Stoichiometry Unit 7

What is it? Study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction Based on the law of conservation of mass which states that

2 slices of bread + 1 slice of cheese  1 grill cheese sandwich Recipe 2 slices of bread + 1 slice of cheese  1 grill cheese sandwich Why is it important to know the recipe? To determine how much is needed or how much will be produced How many sandwiches can you make if you have 20 slices of bread and LOTS of cheese?

Interpreting the Equations When looking at a balanced equation, the coefficients represent the 4 different possible relationships that the reactants and products can have Particles Moles Mass Volume (if only gases are present!!)

4 Different Relationships 2 H2(g) + O2 (g)  2H20 (g) 1. Particles What are the three different types? Atoms Molecules Formula units What are the relationships of the particles in the chemical equation? For every 2 molecules of hydrogen that react with 1 molecule of oxygen, 2 molecules of water are produced Are there equal numbers of each element on both sides? What law does it obey? Conservation of MATTER

2 H2(g) + O2 (g)  2H20 (g) 2. Moles What are the mole relationships in the chemical reaction? For every 2 moles of hydrogen that react with 1 mole of oxygen, 2 moles of water are produced Are the number of moles equal? No, but it indirectly obeys the law of conservation of mass

Mole Ratios = Conversion Factors What is it? A ratio between the numbers of moles of any two substances in a balanced chemical equation 2H2 + O2  2H20 What are the 6 different mole ratios that exist? Hint: 2 mol H2 1 mol O2 1 mol O2 2 mol H2

2 H2(g) + O2 (g)  2H20 (g) 3. Mass Must obey the conservation of mass Previously, we stated that if 2 moles of hydrogen (_____g) reacts with 1 mole of oxygen (_____g), then 2 moles of water are produced. 1 mole of H2 = 2 grams of H2 1 mole of O2 = 32 grams of O2 What is the mass of 2 moles of water? 1 mole of H2O = 18.02 grams of H2O

2 H2(g) + O2 (g)  2H20 (g) Pressure at 1 atm or 101.3 KPa 4. Volume (liters) Must be at STP Pressure at 1 atm or 101.3 KPa Temperature at 0 degrees Celsius or 273K All reactants and products must be gases The equation tells you the volume relationship between the reactants and products

2 H2(g) + O2 (g)  2H20 (g) What are the volume relationships in the chemical reaction? For every 2 liters of hydrogen that reacts with 1 liter of oxygen, 2 liters of water are formed Is the volume conserved?

2 K(s) + 2 H20(l) 2KOH(aq) + H2(g) Interpret the above equation in terms of relative numbers of representative particles, numbers of moles, and masses of reactants and products. Mole relationship 2 mol K + 2 mol H20 2 mol KOH + 1 mol H2

2 K(s) + 2 H20(l) 2KOH(aq) + H2(g) Particles (Be specific!! Three types) 2 atoms K + 2 molecules H20 2 for.u. KOH + 1 molecule H2 Masses ____ g K + _____g H20 ______g KOH + _____g H2 1 mole K = ______ g K 1 mole H2O = _________g H20 1 mole KOH = _______g KOH 1 mole H2 = _________g H2

Mole to Mole Calculations 1N2 (g) + 3H2 (g)  2NH3 (g) What are the mole ratios for this chemical equation? Hint: there are 6 of them

6 mole ratios

1N2 (g) + 3H2 (g)  2NH3 (g) How many moles of hydrogen must react to produce 5.5 moles of ammonia? Setup: 5.5 mol NH3 = _______ mol H2 Conversion Factors and/or Mole Ratios: 2 mol NH3 = 3 mol H2 Work:

1N2 (g) + 3H2 (g)  2NH3 (g) How many moles of nitrogen must react to produce 5.5 moles of ammonia? Setup: 5.5 mol NH3 = _______ mol N2 Conversion Factors and/or Mole ratios: 2 mol NH3 = 1 mol N2 Work:

Moles of A to Mass of B You can never convert directly from moles of A to grams of B You must go through the moles!! One extra step

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen must react with 2.3 moles of hydrogen? Setup: 2.3 mol H2 = _______ g N2 Conversion Factors and/or Mole ratios: 3 mol H2 = 1 mol N2 1 mol N2 = 28g N2 Work:

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of ammonia are produced if 2.3 moles of hydrogen is reacted with excess nitrogen? Setup: 2.3 mol H2 = _______ g NH3 Conversion Factors: 3 mol H2 = 1 mol NH3 1 mol NH3= 17.03g NH3 Work:

Mass A to Mass B You can never go from A to B without moles!! Steps: 1) Convert the initial value to moles 1 mole A = ___ grams A 2) Use Mole A to Mole B ratio from equation 3) Convert Moles B to grams of B 1 mole B= ___ grams B

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of ammonia are produced if 15 grams of hydrogen is reacted with excess nitrogen? Setup: 15 grams H2 = _______ g NH3 Conversion Factors: 1 mol H2 = 2.02 grams H2 3 mol H2 = 1 mol NH3 1 mol NH3= 17.03g NH3 Work:

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen reacted if 100.0 grams of ammonia is produced? Setup: 100.0 g NH3 = _______ g N2 Conversion Factors: 1 mol NH3= 17.04 g NH3 2 mol NH3 = 1 mol N2 1 mol N2 = 28g N2 Work:

Mass A to Particles B or Particles A to Mass B You can never go from A to B without moles!! Steps (mass  particles): 1) Convert the initial value to moles 1 mole A = ___ grams A 2) Use Mole A to Mole B ratio from equation 3) Convert Moles B to particles of B 1 mole B= 6.02x1023 particles B

1N2 (g) + 3H2 (g)  2NH3 (g) How many particles of ammonia are produced if 22 grams of hydrogen is reacted with excess nitrogen? Setup: 20 grams H2 = _______ molecules NH3 Conversion Factors: 1 mol H2 = 2.02 grams H2 3 mol H2 = 1 mol NH3 1 mol NH3= 6.02x1023 molecules NH3 Work:

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen reacted if 7.1x1023 molecules of ammonia is produced? Setup: 7.1x1023 molecules NH3 = _______ g N2 Conversion Factors: 6.02x1023 molecules NH3 = 1 mol NH3 2 mol NH3 = 1 mol N2 1 mol N2 = 28g N2 Work:

Mass A to Volume B (gas) 1 mole A = ___ grams A To find volume, the reaction must occur at STP. To go between A and B, you must first find moles! Steps: 1) Convert the initial value to moles 1 mole A = ___ grams A 2) Use Mole A to Mole B ratio from equation 3) Convert Moles B to Liters of B 1 mole B= 22.4L B @ STP

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen reacted if 33 liters of ammonia is produced? Setup: 33 L NH3 = _______ L N2 Conversion Factors: 1 mol NH3= 17.04 g NH3 2 mol NH3 = 1 mol N2 1 mol N2 = 22.4L N2 @ STP Work:

1N2 (g) + 3H2 (g)  2NH3 (g) How many liters of ammonia are produced if 55 grams of nitrogen is reacted with excess nitrogen? Setup: 55 grams N2 = _______ liters NH3 Conversion Factors: 1 mol N2 = 28.02 grams N2 3 mol H2 = 1 mol NH3 1 mol NH3= 22.4L NH3 Work:

Mixed problems Part A 2 H2(g) + O2 (g)  2H20 (g) How many grams of water are produced if 15 grams of hydrogen are reacted? Setup: Conversion factors: Work:

2 H2(g) + O2 (g)  2H20 (g) Part B How many liters of oxygen is reacted if 3.4x1023 molecules of water is produced? Setup: Conversion factors: Work:

Answers to Mixed problems

Theoretical vs. Actual Yield Theoretical yield Maximum amount of product that can be made from given amounts of reactants What you do on paper Actual yield The amount of product that ACTUALLY forms when the reaction is carried out in the lab

Percent Yield Equation: actual yield theoretical yield X 100

Practice What is the percent yield if the theoretical yield is 120 grams, but in the experiment only 87 grams is produced? What is the actual yield if the percent yield is 87% and the theoretical yield is 45 grams?

Who’s the BOSS? Limiting Reactant vs. Excess What is a limiting reactant? The reactant that LIMITS the reaction The reactant that is completely used up Why the boss? B/c the limiting determines how much product is made What is an excess reactant? The reactant that is not completely used up

How to solve for the limiting 2 H2(g) + O2 (g)  2H20 (g) What is the limiting reactant if 3.2 moles of hydrogen react with 1.55 moles of oxygen react? Organize what you have and what You need

Write the reactants on top Label “Have”, “Need”, “INXS” Fill in for the boxes

Solving for “NEEDS” Reactant 1 to Reactant 2: When solving for the NEED boxes, pretend the other “have” value is NOT present Reactant 1 to Reactant 2: Reactant 2 to Reactant 1:

Practice Regardless of how the question is stated, fill in the boxes with moles Why? Easier to convert from moles and saves time Identify the limiting reactant when 1.22 gram of oxygen reacts with 1.05 grams of hydrogen to produce water

2 H2(g) + O2 (g)  2H20 (g) Make a box (take up this whole slide!)

2 H2(g) + O2 (g)  2H20 (g) Convert to MOLES for “HAVE”!! Moles of hydrogen? Moles of oxgen?

Find the “NEEDS” Fill in the box afterwards and solve for the INXS INXS can NOT be negative Reactant 1 to Reactant 2: Reactant 2 to Reactant 1:

How many grams of water are produced? What determines how much product is made? Solve for moles A (BOSS) to mass B

Practice If 4.1 grams of Cr is heated with 9.3 grams of Cl2, what mass of CrCl3 produced? What is the balanced equation?

Make a box (take up this whole slide!) Label for “HAVE”, “NEED”, “INXS” & reactants

Convert to MOLES for “HAVE”!! Moles of reactant 1? Moles of reactant 2?

Find the “NEEDS” Moles of reactant 1 to Moles of reactant 2

How many grams of chromium III chloride are produced? Who’s the BOSS? Solve for moles A (BOSS) to mass B

INXS What is the excess reactant? How much was used? How much was left over?