Ch. 8: Quantities in Chemical Reactions Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry

I. Chapter Outline Introduction Calculations w/ Equations Limiting Reactants Reaction Yields Enthalpy

I. Introduction

I. Introduction Global consumption of fossil fuels results in CO2 production. We can estimate how much CO2 is produced using balanced chemical equations. 2C8H18(l) + 25O2(g)  16CO2(g) + 18H2O(g)

II. Relating Parts to the Whole In Chapter 6, we learned about part to whole relationships.

II. Relating Ingredients We can find the same kinds of relationships between ingredients of a recipe.

II. How Many Pancakes? If we had a dozen eggs, and enough of the other ingredients, how many pancakes could we make?

II. How Much NH3? Doing calculations with a recipe is analogous to doing calculations with a balanced chemical equation.

II. Calculations w/ Equations A balanced equation allows calculations of amounts of reactants or products. If you know the # of moles of one substance in a balanced equation, you know the # of moles of any of the other substances. Numerical relationships between chemical amounts in a balanced chemical equation are called reaction stoichiometry.

II. Mole Ratios Many stoichiometric relationships exist in any one balanced equation. e.g. C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) 1 mole C3H8 reacts w/ 5 moles O2 1 mole C3H8 produces 3 moles CO2 5 moles O2 leads to 4 moles H2O These can be converted to mole ratios. 1 mole C3H8 5 moles O2 3 moles CO2 4 moles H2O

II. Possible Calculations With a balanced chemical equation, there are two common calculations. Convert moles of one substance to moles of another substance. Convert grams of one substance to grams of another substance.

II. Sample Problem If 0.026 mole of molecular oxygen reacts with excess octane according to the reaction below, how many moles of water would form? 2C8H18(l) + 25O2(g)  16CO2(g) + 18H2O(g)

II. Sample Problem How many grams of iron form when 135 g of aluminum reacts with excess iron(III) oxide according to the reaction below? Fe2O3(s) + 2Al(s)  Al2O3(s) + 2Fe(l)

II. Sample Problem If 28.6 g of dinitrogen tetrahydride is used in the reaction below with excess dinitrogen tetroxide, how many grams of molecular nitrogen can be obtained? 2N2H4(g) + N2O4(g)  3N2(g) + 4H2O(g)

II. Sample Problem How many grams of hydrogen chloride are needed to completely react with 30 grams of molecular oxygen according to the reaction below? 4HCl(g) + O2(g)  2H2O(g) + 2Cl2(g)

III. Not Enough Ingredients 1 cup flour + 2 eggs + ½ tsp baking powder = 5 pancakes

III. Limiting Reactants Consider the following reaction. 2CO(g) + O2(g)  2CO2(g) Submicroscopically, 2 molecules CO react with 1 molecule O2 to give 2 molecules CO2. How much CO2 forms if we have 2 molecules CO and 10 molecules of O2? We call CO the limiting reactant, the reactant that is completely used up.

III. Double Stoichiometry Problem Limiting reactant problems are characterized by giving you information about more than one reactant! They are solved by running two calculations and circling the smallest answer.

III. Sample Problem How many grams of Al2S3 can be formed according to the reaction below if 10.0 g Al is reacted with 15.0 g S8? 16Al(s) + 3S8(s)  8Al2S3(s)

III. Sample Problem What is the maximum amount of ammonia (in grams) that can be synthesized from 25.2 g of N2 and 8.42 g of H2 according to the reaction below? 3H2(g) + N2(g)  2NH3(g)

IV. Real-life Reactions In reality, we cannot have complete conversion to products. Even if there was complete conversion, difficult to actually collect all of the product.

IV. Reaction Yields Stoichiometry gives us theoretical yield. What is recovered in the lab is the actual yield. The efficiency of a reaction is commonly expressed as percent yield.

IV. Sample Problem The reaction of 200.0 g of PCl3 with excess water according to the reaction below leads to the collection of 113 g of HCl. Calculate the percent yield. PCl3(g) + H2O(l)  HCl(aq) + H3PO3(aq)

IV. Sample Problem Determine the percent yield of a reaction in which 41.5 g of tungsten(VI) oxide reacts with excess molecular hydrogen to produce atomic tungsten and 9.50 g of water.

V. Heats of Reaction In Chapter 3, we learned about exothermic and endothermic reactions. The amount of thermal energy emitted or absorbed under constant pressure is known as the enthalpy. Specifically, we have the enthalpy of reaction (a.k.a. heat of reaction), ΔHrxn.

V. The Sign of ΔHrxn The sign of the heat of reaction determines whether the reaction is exothermic or endothermic. For exothermic reactions, ΔHrxn is negative. For endothermic reactions, ΔHrxn is positive.

V. Exothermic vs. Endothermic

V. Thermochemical Equations A balanced equation with a ΔHrxn is called a thermochemical equation. 2H2O(l)  2H2(g) + O2(g) ΔHrxn = 572 kJ

V. Stoichiometry with Reaction Enthalpies The ΔHrxn can be used in stoichiometry calculations. For the thermochemical equation: NO(g) + ½ O2(g)  NO2(g) ΔH = -57.1 kJ there are multiple ratios. 1 mole NO : -57.1 kJ ½ mole O2 : -57.1 kJ 1 mole NO2 : -57.1 kJ

V. Sample Problem What mass of propane (C3H8) is needed to produce 1.2 x 103 kJ of heat? How many grams of carbon dioxide are produced in the process? C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) ΔHrxn = -2044 kJ