Lecture notes: CHEM103 November 20, 2008
A mass of 0.15 g NaCl is dissolved in 100 mL of water: what is the molarity of the product(s)?
If you have 100 mL of 0.15 M CaCl2 : how many moles of chloride (Cl-) are in the solution?
One last detail regarding solution calculations…
Dilution: Mi Vi = Mf Vf
You have 2.0 L of 0.85 M commercial bleach available. You need a 0.20 M solution of sodium hypochlorite (NaOCl).
How much can be made from what you have (and extra disilled water)?
You need 100.0 mL of 5.0 M HCl for an experiment. How much 16 M HCl do you need to dilute?
If you inadvertently spill 5 mL of 0.250 M hydrochloric acid into your full coffee cup (420 mL),
what would the final concentration of H+ be?
FINALLY, COVALENT (MOLECULAR) COMPOUNDS!
FIRST, THE DETAILS OF BONDING…
I. KEY IDEA: All chemical reactions driven by electron interactions - specifically VALENCE electrons!
II. KEY IDEA: Correlation of microscopic structures to macroscopic properties!
Covalent vs. Ionic compounds:
Ionic compounds lose (or gain) electrons from involved atoms (combination of metal & non-metal) w/in valence shell to attain a "noble gas configuration". Then they combine in predictable, consistent ratios to form a lattice.
NOTES: ionic compounds are held together by the attraction between adjacent ions – “lattice energy”
because a lattice is a endlessly repeating structure, the concept of a “molecule” of an ionic compound isn’t really appropriate
WHAT property determined the properties of ionic compounds (e.g. mp, hardness, solubility)?
Covalent compounds (composed entirely of non-metals) share, not exchange electrons; but the outcome is the same: filled valence shell.
(Note: these are NOT always shared equally.)
NOTES: we also call these “molecular compounds” because bonding occurs between a specific number of atoms
how many electrons fit in the valence shell of a non-metal? 8 (or sometimes 2)
This determines how electrons are shared between atoms in a molecular compound.
Similarly, the structure or "shape" of covalent molecules is crucial to their properties too.
Structures of covalent molecules are often more complex than the regular lattice patterns of an ionic compound.
Unlike ionic compounds, MANY, MANY many different molecules can be made by combining the same non-metals.
B & H result in BH2, BH3, B2H5, etc.
(contrast this with Ca and F, for example)
This makes the rules for naming covalent compounds a bit different than for ionic compounds…
0) some compounds just make no sense (ammonia, water, nitric oxide, methane, hydrazine)
plus, anything in its elemental form (N2, O2, H2, etc.) keeps the name of the element
1) suffix –ide replaces the latter element’s usual suffix (same as ionic compounds)
2) prefix indicating number is added to each atom
note: prefix “mono” in the leading element is eliminated: nitrogen dioxide, sulfur monoxide, dinitrogen oxide
mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, etc…
* exeptions: some multiple vowels removed: dinitrogen pentoxide
3) covalent compounds in same order as written in abbreviation
lower group # before higher group # (like ionic compounds: metal before non-metal)
examples: carbon dioxide, boron trifluoride
* exception: halogens before oxygen: chlorine dioxide
when in the SAME group, higher period number before lower period number
examples: sulfur dioxide, silicon carbide
CREATING MODELS OF COVALENT COMPOUNDS
As with everything so far, we generate models with predictive power. We will see 2 or 3 different models of varying complexity and sophistication used to describe covalently bonded molecules.
First model of covalent bonding: LEWIS DOT STRUCTURES or THE OCTET RULE
What can this do for us?
1. Generate molecular formulas (how do different elements combine to form compounds?)
2. First step toward understanding molecular structure of non-ionic (covalent) molecules.
3. BUT – this is a deeply flawed model (overly simple)
4. (Don’t worry - we’ll get on to more detailed, better models soon!)