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Molecules, Moles, and Chemical Equations
Chapter 3
Law of Conservation of Matter
Matter is neither created nor destroyed.
In chemistry the number of atoms going into a
chemical reaction must be the same as the
number of atoms coming our of the reaction.
Reactions must be balanced.
Chemical Equation or Reaction: Statement in
formulas depicting a chemical change.
Reactants � Products
(initial) (final)
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electricity
electricityMg Mg
2MgO(s)
O2
O2–
O2–
Mg2+
Mg2+
+ O2(g)2Mg(s)
MACROSCOPICVIEW
ATOMIC-SCALEVIEW
BALANCEDEQUATION
translate the statement
balance the atoms
specify states of matter
adjust the coefficients
check the atom balance
Write a balanced chemical equation for the following: If aqueous silver nitrate is mixed with
aqueous sodium phosphate a chemical reaction occurs and solid silver phosphate and aqueous sodium nitrate are formed.
Take out and complete Exercise 5 pg 26 of Lab
Manual.
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Interpreting Equations and the Mole
Common Terms that Represent Specific
Quantities
1 pair = 2
1 dozen = 12
1 baker’s dozen = 13
1 case = 24
1 gross = 144
1 mole = 6.022 X 1023
mole(mol) - the amount of a substance that contains the same number of entities as there are atoms in exactly 12 g of carbon-12.
This amount is 6.022x1023. The number is called Avogadro’s number and is abbreviated as N.
One mole (1 mol) contains 6.022x1023 entities
(to four significant figures)
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Water
18.02 gCaCO3
100.09 g
Oxygen
32.00 g
Copper
63.55 g
One mole of
common substances.
Summary of Mass Terminology
Term Definition Unit
Isotopic mass Mass of an isotope of an element amu
Atomic mass
Molecular
(or formula) mass
(also called molecular weight)
Molar mass (M)
(also called
atomic weight)
(also called gram-
molecular weight)
amu
amu
g/mol
Average of the masses of the naturally occurring isotopes of an element weighted
according to their abundance
Sum of the atomic masses of the atoms (or ions) in a molecule (or formula unit)
Mass of 1 mole of chemical entities (atoms, ions, molecules, formula units)
Let’s Consider 1.00 mole of Si
1 Si atom = 28.09 amu
1 mole Si atoms = 28.09 g
Here’s the Proof!
1.00 mol Si X 6.022X1023 Si X 28.09amu X 1.66054X10-24 g = 28.09 g Si
1 mol Si 1 Si atom 1 amu
Now you have two unit factors depending upon whether you are talking about individual atoms or a group of atoms.
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Information Contained in the Chemical Formula of Glucose C6H12O6 ( M = 180.16 g/mol)
Oxygen (O)
Mass of atomsper mole of
compound
6 atoms
96.00 g
Carbon (C) Hydrogen (H)
Atoms per moleculeof compound
Moles of atoms permole of compound
Atoms per mole ofcompound
Mass of atomsper molecule
of compound
6 atoms 12 atoms
6 moles
of atoms
12 moles
of atoms
6 moles
of atoms
6(6.022 x 1023)
atoms
12(6.022 x 1023)
atoms
6(6.022 x 1023)
atoms
6(12.01 amu)
=72.06 amu
12(1.008 amu)
=12.10 amu
6(16.00 amu)
=96.00 amu
72.06 g 12.10 g
Molar Mass of Compound: 72.06g + 12.10g + 96.00g = 180.16 g/mol
Interconverting Moles, Mass, and Number of Chemical Entities
Mass (g) = no. of moles x no. of grams
1 mol
No. of moles = mass (g) x
no. of grams
1 mol
No. of entities = no. of moles x6.022x1023 entities
1 mol
No. of moles = no. of entities x 6.022x1023 entities
1 mol
g
n
Another Example:
What is the formula unit mass of K2S?
What is the molar mass of K2S?
What is the mass of 0.250 moles of K2S?
