Stoichiometry Chemical Equations Dr. Ron Rusay Spring 2004 © Copyright 2004 R.J. Rusay

Chemical Reactions Atoms, Mass & Balance: eg. Fe + S -->

Chemical Equation Representation of a chemical reaction: _ C2H5OH + _ O2  _ CO2 + _ H2O reactants products C=2; H =5+1=6; O=2+1 C=1; H=2; O=2+1 1 C2H5OH + 3 O2  2 CO2 + 3 H2O

Chemical Equation C2H5OH + 3 O2  2 CO2 + 3 H2O The equation is balanced and the reaction can be completely stated as: 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water.

The Chemical Equation: Mole & Masses C2H5OH + 3 O2  2 CO2 + 3 H2O 46g (1 mole) of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water. How many grams of carbon dioxide and water are respectively produced?

The Chemical Equation: Moles & Masses (Alcohol) C2H5OH + 3 O2  2 CO2 + 3 H2O 46g (1 mole) of ethanol requires 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water. How many grams of oxygen are needed to react with all of the 46g (1 mole) of ethanol ? How many grams of oxygen are needed to react with 15.3g of ethanol in a 12oz. beer ?

Gases WKS [Extra Credit] An average pair of human lungs contains about 3.5 L of air after inhalation and about 3.0 L after exhalation. Assuming that air in your lungs is at 37°C and 1.0 atm, determine a) the number of moles of O2 and b) the mass of O2 and the total mass of air in a typical breath. How many breaths would it take to oxidize the alcohol in 1 beer if the oxygen inhaled was used only for that purpose?

Limiting Reagent An Ice Cream Sundae

Limiting Reagent

Mass Applications: Limiting Reagent How many FeS molecules are produced in each case? What is left over? © Copyright 1995-2004 R.J. Rusay

Mass Applications: Limiting Reagent How many ZnS molecules can be produced in each case? What is left over? © Copyright 1995-2004 R.J. Rusay

Mass Applications: Limiting Reagent How do masses of reactants relate? Is there enough mass of each reactant for the reaction to consume all of both of them or will there be some left of one of them? C2H5OH + 3 O2  2 CO2 + 3 H2O What would happen if 24. g of O2 (0.75 mol) were available for the reaction of 15.3 g of ethanol (0.33 mol)? How much O2 is needed to react with 0.33 mol of ethanol? = 0.33 mol C2H5OH x 3 mol O2 / 1 mol C2H5OH © Copyright 1995-2004 R.J. Rusay

Mass Applications: Determining a Limiting Reagent One of the reactants, oxygen, has fewer stoichiometrically adjusted moles than the other reactant, ethanol. Therefore, the reactant with the smaller value is the limiting reagent. In this case oxygen. Not all of the ethanol can react. Once the oxygen is depleted, the chemical reaction stops until more oxygen becomes available. The unreacted amount of alcohol remains in the blood.

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Mass Calculations: Products Reactants 1. Balance the chemical equation. 2. Convert mass of reactant(s) or product(s) to moles. 3. Identify mole ratios in balanced equation: They serve as the “Gatekeeper”. 4. Calculate moles of desired product or reactant. (Use the Limiting Reagent as basis) 5. Convert moles to grams. © Copyright 1995-2004 R.J. Rusay

Mass Calculations: Products Reactants © Copyright 1995-2004 R.J. Rusay

Mass Calculations: Products Reactants © Copyright 1995-2004 R.J. Rusay

Mass Calculations: Reactants Products How many grams of salicylic acid are needed to produce 1.80 kg of aspirin? Balanced Equation:

Mass Calculations: Reactants Products grams (Aspirin) grams (Salicylic Acid) Avogadro's Number Atoms Molecules Stoichiometry grams (SA) (Molecular 1800 grams (A) 1 mol (A) 1 mol SA Weight SA) 1380 grams (SA) 1 mol A grams (A) 1 mol (SA) (Molecular Weight A) "Gatekeeper" Calculation: http://ep.llnl.gov/msds/Stoichiometry/Aspirin-Calc.html © Copyright 1995-2004 R.J. Rusay

Percent Yield In synthesis, the actual yield (g) is measured and compared to the theoretical yield (g). This is the percent yield: % = actual / theoretical x 100 If a reaction produced 2.45g of Ibogaine, C20H26N2O, a natural product with strong promise in treating heroin addiction, and the theoretical yield was 3.05g, what is the % yield? © Copyright 1995-2004 R.J. Rusay

Percent Yield A reaction was conducted that theoretically would produce 0.0025 moles of quinine, C20 H24 N2 O2 . The actual amount of isolated quinine was 780 mg. What is the percent yield of quinine? 324 g/mol x 0.0025 mol = 81g = 810mg(theoretical) % Yield = 780 mg/ 810 mg x 100 % Yield = 96%