Transcript WU_Chem101_F10_Ch4 - Chemistry at Winthrop University

INVESTIGATING CHEMISTRY
A FORENSIC SCIENCE PERSPECTIVE
CHAPTER 4:
CHEMICAL EVIDENCE
George Trepal, angry at neighbors, when their dog irritated him.
When one of the family was poisoned with thallium, detectives
asked neighbors why.
Trepal speculated it was to get them to move, consistent with a
note they received.
But the thallium evidence was mishandled.
Why would water soluble thallium nitrate be more dangerous
than a chunk of the metal?
CHEMICAL EVIDENCE
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4.1 Chemical Nature of Evidence: Regions of the Periodic Table
4.2 Types of Compounds: Covalent Compounds
4.3 Types of Compounds: Ionic Compounds
4.4 Common Names & Diatomic Elements
4.5 Basics of Chemical Reactions
4.6 Balancing Chemical Equations
4.7 Mathematics of Chemical Reactions: Mole Calculations
4.8 Mathematics of Chemical Reactions: Stoichiometry Calculations
The periodic table of the elements is a very powerful
organizing principle in chemistry, allowing us to predict trends
in properties.
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4.1 CHEMICAL NATURE OF EVIDENCE: Notice that the group number
tells us the number of valence
REGIONS OF THE PERIODIC TABLE
electrons.
There are seven rows or “periods” in the
periodic table of the elements.
There are 32 columns, called “groups” or
“families”, 18 obvious ones plus the
lanthanides and actinides.
Group 1 is the Alkali Metals.
Groups 2 is the Alkaline Earth Metals.
Groups 3-12 are the Transition Metals.
Groups 13-16 are referred to by the first
element or simply the group number.
Group 17 is the Halogens.
Group 18 is the Noble Gases.
Elements 58-71 and 90-103 are called the
Inner Transition Metals.
4.2 TYPES OF COMPOUNDS: COVALENT COMPOUNDS
• In a covalent bond, electrons are shared, not transferred
as in ionic bonds.
• When two nonmetals react to form a compound, it is
molecular and has covalent bonds.
• When two elements from the upper right corner of the
periodic table combine, we use a different system for
naming these covalent compounds.
• This results in discrete molecules with directional bonds.
For example, H2O.
• It can also result in an infinite network of covalently
bonded atoms as in diamond, C, or sand, SiO2, which is
not a discrete molecule despite its simple formula.
NAMES AND FORMULAS OF COVALENT COMPOUNDS
When two nonmetals react to form a compound, it is molecular
and has covalent bonds, electrons are shared
• We use prefixes such as mono-, di-, etc. when naming
• MonoHexa• DiHepta• TriOcta• TetraNona• PentaDeca• Examples:
– CO2 is carbon dioxide - CO is carbon monoxide
– N2O4 is dinitrogen tetr(a)oxide (The “a” is optional, often
omitted.
• They do not conduct electricity since they have no ions to
carry the current.
• Diatomic elements: H2, O2, N2, F2, Cl2, Br2 and I2
4.3 TYPES OF COMPOUDS: IONIC
COMPOUNDS
• Ionic compounds are salts.
• Ionic bonds occur when one element, a metal, transfers one or
more of its electrons to another element, a nonmetal.
• An ionic bond involves the attraction between a positively
charged ion (cation) and a negatively charged ion (anion).
• If there are six cations around each anion in the crystal lattice,
then each attraction is 1/6th of the ionic bond.
• The crystal lattice is the 3D, repeating pattern exhibited by
solids.
• 4Na + O2  2Na2O
– Metals form cations, Na1+, above.
– Nonmetals form anions, O2-, above.
Metals lose electrons to form cations(+).
Nonmetals gain electrons to form anions(-).
Why? To gain a noble gas electron configuration (very stable).
Naming flow chart
Ionic Compounds and Naming
When a metal and a nonmetal or polyatomic ion form a
compound, it is a salt .
– Transfer of electrons
– Ionic cation (+) & anion (-)
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Metal & nonmetal- sodium and chlorine
(no change) & -ide
Sodium chloride
Transition metals (roman numeral)
Balance the compound to net 0 charge
• The crystal lattice is the 3D, repeating pattern
exhibited by solids; If there are six cations around
each anion, each attraction is 1/6th of the ionic bond.
POLYATOMIC IONS
• Ions are charged species.
• Simple positive ions are named as the original atoms.
• Simple negative ions take the ending -ide. Chlorine
becomes chloride.
• Polyatomic ions are groups of atoms tightly bound
together acting as a unit. Many take the –ate ending.
PROPERTIES OF IONIC COMPOUNDS
• Ionic compounds, salts, are solids and most have very
high melting points. Salt, NaCl, melts at 803oC.
Many are water soluble, but not soluble in hexane, a
nonpolar solvent. Salts conduct electricity in the
molten state, but not as solids. Why not? If soluble,
their water solutions also conduct. Why?
• Because ions are free to move toward the electrodes
once the crystal lattice is destroyed in the molten state
or solutions.
Among transition metals it is common to see two cations for
the same metal.
• Iron(II) or Ferrous ion, Fe2+ and Iron(III) or Ferric
ion, Fe3+.
• Chromium(II) or Chromous ion, Cr2+
• Chromium(III) or Chromic ion, Cr3+
• Copper(I) or Cuprous ion, Cu1+ and Copper(II) or
Cupric ion, Cu2+.
