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Chemistry Chapter 12 Notes #3 Limiting and Excess.

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Presentation on theme: "Chemistry Chapter 12 Notes #3 Limiting and Excess."— Presentation transcript:

1 Chemistry Chapter 12 Notes #3 Limiting and Excess

2 Reactions 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

3 Example How many sets of blue/red/yellow crayons can you make with the following? –10 red crayons –12 blue crayons –6 yellow crayons What is the limiting reactant? Which were in excess (those not used up)?

4 Example Hydrogen + Oxygen 4 Water molecules get created and an oxygen molecule is left over (excess), which means that Hydrogen is the limiting reactant

5 The Math! CsF + XeF 6 -> CsXeF 7 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 ratio 10 mol CsXeF 7 You can only make as much as your least amount!

6 The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe If g Na and g Fe 2 O 3 are used, determine the limiting reactant 1) reactants = grams to moles 2) Mole to Mole comparison (ratio) 3) Limiting reactant becomes “given” 4) Go to Stoichiometry step #3 Excess reactant: Start Mass – Mass used

7 The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe If g Na and g Fe 2 O 3 are used, determine the limiting reactant

8 The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe What is the mass of solid iron produced?

9 The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe What is the mass of the excess reactant after the reaction?

10 Percent Yield If 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 100 Theoretical yield (math)

11 Percent Yield How might a business use percent yield in production for overall cost effectiveness? Overall, you want percent yield to be as close to 100% as possible –Might result in changing conditions –Or more excess (which speeds up the reaction or help make it go through to completion)

12 The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe If we ran this experiment and produced 62.0 g of Fe, what would the percent yield be?


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