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

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

4. Chemical Equation » Representation of a chemical reaction: _ C 2 H 5 OH + _ O 2  _ CO 2 + _ H 2 O reactants products reactants products » C=2; H =5+1=6; O=2+1C=1; H=2; O=2+1 C 2 H 5 OH + O 2  CO 2 + H 2 O C 2 H 5 OH + O 2  CO 2 + H 2 O

5. Chemical Equation C 2 H 5 OH + 3 O 2  2 CO 2 + 3 H 2 O 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.

6. The Chemical Equation: Mole & Masses C 2 H 5 OH + 3 O 2  2 CO 2 + 3 H 2 O 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?

7. The Chemical Equation: Moles & Masses (Alcohol) Alcohol C 2 H 5 OH + 3 O 2  2 CO 2 + 3 H 2 O 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 ?

8. Chemical Equations / Stoichiometric Calculations Example:Example: C 8 H 18(l) + O 2(g) -----> CO 2(g) + H 2 O (l)  For the combustion of octane which produces carbon dioxide and water.  The equation must conserve mass,i.e. account for the mass in product and reactant by having a “balance” which is an equal number of individual atoms in reactant and product. © Copyright 1995-2004 R.J. Rusay

9. Chemical Equations  The equation is “balanced” as before by placing stoichiometric factors before each of the molecules in the reaction so that the atoms equal ? C 8 H 18(l) + ? O 2(g) -----> ? CO 2(g) + ? H 2 O (l) Remove fraction: C 8 H 18(l) + O 2(g) -----> CO 2(g) + H 2 O (l) © Copyright 1995-2004 R.J. Rusay

10. Mass Calculations  The Balanced Equation is a balance on a molar basis which can be related to mass. 2 C 8 H 18(l) + 25 O 2(g) -----> 16 CO 2(g) +18 H 2 O (l) 228 g of octane (2 moles)* will react with 800 g of oxygen (25 moles) to produce 704 g of carbon dioxide and 324 g of water. *(2 moles octane x 114 g/mol = 228 g ) © Copyright 1995-2004 R.J. Rusay

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

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

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

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

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

16. Limiting Reagent An Ice Cream Sundae

17. Limiting Reagent

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

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

20. 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? 2 C 8 H 18(l) + 25 O 2(g) -----> 16 CO 2(g) +18 H 2 O (l) What would happen if 600. g of O 2 (18.75 moles) were available for the reaction of 228 g of octane (2 moles) instead of 800. g (25 moles) of oxygen? © Copyright 1995-2004 R.J. Rusay

21. Mass Applications: Determining a Limiting Reagent � One of the reactants, oxygen, has fewer stoichiometrically adjusted moles than the other reactant, octane. � Therefore, the reactant with the smaller value is the limiting reagent. In this case oxygen. � Not all of the octane can react. Once the oxygen is depleted the chemical reaction stops.

22. 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, C 20 H 26 N 2 O, 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

23. Percent Yield � A reaction was conducted that theoretically would produce 0.0025 moles of quinine, C 20 H 24 N 2 O 2. 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 =