• Note: –ous for Low and –ic for High.
Iron(II) Sulfate (green) and Copper(II) Sulfate Pentahydrate
Unnumbered Figure, pg. 105
Investigating Chemistry, 2nd Edition
© 2009 W.H. Freeman & Company
Seven Diatomic Elements.
All are gases except Br2(l) and I2(s)
Figure 4.5, pg. 112
Investigating Chemistry, 2nd Edition
© 2009 W.H. Freeman & Company
Compounds
• Covalent – mono, di, tri, …; end with ide
• Ionic –
– main group (1-8A) – set charges, net 0 charge, anion ends with ide
– Polyatomic (see chart, no name change), net 0 charge on compound
– Naming with transition metals (B groups, many charges, use Roman
numerals)
• Converting mass (g) to moles
Practice
• 4Na + O2  2Na2O
• tetraphosphorus decaoxide 
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Na2CO3
Lead (IV) oxide
calcium chloride
PF5
4.7 THE MATH OF CHEMICAL REACTIONS:
MOLE CALCULATIONS
• Like a dozen eggs, a gross of pencils, or a ream of
paper, the mole is a definite amount of a chemical
substance.
• One definition is this:
• One mole = 6.022 x 1023 particles (atoms, ions,
molecules, formula units)
• Another definition is: one mole = sum of the atomic
masses for all the atoms in the formula expressed in
grams. For H2O, 1 mol = 18.0 g.
What is the mass of one mole of I2? Of
Sodium carbonate?
One mole = the formula weight in grams. Add the
atomic weights of all the atoms in the formula.
4.8 THE MATH OF CHEMICAL REACTIONS:
STOICHIOMETRY CALCUATIONS
• Stoichiometry uses balanced chemical equations to
quantify the masses of reactants and products for a
given reaction.
• Consider the reaction involved in a natural gas
explosion: CH4 + O2  CO2 + H2O
• Notice that there is more H on the left than on the
right side. How could we fix that?
• Now note that we have too many O atoms. Can you
fix that?
• Does that balance the equation?
STOICHIOMETRY CALCULATIONS
• The mole concept is at the heart of our calculations in
chemistry.
• Use these steps:
• 1. Convert grams to moles using the equation, 1 mole
= the molecular weight in grams. (formula weight for
salts)
• 2. Use the balanced equation to convert between moles
of one reactant or product and another.
• 3. Convert moles to grams for the desired compound
using its molecular or formula weight.
• grams A  moles A  moles B  grams B
STOICHIOMETRY
• CH4 + 2O2  CO2 + 2H2O
• 1 mol 2 mol  1 mol 2 mol
• One mole of CH4 has a mass of 12.0 + 4(1.01) = 16.0
grams.
• One mole of O2 has a mass of 2(16.0) or 32.0 grams.
Two moles = 64.0 g.
• One mole of CO2 has a mass of 12.0 + 2(16.0) = 44.0
g.
• And one mole of water is 2(1.01) + 16.0 = 18.0 g. So
2 mol = 36.0 grams.
STOICHIOMETRY
CH4 + 2O2  CO2 + 2H2O
1 mol 2 mol  1 mol 2 mol
16.0 g + 64.0 g = 44.0 g + 36.0 g
80.0 grams of reactants must yield 80.0 grams of
products. It’s the law!
• The law of the conservation of matter applies.
• There is no law of conservation of moles, but it may
appear so in this case.
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STOICHIOMETRY CALCULATIONS
• What mass of water could be formed by burning 28.2
grams of methane?
• CH4 + 2O2  CO2 + 2H2O
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28.2 g CH4 x (1 mol CH4/16.0 g CH4) x
(2 mol H2O /1 mole CH4) x
(18.0 g H2O/1mol H2O) =
63.45, or rather 63.5 g water (A coin toss. If heads round
5 up, but if tails, round 5 down.)
• This mass is called the theoretical yield. It is the largest
mass of product that can be obtained from the given
masses of reactants.
A SAMPLE PROBLEM: NOMENCLATURE
• Can you name CaCl2 and PF5 ? They differ.
• Note that one is a salt and one is not.
• We don’t use prefixes for naming salts (made from a
metal plus a nonmetal).
• We do use them for molecules.
• CaCl2 is just calcium chloride.
• But PF5 is phosphorus pentafluoride.
PROPERTIES OF COVALENT COMPOUNDS
• Covalent compounds may be solids, liquids, or gases.
So the m. p.’s are widely variable, but much lower than
those of salts. This is because forces between
molecules are much weaker than forces within them.
• They do not conduct electricity since they have no ions
to carry the current.
• Paraffin wax contains organic molecules and is a
nonconductor in the solid state, (s), in the melted state,
(l), and in solutions, (aq). [Insoluble in H2O]
PROPERTIES OF IONIC COMPOUNDS
• Ionic compounds, salts, are solids and most have very
high melting points. Salt, NaCl, melts at 803oC.
Many are water soluble, but not soluble in hexane, a
nonpolar solvent. Salts conduct electricity in the
molten state, but not as solids. Why not? If soluble,
their water solutions also conduct. Why?
• Because ions are free to move toward the electrodes
once the crystal lattice is destroyed in the molten state
or solutions.