Stoichiometry Notes

Stoichiometry - shows the relationship between reactants & products in a chemical reaction. Now we will use our knowledge of chemical reactions and equations to solve problems.

a. One new conversion we need to know is the mole ratio – it relates the amounts of moles of any two substances involved in a chemical reaction. 2Al2O3  4Al + 3O2 2 mol Al2O3 2 mol Al2O3 4 mol Al 3 mol O2 4 mol Al 3 mol O2 Writing the inverse(the fraction “upside down”)of any of the above would make a total of 6 mole ratios.

Practice Problem: For the following chemical equation write all possible mole ratios. 2HgO  2Hg + O2 (hint: use the coefficients (front numbers) as your numbers in your ratios (fraction)) _______________ _______________ _______________ (write 3 fractions/ratios on the given fraction bars above)

Answers: 2 mol HgO 2 mol HgO 2 mol Hg 1 mol O2 2 mol Hg 1 mol O2 (The reciprocal (Flip Flop Fraction) is also correct for each of these.)

b. Three types of problems involving stoichiometric calculations (mole ratios) are: Mole to mole problems (1 step problems) *(involves the least amount of work) 2. Mole to mass problems (2 step problems) 3. Mass to mass problems (3 step problems)

To solve these problems: 1st - write a balanced equation for the reaction. 2nd - set-up a dimensional analysis problem, starting with the known quantity written as a fraction. Use correct mole ratios to solve the problem.

Practice Problems (solve these): 8. According to the following equation how many moles of lithium hydroxide are required to react with 20.0 moles of CO2? CO2 + 2LiOH  Li2CO3 + H2O ? 9. How many moles of ammonia, NH3 are produced when 6.00 moles of hydrogen gas react with excess nitrogen gas? 3H2 + N2  2NH3 ?

10. What mass, in grams, of glucose is produced when 3 10. What mass, in grams, of glucose is produced when 3.00 moles of water react with carbon dioxide? 6CO2 + 6H2O  C6H12O6 + 6O2 What mass of aluminum is produced by the decomposition of 5000. grams of Al2O3? ?

III. Percent Yield a. Theoretical yield – maximum amount of product that can be produced from a given amount of reactant. This value can be calculated from working mass to mass problems.

Example: In Practice Problem 14 you determined that 2647 grams (your answer to #14) of Al could be produced from 5000. grams of Al2O3. This will occur if the reaction occurs without ANY type of error!!

b. Actual yield – actual amount of a product obtained from a reaction b. Actual yield – actual amount of a product obtained from a reaction. It must be experimentally obtained. It cannot be more than the theoretical yield.

Example: Let’s say we carry out the reaction in Practice Problem 14 in a lab and we only obtain 2100. grams of Al from 5000. grams of Al2O3.

c. Percent yield = Actual yield X 100 Theoretical yield 2100. grams X 100 = 79.36% 2647 grams

15. When 36.8 grams of C6H6 react with an excess of Cl2, the actual yield of C6H5Cl is 38.8 grams. What is the percent yield of C6H5Cl? C6H6 + C12  C6H5Cl + HCl Actual Yield (given)= _?_ (in green) (1)Theoretical Yield (calculated from 36.8)= _?_ (2) Percent Yield = _?_

IV. Limiting Reactants- The reactant that limits the amounts of the other reactants that can combine and the amount of the product that can form in a chemical reaction. The substance that is not used up completely in a reaction is called the excess reactant.

Let’s say you need the following to make one hamburger: 2 buns 1 piece of meat 1 piece of lettuce

If you have 16 buns, 16 pieces of meat, and 20 pieces of lettuce, how many hamburgers can we make?_________ What limits how many we can make (what do we run out of first)? _________ What is in excess (what do we have leftovers of)? _________

You should be able to recognize a limiting reactant problem because they have 2 amounts given – one for each reactant! To solve these problems first write the balanced equation for the reaction. Second determine which of the reactants the limiting reactant is. (To determine which the limiting reactant is use both amounts of reactants given and setup two dimensional analysis problems to solve for the product. The smaller answer will come from the limiting reactant and will be the correct answer.)

Practice Problems: 17. Silicon dioxide (quartz) is usually quite uncreative but reacts readily with hydrogen fluoride according to the following equation. SiO2 (s) + 4HF (g)  SiF4 (g) + 2H2O (l) If 2 moles of HF are exposed to 4.5 moles of SiO2, which is the limiting reactant? _?_ What is the excess reactant? _?_

Answer:

Go to pHet and complete the Sandwiches, Molecules and Game as seen in the picture below. Link: https://phet.colorado.edu/sims/html/reactants-products-and-leftovers/latest/reactants-products-and-leftovers_en.html