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Conversion Problem Solving
1. How many atoms of lead are present in 15 g of lead?
2. What is the mass of 1.36X 1021 atoms of silver?
3. How many moles of Ba(NO3)2 are in 20.0 grams of Ba(NO3)2?
4. How many atoms of oxygen are present in 20.0 grams of Ba(NO3)2?
5. How many moles of nitrate ions are present in 20.0 grams of Ba(NO3)2?
EXERCISE 4
Work in Groups and complete this exercise.
Empirical and Molecular Formulas
Empirical Formula -
Molecular Formula -
The simplest formula for a compound that agrees with the elemental analysis and gives
rise to the smallest set of whole numbers of atoms.
The formula of the compound as it exists, it may be a multiple of the empirical formula.
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Example
Acetylene C2H2
Benzene C6H6
What is the empirical formula for each of the above compounds?
Note: Two completely different compounds can have the same empirical formula.
Determination of Empirical and Molecular
Formulas
Example: A compound of nitrogen and oxygen is
analyzed and a sample weighing 1.587 grams
is found to contain 0.483 grams of nitrogen
and 1.104 grams of oxygen. Determine the
empirical formula of this compound.
Example: A compound contains 68.8% C, 5.0%
H, and 26.2 % O by mass. The molecular mass
of the compound is determined to be 122.13
amu. Determine the empirical and molecular
formulas of the compound.
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Example: A vitamin is analyzed and found to
contain 40.9%C, 4.58%H, and 54.5% O. The
molar mass of the compound is 176.13 g/mol.
Determine the empirical and molecular
formula.
Combustion Analysis: a procedure used to determine the chemical composition of organic
compounds.
General Equation for Combustion Analysis Reactions
Compound + n O2(g) ���� n CO2(g) + n H2O(g) + other elements
Example: A compound is combusted and the products are massed. Using the information provided below, determine the empirical and molecular formulas of the compound.
Products: 6.21 mg CO2 2.54 mg H2O
Compound Mass: 4.24 mg
Molar Mass of Compound: 60.6 g/mol
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Some Compounds with Empirical Formula CH2O
(Composition by Mass: 40.0% C, 6.71% H, 53.3% O)
Name
Molecular Formula
Whole-Number Multiple (g/mol) Use or Function
formaldehyde
acetic acid
lactic acid
erythrose
ribose
glucose
CH2O
C2H4O2
C3H6O3
C4H8O4
C5H10O5
C6H12O6
1
2
3
4
5
6
30.03
60.05
90.09
120.10
150.13
180.16
disinfectant; biological preservative
acetate polymers; vinegar(5% soln)
part of sugar metabolism
sour milk; forms in exercising muscle
component of nucleic acids and B2
major energy source of the cell
CH2O C2H4O2 C3H6O3 C4H8O4 C5H10O5 C6H12O6
Solutions
Solute + Solvent = Solution
x
x x
x x
x x
x
x
x
x x
x
x
More Dilute More Concentrated
x = solute particles
Molarity
Molarity is a form of representing concentration
of a solution.
Molarity (M) = moles of Solute
liters of Solution
Units of Molarity: moles/L
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Calculating Molarity
Example: A sample of sodium nitrate weighing
0.38 grams is dissolved in 50.0 mL of
solution. Determine the molarity of the
solution.
Laboratory preparation of molar solutions.
A•Weigh the solid needed.
•Transfer the solid to a
volumetric flask that contains
about half the final volume of
solvent.B Dissolve the solid
thoroughly by swirling.
C Add solvent until the solution
reaches its final volume.
Problem Solving with Molarity
1. A student must prepare 250.0 mL of a 0.250
M sodium hydroxide solution. What mass of
solute is required?
2. How many mL of 0.163 M sodium chloride
solution are required to contain 0.0958
grams of solute?
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Converting a concentrated solution to a dilute solution.
M1V1 = M2V2
Or
MdVd = McVc
Dilution Problem Solving
Explain how you would prepare 100.0 mL of a
0.100 M ammonia solution from 14.8 M
ammonia.
The Role of Water as a Solvent
The Universal Solvent: H2O
Let’s consider the
(1) shape of the water molecule.
