Stoichiometry+of+Excess+Quantities

Stoichiometry of Excess Quantities //__Text Book p:132-134__//

**Stoichiometry** Mathematical relationships among the quantities of reactants and products involved in chemical reactions We have assumed that a given reactant is //completely// used up during the reaction **Definitions** **Ex. #1** ** If 20.0 g of hydrogen gas react with 100.0 g of oxygen, which reactant is present in excess and by how many grams? ** The balanced equation: First PREDICT which reactant is limiting (it’s ok if you predict wrong) USUALLY the reactant with the least number of moles is limiting (but not always) = 20.0 gH2 x __1 mol H2__ = 10 mol H2  --2.0 g H2  = 100.0 gO2 x __1 mol O2__ = 3.125 mol O2   ---32.0 g O2  ** Mass of H2 that reacts with 100.0 g O2 ** = 100.0 g O2 x __1 mol O2__ x __2 mol H2__ x __2.0 g__ __H2__ = 12.5 g H2 ---_32.0 g O2 _1 mol O2 _1 mol H2 What we have: 100.0 g O2 and 20.0 g H2 We predict O2 is limiting (i.e. all 100.0 g reacted) We calculated that we would need 12.5 g H2 Only 12.5 g H2 is required, so we have an excess of 7.5 g H2 (20.0 g - 12.5 g) So H2 is in EXCESS of 7.5 g. = 20.0 g H2 x __1 mol H2__ x __1 mol O2__ x __32.0 g__ __O2__ = 160.0 g O2 ---2.0 g H2__ 2 mol H2_ 1 mol O2  **Example #2 Limiting Reactant Calculation:** A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen. Which is the limiting reactant and how much excess reactant remains after the reaction has stopped? First, we need to create a balanced equation for the reaction: Next we can use stoichiometry to calculate how much product is produced by each reactant. NOTE: It does not matter which product is chosen, but the same product must be used for both reactants so that the amounts can be compared. The reactant that produces the lesser amount of product: in this case the oxygen. Next, to find the amount of excess reactant, we must calculate how much of the non-limiting reactant (oxygen) actually did react with the limiting reactant (ammonia). We're not finished yet though. 1.70 g is the amount of ammonia that **//reacted//**, not what is left over. To find the amount of excess reactant remaining, subtract the amount that reacted from the amount in the original sample.
 * Introduction: So far ...**
 * EXCESS REACTANT** = the reactant in excess
 * LIMITING REACTANT =** the reactant that completely reacts
 * THE LIMITING REACTANT** determines the yield of the product (how much product(s) will form)
 * -2H2（g）+O2(g) → 2H2O(l)- **
 * Number of moles of H2 present **
 * Number of moles of O2 present **
 * Mass of O2 that reacts with 20.0 g H2 **
 * 4 NH3(g) + 5 O2(g)[[image:http://www.chem.tamu.edu/class/majors/tutorialnotefiles/arrow.gif width="36" height="13"]]4 NO(g) + 6 H2O(g) **