Investigating Chemistry

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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?
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CHEMICAL EVIDENCE

• 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

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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 REGIONS OF THE
PERIODIC TABLE
Notice that the group number tells us the number
of valence 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.

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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.

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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
• Mono- Hexa-
• Di- Hepta-
• Tri- Octa-
• Tetra- Nona-
• Penta- Deca-
• 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

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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.

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Metals lose electrons to form cations().
Nonmetals gain electrons to form anions(-).
Why? To gain a noble gas electron configuration
(very stable).
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Naming flow chart
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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 (-)
• 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.

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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.

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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.

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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.

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Iron(II) Sulfate (green) and Copper(II) Sulfate
Pentahydrate
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Seven Diatomic Elements. All are gases except
Br2(l) and I2(s)
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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

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Practice

• 4Na O2 ? 2Na2O
• tetraphosphorus decaoxide ?
• Na2CO3
• Lead (IV) oxide
• calcium chloride
• PF5

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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.

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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.
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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?

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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

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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.

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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. Its 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
• 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.

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A SAMPLE PROBLEM NOMENCLATURE

• Can you name CaCl2 and PF5 ? They differ.
• Note that one is a salt and one is not.
• We dont 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.

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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

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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.