CHAPTER 12.2 BASIC STOICHIOMETRY MRS RAGSDALE CHEMISTRY I

MOLAR RATIO REALLY SUPER BIG IMPORTANT! OBTAINED FROM A PROPERLY BALANCED CHEMICAL EQUATION BALANCE THE EQUATION WRONG? MOLAR RATIO IS WRONG! FAIL. COEFFICIENTS FROM BALANCED EQUATION = MOLAR RATIO

MOLAR RATIOS N₂ + 3H₂ → 2NH₃ BASED ON THE COEFFIECIENTS: Molar Ratio is 1:3:2 FOR EVERY 1 MOLE OF N₂ I WILL NEED 3 MOLES OF H₂ THIS WILL PRODUCE 2 MOLES OF NH₃

Moles to Moles N₂ + 3H₂ → 2NH₃ IF YOU HAVE 5 MOLES OF N₂, HOW CAN YOU CONVERT THIS TO NH₃? N₂ AND NH₃ ARE IN A 1 : 2 RATIO FOR EVERY 1 MOLE OF N₂, YOU WILL MAKE 2 MOLES OF NH₃ 5 MOLES N₂ X = 10 MOLES NH₃

Grams to Moles N₂ + 3H₂ → 2NH₃ IF YOU HAVE 25 GRAMS OF N₂, HOW CAN YOU CONVERT THIS TO MOLES OF NH₃? Remember – to convert grams of a substance into moles, you will need the molar mass (g/mol) There are 28g of N₂ in 1 mol of N₂ For every 1 mol of N₂ there are 2 moles of NH₃ 25gN₂ X X = 1.79 moles NH₃

MRS RAGSDALE CHEMISTRY I 12.3 LIMITING REAGENTS MRS RAGSDALE CHEMISTRY I

WHAT EXACTLY IS A LIMITING REAGENT? LIMITING REAGENTS – WHATEVER REACTANT THAT YOU RUN OUT OF FIRST EXCESS REAGENT – WHATEVER REACTANT YOU HAVE LEFT OVER

HOW IS A LIMITING REAGENT USEFUL? YOU CAN PREDICT THE AMOUNT OF PRODUCE YOU CAN EXPECT TO FORM IN A REACTION BASED ON THE LIMITING REAGENT

ENOUGH ALREADY… I UNDERSTAND WHAT IT IS BUT HOW THE HECK AM I SUPPOSED TO DO THIS????

MRS RAGSDALE’S WAY… 2Cu + S → Cu₂S LET’S SAY I HAVE 80.0g OF Cu REACTING WITH 25.0g OF S. HOW CAN I FIND OUT WHAT THE LIMITING REAGENT IS? CONVERT 80.0g OF Cu INTO ?g OF S AFTER DOING THE MATH I GET 20.2g OF S USE LOGIC!!! I HAVE 25g OF S AVAILABLE… HOW MUCH DO I ACTUALLY NEED THOUGH? THAT’S RIGHT! I NEED 20.2g BUT HAVE 25g – S IS IN EXCESS THAT MEANS THAT Cu IS MY LIMITING REAGENT

WHAT IS % YIELD? WE’RE JUST NOT PERFECT – EVEN THE BEST SCIENTIST WILL LOSE SOME PRODUCT % YIELD – MEASUREMENT OF HOW MUCH PRODUCT YOU OBTAINED VS HOW MUCH YOU SHOULD HAVE OBTAINED

HOW TO CALCULATE % YIELD STEP 1: THEORETICAL VS ACTUAL THEORETICAL YIELD: WHAT YOU CALCULATED USING STOICHIOMETRY ACTUAL YIELD: WHAT YOU PHYSICALLY OBTAINED DOING AN EXPERIMENT STEP 2: DO THE MATH