Molar Concentration Substances enter into chemical reactions according to certain molar ratios. Volumes of solutions are more convenient to measure than their masses. The Molarity (M), or molar concentration, is the amount of solute, in moles, per liter of solution. M = mol of solute/1 L of solution Keep in mind that molarity signifies moles of solute per liter of solution, not liters of solvent. Slide 44 of 48
Dilution Of A Solution 500 ml beaker Slide 45 of 48
Dilution of Copper (II) sulfate pipet volumetric flask Slide 46 of 48
Summary A mole is an amount of a substance containing Avogadro s number of elementary entities. Avogadro s number (N A ) = 6.022 x 10 23. The molar mass of a substance is the mass in grams of one mole of that substance. Formulas and molar masses can be used to calculate mass percent composition. Slide 47 of 48
Summary (continued) A chemical equation uses symbols and formulas for the elements and/or compounds involved in a reaction. Calculations involving reactions use stoichiometric factors based on stoichiometric coefficients in the balanced equation. The limiting reactant determines the amounts of products in a reaction. The molarity of a solution is the number of moles of a solute per liter of solution. Slide 48 of 48