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# Stoichiometry

The mathematical relationships of chemical reactions
by

## Nancy Cope

on 28 January 2015

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#### Transcript of Stoichiometry

V
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C
A
B
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L
A
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STOICHIOMETRY
LIMITING REACTANT
THEORETICAL YIELD
ACTUAL YIELD
EXCESS REACTANT
PRODUCTS
REACTANTS
PERCENT YIELD
Stoichiometry
the study of the mathematical relationships of chemical reactions
We know how to...
identify different types of chemical reactions
balance chemical reactions
convert between grams and moles (and reverse)
aka Revenge of the Mole
We will learn how to...
convert between grams of one chemical to grams of a different chemical using the balanced equation and the molar masses
the study of the mathematical relationships in a chemical reaction
a comparison between the amount of product mathematically possible an the amount made in lab
the amount of product produced in the lab
the amount of product mathematically possible
the starting material that still remains after the reaction
the starting material that is completely consumed in the reaction
the ending material in a reaction
the starting material in a reaction
We remember from our last unit....
Reactants are the starting material in a chemical reaction
Products are the ending material in a chemical reaction
All chemical reactions must be balanced (to agree with the Law of Conservation of Mass)
We remember from last semester...
comparing different amounts of chemicals must be done with the unit of moles because each element has a different molar mass
CH + 2O CO+ 2H O
2
2
4
2
{
{
Reactants
Products
the same number of atoms of carbon and oxygen have different masses but are the same number of moles
Now we will put it all together!
There are three types of stoichiometry problems
3-step
2-step
1-step
grams of what you are given to moles of what you want OR moles of what you are given to grams of what you want
moles of what you are given to moles of what you want
grams of what you are given to grams of what you want
To help us there is a new mole map
Step One: Write a balanced equation
There are three steps in the process of solving stoichiometry problems.
Step 2: Identify the want and the given.
How many moles of water are produced from the combustion of 100 moles of methane in excess oxygen?
Step 3: Layout and carry out your plan
one step: mole ratio
CH + 2O CO + 2H O
2
4
2
2
given: 100 moles of methane
want: moles of water
Find your place on the map
Finish!
Start!
CH + 2O CO + 2H O
2
4
2
2
100 moles CH
4
2
1 mole CH
2 moles H O
4
x
=
200 mole H O
2
But what if it's more than one step...
How many grams of water are produced from the combustion of 50 moles of methane?
two step problem: moles to grams
use the mole ratio and the molar mass
CH + 2O CO + 2H O
2
4
2
2
4
2 mole H O
2
1 mole CH
4
18 grams H O
2
1 mole H O
2
50 mole CH
x
=
x
2
180 grams H O
How many grams of carbon dioxide are produced from the combustion of 34 grams of methane?
Three step problem: grams to grams
34 grams CH
2
16 grams CH
4
44 grams CO
4
88 grams CO
1 mole CH
2
1 mole CO
2
1 mole CO
4
1 mole CH
4
=
x
x
x
2
Let's look at a different problem
How many grams of silver phosphate are produced from 200 grams of silver nitrate reacting with 200 grams of sodium phosphate?
because we have two starting materials in this problem, we have to do two stoichiometry calculations
200 grams of silver nitrate to grams of silver phosphate (3 steps)
200 grams of sodium phosphate to grams of silver phosphate (3 steps)
this reaction gives us two starting amounts and asks for one answer
First, write a balanced equation to show the reaction
3AgNO + Na PO Ag PO + 3NaNO
3
3
4
3
4
3
Given
Molar Mass of Given
Mole ratio
Molar Mass of Want
Multiply on the top, divide on the bottom...
The smaller answer the the amount of product that will be made. This is the
theoretical yield
. The reactant that is responsible for the theoretical is the
limiting reactant
, because it limits the amount of product being made.
In lab, when the reaction is completed the actual yield is produced. This amount may be different than the theoretical yield due to experimental error.
The actual yield of the reaction was 100 grams of Ag PO
Theoretical Yield
Limiting Reactant
3
4
There are two ways to determine how "off" the actual yield is from the theoretical yield.
Percent Error
Percent Yield
Percent Yield
a measure of how much product was made
Stoichiometry is beneficial because it allows us to...
...maximize the efficiency of our experiments
theoretical yield
percent yield = actual yield
Percent Error
a measure of the difference between the theoretical and actual yields
percent error = actual yield - theoretical yield
theoretical yield
The percent yield and percent error should add to be 100%
...Estimate how much material will be made in a chemical reaction
...determine the accuracy of our experiments
Stoichiometry may also be used with gas laws, thermodynamics, equilibrium and acid/base reactions, all topics we will cover in the future.
yes, this means stoichiometry will not go away.
CH + 2O CO + 2H O
2
4
2
2
Full transcript