1. Chapter 12 Notes #3 Limiting and Excess
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 + 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 ratio
10 mol CsXeF7
You can only make as much as your least amount !

6. The Math 6Na + Fe2O3 -> 3Na2O + 2Fe
If g Na and g Fe2O3 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 + Fe2O3 -> 3Na2O + 2Fe
If g Na and g Fe2O3 are used, determine the limiting reactant

8. The Math 6Na + Fe2O3 -> 3Na2O + 2Fe
What is the mass of solid iron produced?

9. The Math 6Na + Fe2O3 -> 3Na2O + 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 + Fe2O3 -> 3Na2O + 2Fe
If we ran this experiment and produced 62.0 g of Fe, what would the percent yield be?