WHAT DO THE COEFFICIENTS IN A REACTION TELL US??!?!

• Consider the equation:– 2 H2 (g) + O2 (g) 2 H2O (l)

• The coefficients indicate: “Two molecules of diatomic hydrogen react with one molecule of diatomic oxygen to produce two molecules of water.”

• So, the coefficients also indicate: “2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water.” We can use this interpretation to help us make relationships to help us in reaction stoichiometry!!!

Look at those AMU values! Sure look like

molar masses to me!

USING THE LAW OF CONSERVATION OF MASS - Always assume a reaction proceeds 100% to the product (unless stated otherwise) Example: What does the law of conservation of mass state? Example: Think about the following reaction: 2 Na3N 6 Na + N2. If 500 grams of Na3N completely decompose to form 84.42 grams of N2, how much Na is produced?

• Mass can neither be created nor destroyed, but it can be rearranged.• The mass (amount of atoms) of the reactants must equal the mass (amount of atoms) of the products

STOICHIOMETRY PROBLEMS

KEY NOTES:Stoichiometry calculations can go between any substances in a reaction… reactants or products!Stoichiometry helps us to find a theoretical yield – a maximum amount ofproduct that SHOULD be made. (Not the actual yield!)

MOLE Ratios

Example: Use the balanced equation for the synthesis of water and determine the following mole ratios:

2H2 + O2 2H20

Hydrogen to Oxygen

Hydrogen to Water

Oxygen to Water

Water to Oxygen

Water to Hydrogen

MOLE-MOLE problems Example: Use the balanced equation for the synthesis of water and determine how many moles of water would form if you began with 32.38 moles of oxygen gas.

2H2 + O2 2H20

MASS-MASS and MASS-PARTICLE and VOLUME CONVERSION problems

Volume (1mole = 22.4L)

SAMPLE MAP: How many grams of H2O could be formed by reacting 50.0 grams of H2 with O2?

2 H2 + O2 2 H2O 50.0 g ? g Molar Mass Molar Mass Mole Mole Mole Ratio “Bridge”

STEP 1: According to the chart above, we calculate f rom

grams to moles using molar mass.

STEP 3: According to the chart above, we calculate f rom

grams to moles using molar mass.

STEP 2: I n order to get f rom one substance to another, we need the mole ratio. The mole ratio uses the coeffi cients. I n this case, the ratio is 2 moles of H2 f or every 2 moles of H2O.

SAMPLE SETUP: Here’s what the calculation should look like for the above problem…

Practice: How many grams of sodium chloride could be formed in a synthesis reaction in which 30.00 grams of chlorine gas are reacted with excess solid sodium?

Practice: How many grams of bromine are required to react completely with 45.70 grams of lithium iodide forming lithium bromide and iodine?

Practice: How many molecules of hydrogen gas are required to react with 0.334 grams of nitrogen gas in the synthesis of ammonia (NH3)?

Practice: How many moles of water could potentially be produced by reacting 5.60 grams of oxygen with hydrogen gas?

Practice: You are planning an experiment that requires 0.0580 mol of nitrogen monoxide (NO) gas. What volume of gas would you need at STP?

Practice: How many atoms of O2 are in 45.6 L?

Practice: How many grams of CO2 are in 28.3 L?

•PERCENT YIELD

100ltheoretica

actualyield%

• Percent Yield allows us to express a ratio of how much product we should have gotten in comparison to how much we actually obtained.

• Percent Yield can be above or below 100% - very rarely will you get a 100% yield… unless you are very lucky!

There is no way we’re getting 100% yield on

this lab… 

Example: The reaction of combustion of octane in an engine is known to be only 74.3% efficient. How many grams of water would you actually expect to form if 90.00 grams of liquid octane are burned?

2C8H18 + 25 O2 18H2O + 16CO2

Practice: If 35.6 grams of zinc sulfate are actually produced when excess zinc chloride solution reacts with 47.3 grams of aluminum sulfate, what is the percent yield of this reaction?

3ZnCl2 + Al2 (SO4)3 3ZnSO4 + 2AlCl3

***How do I know I have a limiting reactant problem? You will be given the starting amounts of BOTH reactants, and asked to find the maximum amount of product you can produce. Steps to solve limiting reactant problems?

TO FIND THE THEORETICAL YIELD: Take both of your reactants and find how much product they can produce. Whichever product amount is smallest is the maximum amount of product you can make.TO FIGURE OUT WHICH IS THE LIMITING REACTANT: Look to see which reactant made the least amount of product. This is your limiting reactant. The other reactant is in excess.TO FIGURE OUT THE AMOUNT OF LIMITING REACTANT LEFT OVER: Easy. It will always be NONE. You will use up all of the limiting reactant!TO FIGURE OUT THE AMOUNT OF EXCESS REACTANT LEFT OVER: Take the amount of limiting reactant you used, and use stoichiometry to calculate the amount of excess actually used. Then, subtract this value from how much you originally had. (HAD – USED = LEFT OVER)

LIMITING REACTANTS (aka Limiting Reagent Problems)

Limiting Reactants LIMIT HOW MUCH OF THE PRODUCT CAN BE MADE

Example: 15.00 grams of hydrogen are mixed with 40.00 grams of oxygen to form water. Write the balanced equation. What is the theoretical yield (maximum yield) of water in grams? What is the limiting reactant? How do you know? How many grams of the limiting reactant are left over? How many grams of the excess reactant are left over? This reaction was done in a laboratory environment and 43.00 g of water was actually collected. What was the percent yield of this experiment?

Practice: 35.60 grams of zinc are reacted with 100.00 grams of copper (II) sulfate forming zinc sulfate and copper. Write the balanced equation What is the theoretical yield (maximum yield) of pure copper in grams? What is the limiting reactant? What is the excess reactant? How many grams of the zinc will be left over? How many grams of the copper (II) sulfate will be left over?