Download Stoichiometry

The Arithmetic of
Chemistry
Stoichiometry and chemical
reactions
A process in which one or more substances is changed into one
or more new substances is a chemical reaction
A chemical equation uses chemical symbols to show what
happens during a chemical reaction
3 ways of representing the reaction of H2 with O2 to form H2O
reactants
products
3.7
Remember this?
 Law of mass conservation
 Law of definite proportions
Balancing equations
 Number of atoms conserved: atoms in
reactants equal to atoms in products
 Change coefficients, not subscripts
 Balance atoms that occur in only one
reactant and one product
 Balance the rest
 Check number of atoms on each side
Test yourself
 Test yourself: ethane gas reacts with
oxygen gas to produce carbon dioxide and
water
 Heating potassium chlorate yields potassium
chloride and oxygen gas
 Demo reaction: Pb(NO3)2 and KI
 Atoms: small scale
 Macroscopic scale: the mole
Atomic mass is the mass of an atom in
atomic mass units (amu)
By definition:
1 atom 12C “weighs” 12 amu
On this scale
1H
= 1.008 amu
16O
= 16.00 amu
Average atomic masses:
weighted averages
Natural lithium is:
7.42% 6Li (6.015 amu)
92.58% 7Li (7.016 amu)
The mole (mol) is the amount of a substance that
contains as many entities as there
are atoms in exactly 12.00 grams of 12C
3.2
How Big is Avogadro’s
Number?
 an Avogadro’s number of softdrink cans
would cover the surface of the earth to a
depth of over 200 miles
 if we were able to count atoms at the rate of
10 million per second, it would take about 2
billion years to count an Avogadro’s number
of atoms
eggs
Molar mass is the mass of 1 mole of shoes in grams
marbles
atoms
molecules
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 12C atom = 12.00 amu
1 mole 12C atoms = 12.00 g 12C
1 mole lithium atoms = 6.941 g of Li
For any element
atomic mass (amu) = molar mass (grams)
3.2
One Mole of:
S
C
Hg
Cu
Fe
3.2
Converting small scale to
large (and back)
 Dimensional analysis: method of calculation
utilizing a knowledge of units.
 Conversion factors are used to manipulate
units:
Conversion factor 
desired unit
given unit
Test yourself
 How many atoms are in 0.551 g of K?
 How many molecules of water are in a
15g sample? How many individual H
atoms? O atoms?
Back to chemical
equations
 2H2 + O2  2H2O can be read as:
 2 molecules H2 of react with 1 molecule of
O2 to produce 2 molecules of water, OR
 2 moles of H2 react with 1 mole O2 to form 1
mole of water
 Convert to grams.
Limiting reagent
6 red
green
leftused
overup
3.9
Do You Understand Limiting Reagents?
In one process, 124 g of Al are reacted with 601 g of Fe2O3
2Al + Fe2O3
Al2O3 + 2Fe
Calculate the mass of Al2O3 formed.
g Al
mol Al
g Fe2O3
124 g Al x
mol Fe2O3 needed
OR
mol Al needed
mol Fe2O3
1 mol Al
27.0 g Al
x
g Fe2O3 needed
1 mol Fe2O3
2 mol Al
Start with 124 g Al
160. g Fe2O3
=
x
1 mol Fe2O3
g Al needed
367 g Fe2O3
need 367 g Fe2O3
Have more Fe2O3 (601 g) so Al is limiting reagent
3.9
General types of simple
chemical reactions
1. Combination : A + Z  AZ
2. Decomposition: AZ  A + Z
3. Single replacement (substitution)
A + BZ  AZ +B
4. Double replacement/displacement
5. Neutralization (acids and bases)
6. Combustion: reaction with O2, energy
produced
1. Combination
Reactions
 Two or more substances (elements or
compounds) react to produce one
compound
 Also known as “synthesis” or “addition”

A
+
Z
 AZ





Metal
Nonmetal
Metal
Water
Metal oxide
oxygen
metal oxide
oxygen
nonmetal oxide
nonmetal
salt
metal oxide base
nonmetal oxide
salt
2. Decomposition
 ONE substance reacts to form two or more
substances (elements or compounds).

AZ

A
+
Z




Compound
Metal carbonate
Hydrated salt
Compound
compound/element
metal oxide
salt
compound/element
oxygen
carbon dioxide
water
water
3. Single Replacement
 Also known as single displacement or
substitution.
 A metal (A) replaces a metal ion (B=metal) in its
salt or a hydrogen ion (B=H) in an acid
 A + BZ  AZ +B
 Ex. Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s)
 Ex. Sn(s) + HCl(aq)  SnCl2(aq) + H2(g)
 A nonmetal (X) replacing another nonmetal (Z)
in its salt (B=metal) or acid (B=H)
 Cl2(g) + NaBr (aq)  _____________________________
 Bromine + Potassium Iodide ______________________
4. Double Replacement




Also known as “Double displacement”,
“Metathesis”, or “Double decomposition.”
Two compounds are involved with the cation
of one compound EXCHANGING with the
cation of another compound.
AX + BZ  AZ + BX
These reactions proceed if one of the ff. is
satisfied:
1. An insoluble/slightly soluble product is formed
(PRECIPITATE formation)
2. A weakly ionized species is produced. The most
common species of this type is water.
3. A gas is produced as a product.
5. Neutralization
 Reaction of an acid and a base that
usually produces a salt and water.
 HX + MOH  MX + HOH
 Hydrochloric acid and sodium hydroxide.
 Hydrochloric acid and magnesium hydroxide.
 Sulfuric acid and Barium hydroxide
 To be discussed in a separate lesson
6. Combustion
 Reactions involving oxygen (metal +oxygen),
(nonmetal + oxygen), (organic compounds +
oxygen)
are sometimes called
combustion reactions.
 Energy is given off in combustion reactions.
 For organic compounds, water and carbon
dioxide are usual byproducts.
 Ex. Metabolism of food, fuel combustion:
 Sucrose (C22H12O11) combusts
 Combustion of propane
 Combustion of hydrogen
Quiz
 Insoluble silver chloride is produced in
the reaction between silver nitrate and
hydrochloric acid.
 What type of reaction is this?
 Balance the equation.
Quiz
 Sodium azide produces nitrogen gas and
sodium metal.
 Type of reaction?
 Balanced equation?
 How many grams of nitrogen gas is
produced from 60.0g sodium azide?
Quiz
 Working at 273.15K and 1atm (STP), I
have 10.0g of carbon and 56L of oxygen.
Under these conditions, I know 1mol of
any gas has a volume of 22.4L (Molar
volume at STP). What is the limiting
reagent? If carbon monoxide made sure
to be formed, what will be its volume?