Stoichiometry (Yay!)

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

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 can represent: The number of molecules - a literal interpretation This means 2 molecules of hydrogen will react with 1 molecule of oxygen to give 2 molecules of water. 2 H2 + O2  2H20

Interpreting the Equations The coefficients can also represent: The number of moles of each molecule This means 2 moles of hydrogen will react with 1 mole of oxygen to give 2 moles of water. 2 H2 + O2  2H20

Interpreting the Equations The coefficients can also represent: The number of volumes of gas in a reaction - ONLY if the molecules are gas molecules 2 volumes of hydrogen reacts with 1 volume of oxygen will give 2 volumes of water. 2 H2 (g) + O2 (g)  2H20 (g)

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 3 different equivalencies that exist? 2 mol H2 ≈ 1 mol O2 2 mol H2 ≈ 2 mol H2O 1 mol O2 ≈ 2 mol H2O

Mole Ratios From each equivalency, two mole ratios can be formed: 2 mol H2/1 mol O2 1 mol O2/2 mol H2 2 mol H2/2 mol H2O 2 mol H2O/2 mol H2 1 mol O2/2 mol H2O 2 mol H2O/1 mol O2 Which mol ratio is used depends on which ratio is needed when doing unit analysis, or which one the question asks for.

N2 + H2  NH3 If you start with 3.0 moles of N2, how many moles of H2 are needed? How many moles of NH3 can be produced? 1. Balance the reaction: N2 + 3 H2  2 NH3 2. Write out the equivalencies: 1 mol N2 ≈ 3 mol H2 1 mol N2 ≈ 2 mol NH3 3 mol H2 ≈ 2 NH3 3. Set up the unit analysis

N2 + 3 H2  2 NH3 To solve for H2: 3.0 mol N2 3 mol H2 = 9.0 mol H2 1 mol N2 To solve for NH3: 3.0 mol N2 2 mol NH3 = 6.0 mol NH3 The mole ratios are conversion factors using the coefficients from the balanced equation.

N2 + 3H2  2NH3 How many moles of nitrogen must react to produce 2.18 moles of ammonia?

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!!

Double Flux Capacitor Particles A Mass A Particles B Mass B Mole Ratio Moles B Moles A *draw stuff on lines on board Volume A Volume B

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen must react with 2.3 moles of hydrogen?

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of NH3 are produced if 2.3 moles of hydrogen is reacted with excess nitrogen?

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

N2 (g) + 3H2 (g)  2NH3 (g) How many grams of NH3 are produced if 15 grams of hydrogen is reacted with excess nitrogen?

N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen reacted if 100.0 grams of NH3 is produced?

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

N2 (g) + 3H2 (g)  2NH3 (g) How many particles of NH3 are produced if 22 grams of hydrogen is reacted with excess nitrogen?

N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen reacted if 7.1x1023 molecules of NH3 is produced?

Mass A to Volume B (gas) 1 mole A = grams A 1 mole B= 22.4L B 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

1N2 (g) + 3H2 (g)  2NH3 (g) How many grams of nitrogen are needed if 33 liters of ammonia is produced?

1N2 (g) + 3H2 (g)  2NH3 (g) How many liters of ammonia are produced if 55 grams of nitrogen is reacted with excess hydrogen?

Mixed problems How many grams of water are produced if 15 grams of hydrogen are reacted? H2(g) + O2 (g)  H20 (g)

2 H2(g) + O2 (g)  2H20 (g) How many liters of oxygen is reacted if 3.4x1023 molecules of water is produced?

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 Percent yield = actual yield theoretical yield X 100% The % yield should always be less than 100 % due to experimental error. When plugging in the theoretical yield, always use the UNROUNDED value.

Percent Yield What is the percent yield if the theoretical yield is 120 grams, but in the experiment only 87 grams is produced?

The actual yield will always be one of the products The actual yield will always be one of the products. It can never be one of the reactants. Na + H2O  NaOH + H2 If 18.3 g of sodium can produce 8.23 liters of hydrogen gas, what is the % yield? Balanced equation: 2Na + 2H2O  2NaOH + H2 18.3 g Na 1 mol Na 1 mol H2 22.4 L H2 = 8.91 L H2 23.0 g Na 2 mol Na 1 mol H2 (8.23 L H2 / 8.9113 … L H2) x 100 % = 92.4 %

Limiting Reactant vs. Excess What is a limiting reactant? The reactant that LIMITS the reaction The reactant that is completely used up Limiting reactant determines how much product is made What is an excess reactant? The reactant that is not completely used up

AB+ CD AD2 + C2B Balance equation Label information for each reactant/product Convert reactant (or product) to moles, if needed Solve for one substance on the opposing side of the equation The limiting reagent produces the least amount of product

H2(g) + O2 (g)  H20 (g) Identify the limiting reactant when 1.22 gram of oxygen reacts with 1.05 grams of hydrogen to produce water

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