Mon 3-21 and Tues 3-22 Gas Stoichiometry and Limiting/Excess Stoichiometry MRS. WILSON

Objectives 1.Calculate quantities (# moles, mass, or volume) of reactants and products in a reaction involving gases. 2.Use calculations to determine the limiting reagent/reactant in a problem involving masses/volumes of MORE THAN ONE reactant. 3.Use the limiting reagent/reactant quantity to calculate quantity of product, excess left over, and % yield. Homework: #4 and #5 on pg. 14 of packet. Answers are on website homepage. NOTE: Pg. 13 #1a, b, c answers are wrong online; correct answers are on one of the posters. Daily Quiz next class. Unit 5 Exam on Mon 3-28 and Tues 3-29.

A Real-Life Example of Gas Stoichiometry: Air Bag Design For a car airbag to inflate instantly, the car must come equipped with the correct masses of cost-efficient reactant(s).  To figure this out, the volume of the inflated airbag is more important to determine, than its mass. Early airbags used the decomposition of sodium azide, NaN 3 : 2 NaN 3 (s)  2 Na(s) + 3 N 2 (g)

An Overview: Gas Stoichiometry Problems

A Real-Life Example of Limiting/Excess Stoichiometry: S’mores 2 graham crackers + 1 marshmallow + 1 sq. chocolate  1 s’more If you have 10 graham crackers, 25 marshmallows and 15 chocolate squares… how many s’mores can you make? In this case, the number of graham crackers is “limiting” how many s’mores you can make. It’s the “limiting” reagent. The marshmallows and chocolate are the “excess” reagents. How many marshmallows and chocolate squares do you have left over?

A Real-Life Example of Limiting/Excess Stoichiometry: CO 2 in Space Flight Astronauts exhale CO 2 and it is absorbed by lithium hydroxide: CO 2 (g) + 2 LiOH(s)  Li 2 CO 3 (aq) + H 2 O(l)  During space flights, the shuttle must carry “excess” LiOH because the “scrubber” machinery for this process is not 100% efficient (no machinery or process ever is).  Thus, NASA has to determine the volume of CO 2 potentially exhaled (it “limits” the reaction), the mass of LiOH to carry, and then add more.

Lithium Hydroxide Scrubber Modified by Apollo 13 Mission Astronaut John L. Swigert holds the jury-rigged lithium hydroxide scrubber used to remove excess carbon dioxide from the damaged Apollo 13 spacecraft.

An Overview of Limiting/Excess Stoichiometry  These problems ALWAYS give you the mass and/or P/V/T for BOTH reactants. Step 1: Calculate # moles of each reactant (if not given). Step 2: Do limiting/excess stoich. Calculate # moles of product that would be made if one, then the other, reactant is limiting. ** Yes, you have to do stoichiometry, twice (once for each reactant) Step 3: Whichever reactant produced the LEAST # moles of product is “limiting” or the “limiting reactant/reagent.” Step 4: Use the # moles of product produced by the limiting reagent to calculate the desired quantity (mass or V )

Gas and Limiting/Excess Stoichiometry Posters Take your time! Visit each of the six posters. Work the problems and the problems in the packet. Practice is necessary for you to achieve success in this topic!