Stoichiometry

What is Stoichiometry? The quantitative study of a chemical reaction. A chemical reaction is like a recipe. You need to make brownies. 1 pkg mix + 2eggs + ¼ cup oil →24brownies What if you need 48 brownies?

What is Stoichiometry? 2H2 + O2 →2H2O 2 molecules H2 + 1 molecule O2 → 2 molecules H2O 2 moles H2 + 1 mole O2 → 2 moles H2O mole ratio 4g H2 + 32 g O2 → 36 g H2O

Mole - Mole 1. How many moles of H2 are needed to react with 3.5 mol O2? 2H2 + O2 →2H2O 3.5 mol O2 x 2 mol H2 = 7.0 mol H2 1 mol O2 B) How many moles of water are produced?

Practice 2. How many moles of KClO3 must decompose to produce 9.0 moles of O2? 2KClO3 → 2KCl + 3O2 6 mol KClO3

Mass – Mole 3. How many grams of CO2 are produced by the combustion of 0.750 mol of butane (C4H10)? 2 C4H10 + 13O2 → 8CO2 + 10H2O 0.750 mol x g 0.75 mol C4H10 8 molCO2 2 mol C4H10 0.75 mol C4H10 8 mol CO2 44 g CO2 2 mol C4H10 1 mol CO2 = 132 g

Example 4. How many moles of Fe2S3 are produced by reacting 50.0 g of Fe with S? 0.0446 mol

Mass - Mass 5. How many grams of water are produced by the combustion of 1.00 g of C6H12O6? C6H12O6 + 6O2 → 6CO2 + 6H2O 1.00 g x g 1.00 g C6H12O6 1 mol C6H12O6 6 mol H2O 18 g H2O 180 g C6H12O6 1 mol C6H12O6 1 mol H2O = 0.600 g

Examples 6. 4NH3 + 7 O2 → 4NO2 + 6H2O 122 g x g 194 g

Examples 7. How many molecules of HCl are produced by reacting 125 g of chlorine with hydrogen? 2.12 x1024 molecules

Steps to solve Stoichiometry Problems Write the balanced equation. Show what you are given and what you are trying to find. Convert what you are given to moles. Multiply by the mole-mole ratio (coefficients). Convert to what you are trying to find.

Limiting Reactant The reactant that is completely used up in a reaction. Determines the amount of product formed. Excess reactant – reactant that is left over (too much) does not affect the amount of product formed.

Limiting Reactant 1. We need to make a turkey sandwich. 2 slices bread + 1 piece of turkey→ 1 sandwich 20 slices 15 pieces ? How many sandwiches can we make? 10 What is the limiting reactant? Bread How much turkey will be left over? 5 pieces

2. What if we had 20 slices of bread and 6 pieces of turkey? How many sandwiches could we make? 6 What is the limiting reactant? turkey How much bread would be left over? 8 slices

Limiting Reactant 1) Mg + I2 → MgI2 3 mol 4 mol What is the LR? Mg After the reaction is complete: How many moles of Mg? 0 moles Mg Moles of I2? 1 mol I2 Moles of MgI2? 3 mol MgI2

Limiting Reactant 2) 2Al + 3Br2 → 2AlBr3 LR? Br2 4.0 mol 4.0 mol LR? Br2 After the reaction is complete: How many moles of Br2? 0 mol Br2 Moles of AlBr3? 2.7 mol AlBr3 Moles of Al? (Honors) 1.3 mol Al

Limiting Reactant 3) N2 + 3H2 → 2NH3 LR? 4.0 mol 10.0 mol LR? To determine LR: divide # moles by the coefficient N2: 4.0 mol/1= 4.0 H2: 10.0 mol/3 = 3.3 H2 is LR. This number is only used to determine LR. Always use moles of LR in the stoichiometry calculations.

Limiting Reactant How many moles of NH3 are formed? 6.7 mol NH3 (Honors) Calculate the number of moles of N2 left over. 0.7 mol N2

Limiting Reactant 4) HCl + NaOH → H2O + NaCl 10.0g 10.0g LR? Convert grams to moles HCl 10.0g 1mol = 0.274 mol HCl 36.5 g NaOH 10.0g 1 mol = 0.25 mol NaOH 40 g NaOH is LR

Limiting Reactant Calculate the number of grams of NaCl formed. Use the # moles of the LR 0.25 mol NaOH 1 mol NaCl 58.5 g NaCl 1 mol NaOH 1 mol NaCl = 14.6 g NaCl

Limiting Reactant (Honors)Calculate the number of grams of HCl left over. First calculate the # g of HCl that are used in the reaction. 0.25 mol NaOH 1 mol HCl 36.5 g HCl 1 mol NaOH 1 mol HCl = 9.13 g HCl used 10.0 g – 9.13 g = 0.9 g HCl left over

Limiting Reactant 5) Zn + 2HCl → ZnCl2 + H2 50.0g 42.0g LR? HCl Calculate the # grams of ZnCl2 formed. 78.2 g (Honors)Calculate the # grams of Zn left over. 12.6 g