Presentation on theme: "Chapter 12 Notes #3 Limiting and Excess"— Presentation transcript:
1Chapter 12 Notes #3 Limiting and Excess ChemistryChapter 12Notes #3Limiting and Excess
2Reactions What causes a chemical reaction to stop? The reactants – or one of the reactants- get used up.Limiting reactant – the reactant that gets used up first, therefore limiting (stopping) the reaction
3ExampleHow many sets of blue/red/yellow crayons can you make with the following?10 red crayons12 blue crayons6 yellow crayonsWhat is the limiting reactant?Which were in excess (those not used up)?
4Example Hydrogen + Oxygen 4 Water molecules get created and an oxygen molecule is left over (excess), which means that Hydrogen is the limiting reactant
5The Math! CsF + XeF6 -> CsXeF7 How many moles of cesium xenon heptafluoride can be produced from the reaction of 12.5 mol cesium fluoride with 10.0 mol of xenon hexafluoride?Hint – notice the 1:1:1 ratio10 mol CsXeF7You can only make as much as your least amount !
6The Math 6Na + Fe2O3 -> 3Na2O + 2Fe If g Na and g Fe2O3 are used, determine the limiting reactant1) reactants = grams to moles2) Mole to Mole comparison (ratio)3) Limiting reactant becomes “given”4) Go to Stoichiometry step #3Excess reactant: Start Mass – Mass used
7The Math 6Na + Fe2O3 -> 3Na2O + 2Fe If g Na and g Fe2O3 are used, determine the limiting reactant
8The Math 6Na + Fe2O3 -> 3Na2O + 2Fe What is the mass of solid iron produced?
9The Math 6Na + Fe2O3 -> 3Na2O + 2Fe What is the mass of the excess reactant after the reaction?
10Percent YieldIf the conditions aren’t right, often a reaction will stop before the reactants are used up.Percent yield shows the efficiency of the reaction. (compare to a letter grade on a test)Actual yield (experiment) x 100Theoretical yield (math)
11Percent YieldHow might a business use percent yield in production for overall cost effectiveness?Overall, you want percent yield to be as close to 100% as possibleMight result in changing conditionsOr more excess (which speeds up the reaction or help make it go through to completion)
12The Math 6Na + Fe2O3 -> 3Na2O + 2Fe If we ran this experiment and produced 62.0 g of Fe, what would the percent yield be?