CHEM 1201 Lecture 8: Limiting ReagentsPremium
This preview shows half of the first page. to view the full 3 pages of the document.
Chem 1201 - Lecture 8 - Limiting Reagents
● The amounts of two known reactants are given, one will be used up. The other reactant
is in excess…but which one is it?
● The limiting reagent is completely consumed in the reaction.
● The limiting reagent limit’s the amount of product obtained.
● Therefore, at the end of reaction, those reactants present in excess will still be in the
General Scheme for a Typical Limiting
● GOAL (#1): Identify which reagent is the “limiting reagent”
○ Reagent amounts are given in gram amounts (haven’t covered molarity yet); so
these values must be converted to moles via molar mass bearing in mind the
stoichiometry of the reaction (MOST IMPORTANT!!).
○ The reagent with the smallest mole value, from using stoichiometry, is the limiting
○ Multiply the moles of the limiting reagent times the molar mass of the product
(and preserving stoichiometry) gives us the theoretical yield in grams. (i.e. moles
x molar mass = grams). This number is the theoretical yield assuming all the
limiting reagent is converted to product barring any form of experimental error.
● GOAL (#2): Determine percent yield
○ If given an actual (or TRUE experimental value) yield one can determine a
○ Note: A chemical reaction is no guarantee that all the reagents will be consumed
to produce 100% yield due solubility and various other affects.
● First let’s simplify the concept with something more appetizing before we jump into the
numbers and letters. Suppose you have a job making sundaes and each sundae
requires two scoops of ice cream, one cherry and 50 ml of chocolate syrup:
○ 2 scoops + 1 cherry + 50 ml syrup 1 sundae
● A mob of 25 first graders enters; can you feed them all? You have 50 scoops of ice
cream, 30 cherries, and 1 liter of syrup; a quick calculation shows that:
○ Ice cream: No. of sundaes = 50 scoops (1sundae/2 scoops) = 25 sundaes
○ Cherries: No. of sundaes = 30 cherries (1sundae/1 cherry) = 30 sundaes
○ Syrup: No. of sundaes = 1000ml syrup (1sundae/50 ml syrup) = 20 sundaes
○ The syrup is the limiting “reagent” here because it limits the total amount product
(sundaes) that can be produced. Of the three ingredients, the syrup allowed the
fewest sundaes (20); the rest of the ingredients, ice cream and cherries, are left
over “unreacted” when the syrup is gone, so they are present in excess.
● Basic Example (Given mol’s, short cut a step, but I am emphasizing the importance of
○ 2H2(g) + O2(g) 2H2O(l)
○ For every mol of O2 we need 2mol of H2 to make 2 mol of H2O. Based on
You're Reading a Preview
Unlock to view full version