Chemistry 103 Lecture 15
Feb 12, 2016
Outline
I. Chemical Reactions - balancing (review)II. Stoichiometry - Limiting ReactantIII. Classification of Reaction Types
Avogadro’s Number
Solution = 6.022 x 1023
Avogadro’s number is equal to 1 mole
Makes working with large numbers easier
Concept Review
Do 7.0g of CO and 7.5 g of C2H6 contain the same number of C atoms?
(Prove your answer mathematically)
A. YES B. NO
Chemical ReactionIn a chemical reaction, Old bonds are broken and
new bonds are formed Atoms in the reactants are
rearranged to form one or more different substances
Fe and O2 form rust (Fe2O3)
Chemical’s Shorthand - Chemical Rxn Fe (s) + O2(g) --------> Fe2O3 (s)
2Fe(s) + O2(g) --------> Fe2O3(s)
4Fe(s) + O2(g) --------> 2Fe2O3(s)
4Fe(s) + 3O2(g) --------> 2Fe2O3(s) BALANCED!!!!!
Balancing Practice
Li3N(s) -------> Li(s) + N2(g)
Mg + H3PO4 ------> Mg3(PO4)2 + H2
MgCl2 + AgNO3 ------> Mg(NO3)2 + AgCl
Stoichiometry Chemical Stoichiometry: using mass and quantity
relationships among reactants and products in a chemical reaction to make predictions about how much product will be made.
We can read the equation in “moles” by placing the word “mole” or “mol” between each coefficient and formula.
4Fe(s) + 3O2(g) 2Fe2O3(s)
4 mol Fe + 3 mol O2 2 mol Fe2O3
Moles in Equations
Mole to Mole problems
Using the balanced chemical reaction, how many moles of O2 will react with 4.28 moles of CS2 to produce CO2 and SO2?
CS2 + O2 ------> CO2 + SO2 (unbalanced)
Mole to Mole problems
When 4.28 moles of CS2 reacts according to the balanced chemical equation below, how many moles of CO2 will be produced?
CS2 + 3O2 ------> CO2 + 2SO2
Mole to Mole problems
When 4.28 moles of CS2 reacts according to the balanced chemical equation below, how many moles of SO2 will be produced?
CS2 + 3O2 ------> CO2 + 2SO2
Reaction Interpretation
Using the balanced chemical reaction, how many moles of O2 will react with 4.28 moles of CS2 to produce CO2 and SO2?
CS2 + 3O2 ------> CO2 + 2SO2BEFORE
AFTER
Chemical Stoichiometry2H2 + O2 ----> 2H2O
mole to mole comparisons: How many moles of O2 are needed to produce
1.39 moles of H2O?
Chemical Stoichiometry
2H2 + O2 ----> 2H2O
mole to gram or gram to mole comparisons:How many moles of O2 are needed to produce 7.4g
of H2O?
Chemical Stoichiometry
2H2 + O2 ----> 2H2O
mole to mole comparisons: How many moles of O2 are needed to exactly react
with 4.28 moles of H2?
Chemical Stoichiometry
2H2 + O2 ----> 2H2O
mole to gram or gram to mole comparisons:How many moles of O2 are needed to exactly react
with 12.4g of H2?
How many grams of O2 are needed to produce 0.400 mol Fe2O3?
4Fe(s) + 3O2(g) 2 Fe2O3(s)
A) 38.4 g O2
B) 19.2 g O2
C) 1.90 g O2
Grams-to-Mole
The reaction between H2 and O2 produces 13.1 g water.
How many grams of O2 reacted?
2H2(g) + O2(g) 2H2O(g)
Plan: g H2O mole H2O mole O2 g O2
Calculating the Mass of a Reactant
The reaction between H2 and O2 produces 13.1 g water.
How many grams of O2 reacted?
2H2(g) + O2(g) 2H2O(g)
Before: x g
After: 13.1 g
Plan: g H2O mole H2O mole O2 g O2
Calculating the Mass of a Reactant
Mass calculations
How many grams of CS2 are needed to completely react with 6.0g of O2 according to the following reaction?
CS2 + 3O2 ------> CO2 + 2SO2
“gram to gram” continued….
One way to remove gaseous carbon dioxide from the air in a spacecraft is to let canisters of solid lithium hydroxide absorb it according to the following reaction:
2LiOH(s) + CO2(g) ------> Li2CO3(s) + H2O(l)
How many grams of LiOH must be used to produce 10.0g of Li2CO3?
Learning Check
Acetylene gas C2H2 burns in the oxyacetylene torch for welding. How many grams of C2H2 are burned if the reaction produces 75.0 g CO2?
2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(g)
A) 88.6 g C2H2
B) 44.3 g C2H2
C) 22.2 g C2H2
Limiting Reactant
A limiting reactant in a chemical reaction is thesubstance that: • Is used up first• Stops the reaction• Limits the amount of product that can form
Reacting Amounts
In a table setting, there is 1plate, 1 fork, 1 knife, and 1 spoon.
How many table settings arepossible from 5 plates, 6 forks, 4spoons, and 7 knives?
What is the limiting item?
Reaction Equivalent
1plate + 1fork + 1knife + 1spoon ----> 1place setting
BEFORE 5 6 7 4 0
AFTER 1 2 3 0 4 LR
Reacting Amounts
In a table setting, there is 1plate, 2 forks, 1 knife, and
1spoon.
How many table settings arepossible from 5 plates, 6 forks, 4spoons, and 7 knives?
What is the limiting item?
Reaction Equivalent
1plate + 2fork + 1knife + 1spoon ----> 1place setting
BEFORE 5 6 7 4 0
AFTER 2 0 4 1 3 LR
Limiting Reactants
When 4.00 mol H2 is mixed with 2.00 mol Cl2,how many moles of HCl can form?
H2(g) + Cl2(g) 2HCl (g)
4.00 mol 2.00 mol ??? Mol
STEPS:
• Calculate the moles of product from each reactant, H2 and Cl2.
• The limiting reactant is the one that produces the smallest amount of product.
Checking Calculations
Initially H2
4.00 mol
Cl2
2.00 mol
2HCl
0 mol
Reacted/Formed
-2.00 mol -2.00 mol +4.00 mol
Left after reaction
2.00 mol Excess
0 mol Limiting
4.00 mol
Limiting Reactant
If 4.80 grams Ca mixed with 2.00 grams N2, which is the limiting reactant?
3Ca(s) + N2(g) Ca3N2(s)