2-Stoichiometry of Chemical Reactions-II.pdf

Stoichiometry of Chemical Reactions II Dr. Alya A. Arabi College of Medicine and Health Sciences UAEU Stoichiometry Limiting Reagent & Percent Yield • Objectives • Identify the limiting reagent in a reaction. • Calculate theoretical yield, percent yield, and the amount of excess reagent that rema...

Stoichiometry of Chemical Reactions II Dr. Alya A. Arabi College of Medicine and Health Sciences UAEU Stoichiometry Limiting Reagent & Percent Yield • Objectives • Identify the limiting reagent in a reaction. • Calculate theoretical yield, percent yield, and the amount of excess reagent that remains unreacted given appropriate information. Stoichiometry “Limiting” Reagent • The limiting reagent is the reactant that is fully consumed first. • The excess reagent is the one you have left over. • The limiting reagent determines how much product you can make. A+B→C 1 1 1 2 1 ? Stoichiometry 1 3 ? How do you find out which is limited? • The chemical that makes the least amount of product is the “limiting reagent”. 𝑴𝑶𝑳𝑬𝑺 !!! 𝑪𝒐𝒆𝒇𝒇𝒊𝒄𝒊𝒆𝒏𝒕 (not moles alone, NOT grams, NOT Liters) Stoichiometry Example React solid Zn with 0.100 mol HCl (aq) Zn + 2 HCl ZnCl2 + H2 1 2 3 Rxn 1: Balloon inflates fully, some Zn left * More than enough Zn to use up the 0.100 mol HCl Rxn 2: Balloon inflates fully, no Zn left * Right amount of each (HCl and Zn) Rxn 3: Balloon does not inflate fully, no Zn left. * Not enough Zn to use up 0.100 mol HCl Stoichiometry React solid Zn with 0.100 mol HCl (aq) Zn + 2 HCl ZnCl2 + H2 Molecular Weight of Zn= 65.4 g/mol mass Zn (g) mol Zn/coefficient mol HCl/coefficient Lim Reactant Reaction 1 7.00 0.107 0.100/2 =0.050 HCl Reaction 2 Reaction 3 3.27 1.31 0.050 0.020 0.100/2 =0.050 0.100/2 =0.050 no LR Zn Stoichiometry What is Yield? • Yield is the amount of product made in a chemical reaction. • There are three types of yield: 1. Theoretical yield- what you theoretically expect to get based on the balanced equation (remember stoichiometry calculations) 2. Actual yield- what you actually get in the lab when the chemicals are mixed 3.Percent Yield = Actual Yield Theoretical Yield x 100 Stoichiometry Example: • 6.78 g of copper is produced when 3.92 g of Al are reacted with excess copper (II) sulfate. 2Al + 3 CuSO4 Al2(SO4)3 + 3Cu • What is the actual yield? = 6.78 g Cu • What is the theoretical yield? = 13.8 g Cu • What is the percent yield? = 49.1 % Stoichiometry Example: Al + CuSO4 → Al2(SO4)3 + Cu Subscripts (in red) are constant numbers that CANNOT be changed. If you modify them, you would be modifying the chemical itself which changes all the chemical reaction (wrong!) You are only allowed to change the coefficients (in green) Stoichiometry 2 Al + 3 CuSO4 → Al2(SO4)3 + 3 Cu Details on Yield • Percent yield tells us how “efficient” a reaction is. • Percent yield must not be greater than 100 %. • Theoretical yield must usually be greater than actual yield! (unless sample is not fully dried, or contaminated with byproduct etc.) – Why? Due to impure reactants; competing side reactions; loss of product in filtering or transferring between containers; measuring. Stoichiometry Reaction Types • Combination Reactions • Decomposition Reactions Stoichiometry Combination Reactions • Two or more substances react to form one product • Examples: N2 (g) + 3 H2 (g) ⎯⎯→ 2 NH3 (g) C3H6 (g) + Br2 (l) ⎯⎯→ C3H6Br2 (l) 2 Mg (s) + O2 (g) ⎯⎯→ 2 MgO (s) Decomposition Reactions • One substance breaks down into two or more substances • Examples: CaCO3 (s) ⎯⎯→ CaO (s) + CO2 (g) 2 KClO3 (s) ⎯⎯→ 2 KCl (s) + O2 (g) 2 NaN3 (s) ⎯⎯→ 2 Na (s) + 3 N2 (g) Stoichiometry Example combining the principle of decomposition reaction and percentage yield: PRACTICE If 454 g of NH4NO3 decomposes, how much H2O is formed? What is the theoretical yield of this product? STEP 1: Write the balanced chemical equation STEP 2: Convert mass reactant to moles STEP 3: Convert moles reactant to moles product STEP 4: Convert moles product to mass product 1) NH4NO3 ---> N2O + 2 H2O Stoichiometry General plan for stoichiometry calculations Mass reactant Mass product Stoichiometric factor Moles Reactant/coefficient Moles Product/coefficient Stoichiometry 454 g of NH4NO3 --> N2O + 2 H2O STEP 2 Convert mass reactant (454 g) to moles of reactant 1 mol 454 g • = 5.68 mol NH 4NO3 80.04 g STEP 3 Convert 5.68 moles reactant to mols of products mol NH4NO3/1 = mol N2O/1 = mol H2O/2 11.4 mol H2O produced Stoichiometry 454 g of NH4NO3 --> N2O + 2 H2O STEP 4 Convert moles product (11.4 mol) to mass product 18.02 g 11.4 mol H2O • = 204 g H2O 1 mol This mass is the THEORETICAL YIELD (this is how Stoichiometry much theoretically is expect to be produced). Practice Problem If 454 g of NH4NO3 decomposes, how much N2O is formed? What is the theoretical yield of this product? Stoichiometry