Chemical Stoichiometry Reacting Quantities and Material Balance Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

Resources Available  Fine, Beall & Stuehr: Chapter 3.5  A balanced chemical equation is used to predict the maximum amount of product which can be formed in a chemical reaction.  Chemical nomenclature, molecular weight, mass and moles of each reactant present and product formed are concepts involved in these calculations.

6/13/2015 Chemical Equations  Stoichiometry (atom balance)  Charge (charge balance)  Format Phase annotation (s, l, g, aq) Arrow notations Write an annotated chemical equation for the decomposition of a 30% solution of hydrogen peroxide to give water and oxygen.

Hydrogen Peroxide Decomposition 30% H2O2H2O2 decomposes H2OH2O water from decomposition and original water 2 H 2 O 2 (aq)  2 H 2 O (l) + O 2 (g) 70% water

u Concentrated hydrogen peroxide contains 30% hydrogen peroxide by weight and the balance water. The density of 30% hydrogen peroxide is gmL -1. u Hydrogen peroxide decomposes upon standing, over a period of several months, to form water and oxygen. u Assume an empty 500 mL plastic bottle used to hold hydrogen peroxide has a mass of grams. 30% H2O2H2O2 decomposes H2OH2O

u What would be the difference in mass and the percentage difference in mass between 500 mL of fresh 30% hydrogen peroxide in the 500 mL bottle and a bottle of hydrogen peroxide which is decomposed? u Assume any oxygen gas formed escapes. u The density of water at 25.0 °C is gcm -3. u The density of pure hydrogen peroxide is gcm % H2O2H2O2 decomposes H2OH2O

volume of 30% H 2 O 2 density of 30% H 2 O 2 mass of 30% H 2 O 2 in solution 30% H 2 O 2 by mass mass of H 2 O 2 in solution mass of O 2 produced 32/68 balanced equation mass of H 2 O produced 36/68 mass of H 2 O in original solution 30% H 2 O by mass total mass of H 2 O at end mass is conserved initial mass of bottle & solution final mass of bottle & solution mass of empty bottle volume of H 2 O at end density of H 2 O mass is conserved

Problem Solving Format  Define the problem  Outline the solution to the problem include relevant diagrams & equations  Solve the problem

Stoichiometry  The study of heat, energy and material balances of a chemical system. The amount of reactants and products in a reaction are proportional to each other. The amount of heat energy released or required by a reaction is proportional to the amount of reactants and products involved in the reaction.

19th Century Chemistry 3 grams of carbon 4 grams of oxygen 400 grams 300 grams Analyzed reactions using weights and volumes

Stoichiometry Rules  Law of Conservation of Mass  Law of Constant Composition  Law of Multiple Proportions

Law of Conservation of Mass C(s) + O 2 (g)  CO 2 (g) 12 g32 g44 g In a chemical reaction, mass is neither created nor destroyed.

Law of Constant Composition Law of Definite Proportions C(s) + O 2 (g)  CO 2 (g) 12 g32 g44 g 6 g16 g22 g 3 g 8 g11 g Any sample of a pure compound always consists of the same elements combined in the same proportions by mass.

Law of Multiple Proportions C(s) + O 2 (g)  CO 2 (g) C(s) + 1/2 O 2 (g)  CO (g) 12 g C and 16 g O 2 12 g C and 32 g O 2 If two elements combine to form more than one stable compound, the masses of one element that combines with a fixed mass of the second element will be in the ratio of small whole numbers. 16:32 is 1 to 2

Chemical Equations and Stoichiometry  Moles of reactants and moles of product The balanced chemical equation deals with molecules or moles, not grams  Limiting reagent Every reaction that is complete (100% reaction) has at least one limiting reactant If equal masses of carbon and oxygen are reacted to give carbon dioxide, which is the limiting reactant?

Chemical Equations and Stoichiometry  Theoretical yield The maximum number of moles or mass of a product that could be formed from the given amounts of reactants What is the theoretical yield when 120 grams of carbon reacts with 120 grams of oxygen to give carbon dioxide?

Chemical Equations and Stoichiometry  Percentage yield The amount of product formed divided by the theoretical yield times 100% What is the percentage yield if 120 grams of carbon reacts with 120 grams of oxygen to give 200 grams of carbon monoxide? Show how your results support the Law of Conservation of Mass.

Reaction of Methane and Carbon Dioxide  50. grams of methane reacts with 50. grams of carbon dioxide to give carbon monoxide and water vapor.  If the reaction is 100% complete determine the limiting reagent. the mass of each reactant & product at the end of the reaction.

 Excel spreadsheet Reaction of Methane and Carbon Dioxide

Challenge  Design a spreadsheet to determine the maximum amount of carbon dioxide that could be formed when any amount of sucrose (C 12 H 22 O 11 ) reacts with any amount of oxygen gas. The other product is water.

Limestone u Limestone is a naturally occurring material consisting principally of calcium carbonate. u Upon heating (roasting) at >1000 °C, it decomposes into calcium oxide and carbon dioxide. u You are the chief engineer. How could you determine when this roasting process is completed?

6/13/2015