Chapter 12 Stoichiometr y

STOY-KEE-AHM-EH-TREE Founded by Jeremias Richter, a German chemist Greek orgin: stoikheion: element & metron: measure Stoichiometry—the calculation of quantities in chemical reactions Calculating how much of each reactant is needed and how much of each product is formed. Like a recipe: you need a certain amount of ingredients to get a certain amount of product.

Chemical Equations: What can you tell? 1N 2 + 3H 2  2NH 3 The coefficients in a Balanced Chemical Equation tell the # of particles of each reactant & product 1 molecule of N 2 reacts with 3 molecules of H 2 to produce 2 molecules of NH 3. OR The coefficients can represent the # of moles of each reactant & product. 1 mole of N 2 reacts with 3 moles of H 2 to produce 2 moles of NH 3.

1. You have to use units to understand the process 2. Units include g, mol, L, particles & chemical formulas 3. Convert your given to moles 4. You have to use units to understand the process 5. Use BCE coefficients to convert from one substance to another 6. You have to use units to understand the process 7. Convert answer to desired unit 8. You have to use units to understand the process 9. Units include g, mol, L, particles & chemical formulas 10. It really is not that hard You Should Know About Stoichiometry

3 Steps of Stoichiometry 1. Convert to moles (from particles, mass, or volume). 2. Do a mole to mole conversion using the balanced chemical equation 3. Convert moles to desired unit (particles, mass, or volume). 1 Mole BCE

Stoichiometry Example 1N 2 + 3H 2  2NH 3 If 2.61 L of H 2 gas reacts with the nitrogen in the air, how many grams of NH 3 will be produced? 2.61 L H 2 x 1 mol H 2 _ 22.4 L H 2 2 mol NH 3 3 mol H g NH 3 _ 1 mol NH 3 = 1.32 g NH 3 x x Given 1 Mole BCE 1 Mole

Percent Yield In a lab, the amount of product formed is often less than expected based on the balanced chemical equation. Theoretical Yield—the amount of product that should be formed (from stoichiometry problem) Actual Yield—the amount of product actually formed in a lab (found in an experiment). Percent Yield—the ratio of actual to theoretical yield. Percent Yield = actual yield x 100 theoretical yield

Example 24.8g calcium carbonate is decomposed by heating. 13.1g CaO is actually produced. What is the percent yield? CaCO 3  CaO + CO 2 Actual Yield: 13.1g CaO Theoretical Yield: (use stoichiometry) 24.8g CaCO 3 x 1 mol CaCO 3 x 1 mol CaO x 56.1 g CaO g CaCO 3 1 mol CaCO 3 1 mol CaO =13.9 g CaO % Yield = 13.1 x

Limiting Reagents Limiting Reagent—limits or determines the amount of product that can be formed in a reaction. Excess Reagent—the reactant that is not completely used up in a reaction. Limiting Reactants video

5N 2 and 9H 2 6NH 3 and 2N 2 Hydrogen isOxygen islimitingexcess Hydrogen is limiting Nitrogen is excess

Solving for a Limiting Reagent: 1. Use stoichiometry to convert each reactant (individually) into product (that’s 2 stoichiometry problems) 2. The reactant that gives you the least product is the limiting reagent. 3. The reactant that gives the most product is the excess reagent.

Example: Nickel replaces silver from silver nitrate in solution according to the following equation: 2AgNO 3 + Ni → 2Ag + Ni(NO 3 ) 2 If you have 22.9 g of Ni and 112 g of AgNO 3,what mass of nickel(II) nitrate would be produced? 22.9g Ni x 1 mol Ni x 1 mol Ni(NO 3 ) 2 x 182.7g Ni(NO 3 ) 2 = 71.3 g Ni(NO 3 ) g Ni 1 mol Ni 1 mol Ni(NO 3 ) g AgNO 3 x1 mol AgNO 3 x 1 mol Ni(NO 3 ) 2 x 182.7g Ni(NO 3 ) 2 =60.2g Ni(NO 3 ) g AgNO 3 2 mol AgNO 3 1 mol Ni(NO 3 ) 2 AgNO 3 is limiting & 60.2 g Ni(NO 3 ) 2 can be produced