INVESTIGATING CHEMISTRY A FORENSIC SCIENCE PERSPECTIVE

CHAPTER 4:

CHEMICAL EVIDENCEGeorge Trepal, angry at neighbors, when their dog irritated him.George Trepal, angry at neighbors, when their dog irritated him.When one of the family was poisoned with thallium, detectives When one of the family was poisoned with thallium, detectives asked neighbors why.asked neighbors why.Trepal speculated it was to get them to move, consistent with a Trepal speculated it was to get them to move, consistent with a note they received.note they received.But the thallium evidence was mishandled.But the thallium evidence was mishandled.Why would water soluble thallium nitrate be more dangerous Why would water soluble thallium nitrate be more dangerous than a chunk of the metal?than a chunk of the metal?

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

The periodic table of the elements is a very powerful organizing principle in chemistry, allowing us to predict trends

in properties.

4.1 CHEMICAL NATURE OF EVIDENCE: REGIONS OF THE PERIODIC TABLE

• 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.

Notice that the Notice that the group numbergroup number tells us the number of tells us the number of valence valence electronselectrons. .

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.

When two nonmetals react to form a compound, it is molecular and has covalent bonds, electrons are shared

• We We use prefixesuse prefixes such as mono-, di-, etc. when naming such as mono-, di-, etc. when naming • Mono- Hexa-• Di- Hepta-• Tri- Octa-• Tetra- Nona-• Penta- Deca-• Examples:Examples:

– COCO22 is carbon is carbon didioxideoxide - CO is carbon - CO is carbon monmonoxideoxide– NN22OO44 is dinitrogen tetr(a)oxide (The “a” is optional, often omitted. 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

NAMES AND FORMULAS OF COVALENT COMPOUNDS

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 chartNaming flow chart

Ionic Compounds and NamingWhen 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.

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.

Unnumbered Figure, pg. 105

Investigating Chemistry, 2nd Edition

© 2009 W.H. Freeman & Company

Iron(II) Sulfate (green) and Copper(II) Sulfate Pentahydrate

Figure 4.5, pg. 112

Investigating Chemistry, 2nd Edition

© 2009 W.H. Freeman & Company

Seven Diatomic Elements. All are gases except Br2(l) and I2(s)

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

• 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

STOICHIOMETRYSTOICHIOMETRY

• 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.

• 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.

STOICHIOMETRYSTOICHIOMETRY

• 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.

STOICHIOMETRY CALCULATIONS

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.