chapter 3 - linn–benton community collegecf.linnbenton.edu/mathsci/physci/backusb/upload/ch 201...
TRANSCRIPT
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)
+ O2(g)2Mg(s)BALANCED
EQUATION
MACROSCOPIC
VIEW
Example: What chemical reaction occurs when a flashbulb is
used to take a photograph?
electricity
electricityMg Mg
2MgO(s)
O2
+ O2(g)2Mg(s)
MACROSCOPIC
VIEW
ATOMIC-SCALE
VIEW
BALANCED
EQUATION
electricity
electricityMg Mg
2MgO(s)
O2
O2–
O2–
Mg2+
Mg2+
+ O2(g)2Mg(s)
MACROSCOPIC
VIEW
ATOMIC-SCALE
VIEW
BALANCED
EQUATION
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.
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)
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.
Information Contained in the Chemical Formula of Glucose
C6H12O6 ( M = 180.16 g/mol)
Oxygen (O)
Mass of atoms
per mole of
compound
6 atoms
96.00 g
Carbon (C) Hydrogen (H)
Atoms per molecule
of compound
Moles of atoms per
mole of compound
Atoms per mole of
compound
Mass of atoms
per 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?
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?
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.
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.
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
Some Compounds with Empirical Formula CH2O
(Composition by Mass: 40.0% C, 6.71% H, 53.3% O)
Name
Molecular
FormulaWhole-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
preservativeacetate polymers; vinegar(5%
soln)
part of sugar metabolism
sour milk; forms in exercising
muscle
component of nucleic acids and
B2major 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
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?
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.
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.
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).
HCl HNO3 HI etc.
Water dissociates soluble ionic compounds completely into ions (strong electrolytes).
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?
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
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.
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. Dissociate strong acids and bases. (Solubility Rules) Do not dissociate weak acids, weak bases, solids, or gases. (Electrolyte Rules)
Determine the spectator ions.
Write the balanced net ionic equation (w/o spectators).
Problem Solving
Aqueous barium chloride reacts with aqueous sodium phosphate to produce aqueous sodium chloride and barium phosphate. Write the net ionic equation for this reaction.
Another Example: An acid-base reaction that forms a gaseous
product.
Sodium carbonate reacts with acetic acid