Yesits still stoichiometry Think about making grilled cheese To make a decent grilled cheese sandwich you need two pieces of bread 2 pieces of cheese and 1 pat of butter Write an equation that describes making a grilled cheese sandwich ID: 556159
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Slide1
Limiting Reagents and Percent Yield
Yes…it’s still stoichiometry…Slide2
Think about making grilled cheese…
To make a
decent
grilled cheese sandwich, you need two pieces of bread, 2 pieces of cheese, and 1 pat of butter. Write an equation that describes making a grilled cheese sandwich.
2 Bread + 2 Cheese + 1 Butter
1 Grilled Cheese
SandwhichSlide3
Grilled Cheese Example Continued
If you had 6 pieces of bread, 4 pieces of cheese, and 3 pats of butter, which of the ingredients would limit how many sandwiches could be made?
The cheese. 6 Bread could make 3 sandwiches, 3 butters could make 3 sandwiches, but 4 cheese can only make 2 sandwiches.
Which of the ingredients would be leftover?
Bread and ButterSlide4
Limiting and Excess Reagent
Limiting Reagent: reactant that determines the amount of product that can be formed
The reaction only occurs until the limiting reagent is used up (in our example, that was the cheese
)
Excess Reagent: the reactant that is not completely used up
Some of the reactant is leftover after the reaction occurs (in our example, this was the bread and butter)Slide5
How to solve limiting and excess reagent problems
You will be given 2 starting points, so you will solve 2
stoichiometry
problems.
Example:
Copper
reacts with sulfur to form copper(I) sulfide. Identify the limiting and excess reagents when 80.0 g Cu reacts with 25.0 g S.
Cu
+ S
Cu
2
SSlide6
Practice
Problem
For each problem, indicate the limiting reagent, the excess reagent, and the amount produced.
1. The combustion of
ethene
is described below:
C
2
H
4
+ 3O
2
2CO
2
+ 2H
2
O
If 2.70 mol C
2
H
4
is reacted with 6.30 mol O
2
, identify the limiting reagent. Slide7
Another Practice Problem
2. When 6.00 g
HCl
reacts with 5.00 g Mg, how many grams of hydrogen gas can be produced?
2HCl + Mg
MgCl
2
+ H
2Slide8
Percent Yield
Theoretical Yield = what you could get according to
stoichiometry
Actual Yield = what is really made in the lab (usually lower than theoretical due to error/loss of efficiency in system)
Percent Yield = Actual Yield/Theoretical Yield (100)Slide9
To Solve Percent Yield Problems
Calculate the theoretical yield
Will require a stoichiometry problem
May be a limiting reagent problem (aka: 2 or more stoichiometry problems)
Find the actual yield (given to you in the problem, or found during lab)
Divide the actual yield by the theoretical yield
Multiply by 100 (it’s a percentage!)Slide10
Example Problem
If in the following reaction 0.112 g H
2
produces 0.745 g of water, what is the percent yield?
Fe
3
O
4
+ H
2
Fe + H
2
OSlide11
Different Type of Percent Yield Problem
Sometimes, you’ll know the percent yield and need to work backwards to figure out actual yield, for example:
If the reaction of 91.3 g C
3
H
6
produces an 81.3% yield, how many grams of CO
2
would be produced?
C
3
H
6
+ O
2
CO
2
+ H
2
O