(2) bond angle.
(3) unequal sharing of electrons.
(4) partial charges.
(5) dipole moment.
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Electron distribution in molecules of H2 and H2O.
The dissolution of an ionic compound
in water.
• The separation of an ionic compound caused
by water molecules results in a solution
containing solvated ions.
–
+
––
––
+
++
+–––
–
–+
+
+
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The electrical conductivity of ionic solutions.
Solvation of Covalent Compounds by
H2O
Water does not dissociate some covalent compounds (nonelectrolytes).
Examples: sucrose, ethanol, acetone, etc.
Water can slightly dissociate some covalent compounds (weak electrolytes).
Examples: HC2H3O2 HC7H5O2 etc.
Note: Why is there one H in the front of the chemical formula?
Water can completely dissociate some covalent
compounds (strong electrolytes) such as
strong acids.
HCl HBr HI HNO3
H2SO4 HClO3 HClO4
Water dissociates soluble ionic compounds
completely into ions (strong electrolytes).
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Useful Tables
Page 17 of lab manual: Summary of Electrolyte
Rules
Page 16 of lab manual: Solubility Rules
Do not memorize. However, be prepared to use
similar tables on the exam.
Problem Solving
When one mole of the following compounds
dissociates in H2O, how many moles of each ion are
present?
Aluminum chloride
Copper (II) nitrate
Sodium phosphate
Problem Solving
If 150.00 mL of 0.200 M iron (III) sulfate are
present in a beaker, how many moles of each
ion are present?
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Acid-Base Reactions (Neutralization)
Acids
Release H+
Turn blue litmus paper pink
pH < 7
Electron pair acceptors
React with some metals to produce H2(g)
Taste Sour
Bases
Release OH-
Turn pink litmus paper blue
pH > 7
Electron pair donors
React with many metal ions to produce
precipitates (ppt’s)
Taste Bitter
Dissociation of Strong Acids in H2O
HCl(g) + H2O(l) � H3O+ (aq) + Cl-(aq)
(hydronium ion)
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The hydrated proton.
Dissociation of Weak Acids in H2O
HF(g) + H2O(l) � H3O+(aq) + F- (aq)
F- = Anion of a weak acid
GENERAL Neutralization Reaction
Acid + Base � H2O(l) + Salt
HCl (aq) + KOH (aq) � H2O(l) + KCl (aq)
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An aqueous strong acid-strong base reaction on the atomic scale.
Determining the Molarity of H+ Ions in Aqueous Solutions of Acids
Nitric acid is a major chemical in the fertilizer and explosives industries. Nitric acid is a strong
acid/strong electrolyte. What is the molarity of H+(aq) in 0.900M nitric acid?
Writing Equations for Aqueous Ionic Reactions
The molecular equation: shows all of the reactants and
products as intact, undissociated compounds.
The total ionic equation: shows all of the soluble ionic substances
dissociated into ions.
The net ionic equation: eliminates the spectator ions and shows the
actual chemical change taking place.
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How to Begin Writing Net Ionic Equations
Write the molecular equation and balance.
Determine the solubility of each compound.
Write the total ionic equation. Dissociate ionic compounds that are soluble (Solubility Rules) . Dissociate strong acids and bases. Do not dissociate weak acids, weak bases, (Electrolyte Rules) solids, or gases.
Determine the spectator ions.
Write the balanced net ionic equation (w/o spectators).
A precipitation reaction and steps to writing its net ionic equation.
Problem Solving
Aqueous barium chloride reacts with aqueous
sodium phosphate to produce aqueous
sodium chloride and solid barium phosphate.
Write the net ionic equation for this reaction.
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Another Example: The reaction of Pb(NO3)2 and NaI.
double displacement reaction (metathesis)
Write a net ionic reaction
Another Example: An acid-base reaction that forms a gaseous
product.
Sodium carbonate reacts with acetic acid
Write a net ionic equation for this reaction
Practice Writing Net Ionic Equations
Exercise 7 of Lab Manual, page 30.