Presentation on theme: "Chemistry Chapter 12 Notes #3 Limiting and Excess."— Presentation transcript:
Chemistry Chapter 12 Notes #3 Limiting and Excess
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
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)?
Example Hydrogen + Oxygen 4 Water molecules get created and an oxygen molecule is left over (excess), which means that Hydrogen is the limiting reactant
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!
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
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
The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe What is the mass of solid iron produced?
The Math 6Na + Fe 2 O 3 -> 3Na 2 O + 2Fe What is the mass of the excess reactant after the reaction?
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)
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)
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?