Limiting Reactants Very rare that reactants are present in the same ratio that they are used in the rxn. This means one reactant will run out first. This means the reaction will stop and one reactant will be left in excess.

Limiting Reactants Limiting Reactant: –The reactant you run out of –Determines how much product can form Excess Reactant: –The reactant you have left over –Often used to speed up reaction or to be sure rxn continues until the limiting reactant is completely used up

Try It S 8 + 4Cl 2  4S 2 Cl 2 1.If you had 30 moles of sulfur and 100 moles of chlorine, which is the limiting reactant? How many moles of sulfur dichloride would form? 2.If you had g of sulfur and g of chlorine, which is the limiting reactant? How many grams of sulfur dichloride would be produced?

Try It When magnesium is burned, it forms magnesium oxide. How much magnesium oxide should you expect to form if 48.6 g of magnesium reacts with 64.0 g of oxygen?

Percent Yield Indicates the success or efficiency of a rxn Rxns rarely produce the expected amount of product (liquid sticks to container, solid sticks to filter paper, rxn stops early, competing rxns interfere) Compare the actual yield (what you measure when done) to the theoretical yield (what you calculated or predicted) Important in industry to balance efficiency, cost and time

Try It What is the percent yield if 10.0 g of methane (CH 4 ) is burned producing 19.5 g of water? What is the percent yield if g of octane (C 8 H 18 ) produces g of carbon dioxide?