Author: WISD
Last saved by: ecalhoun
Company: Weatherford ISD

CHEMISTRY I - Lecture
“Stoichiometry”

*Stoichiometry:	

*To complete a basic stoichiometric calculation, you must include:

•	________________ ____________________ to convert between units.

•	________________ ____________________ to convert between substances.
                (The coefficients of the “given” and the “unknown” from the balanced equation)

          *Use the same “box” set up as we learned before…it’s just a “3 step box” now.

**Write and balance the following equation, then use it to answer Examples 1,2,3 :

			         nitrogen and hydrogen react to produce gaseous ammonia (NH_3). 

EXAMPLE 1: When excess nitrogen reacts with 38.5 grams of hydrogen, how many molecules 
  of ammonia can be made?

EXAMPLE 2: What volume, in L, at STP of ammonia can be produced when 5.40 grams of 
					  nitrogen completely reacts with hydrogen?

EXAMPLE 3: How many moles of nitrogen are needed to react with 3.84 x 10 ^23 molecules of 
                   		 hydrogen when ammonia is produced?

**Percent Yield**

•	Measures the “efficiency” of a chemical reaction by comparing the amount of product you __________________ produce to the amount you ___________  ___  ______ to produce.

			                 ACTUAL YIELD
•	% YIELD  =  --------------------------------------     X  100
						       THEORETICAL YIELD

•	ACTUAL YIELD  is _______________  _____________ .  (“is produced”)
		  (is experimentally determined)

•	THEORECTICAL YIELD _______  ___  ______________. (use other given amount)
		  (assumes a perfect reaction has occurred)	

***To calculate:
1)	Write and balance the equation for the reaction.
2)	Identify ACTUAL YIELD and plug directly into top of % Yield formula.
3)	Calculate THEORETICAL YIELD using other amount given in problem.  
Plug answer into bottom of % Yield formula.
		4)	Calculate % Yield.

EXAMPLE:	24.8 grams of solid strontium carbonate decomposes, when heated, to produce 
carbon dioxide and 13.1 grams of solid strontium oxide.  Calculate the % yield of strontium oxide?

EXAMPLE:	When 41.25 grams of acetylene gas (C_2H_2) is reacted with an excess of oxygen, 
water and 86.9 grams of carbon dioxide is produced.  What is the % yield of 
carbon dioxide?

**Limiting and Excess Reactants**

•	The amount of product that can be made in any reaction depends on:

		1)	

		2)

•	It is impossible to make a certain amount of product if you have insufficient amounts of reactants.

**Limiting reactant:	

•	Causes the reaction to stop because it is completely used up.

•	Determines how much of all other reactants are needed.

•	Determines how much product can be made.

**Excess reactant:

•	Some (but not all) of it used up in reaction.

•	How much is left over depends on how much you start with as well as the mole ratio between it and the limiting reactant.

**To Calculate:
1)	Write and balance equation for the reaction.
2)	To determine the LIMITING/EXCESS reactant:
•	Set up 2 “boxes” (one for each KNOWN) and calculate the amount of product that should be made.
•	The smaller of the 2 answers is CORRECT, and the other is WRONG and can be disregarded.
5)	To determine the amount of EXCESS reactant USED:
•	Set up a “box”, starting with the KNOWN amount of the LIMITING reactant, and convert to the EXCESS reactant. 
•	This answer tells you the amount of EXCESS reactant that gets used up during the reaction. 
6)	To determine the amount of EXCESS reactant LEFT OVER:
•	Subtract the USED amount of the EXCESS REACTANT from the GIVEN 
amount.
•	Tells you how much didn’t get used in the reaction.   

EXAMPLE:	

11.5 grams of chlorine reacts with 20.8 grams of sodium to produce sodium chloride.

	a)	Calculate the mass of sodium chloride that can be produced.
	b)	Determine the LIMITING and EXCESS reactants.
		c)  How much of the limiting reactant is used up in the reaction?
		d)	Determine the amount of EXCESS reactant that is used.
		e)	How much of the EXCESS reactant is left over (un-reacted)?

EXAMPLE:	
	
14. 5 grams of sodium metal reacts with 45.0 grams of water to produce hydrogen and sodium hydroxide. 

a)	Determine the LIMITING and EXCESS reactants.
b)	How many Liters of hydrogen gas can be produced?
c)  How much of the limiting reactant is left over?
d)	Determine the amount of EXCESS reactant that was left over.