Covalent bonds, moles, theoretical/% yields, Excel (Oct. 7)
- Homeworks
will be available to pickup outside my office tomorrow
- Answer
key will also be posted there and eventually online
- Talk
about lab: expected results vs
actual
- Assumptions/errors
- Add
to spreadsheet color of solution when filtered as indication of reaction
progression (brown indicates presence of I2)
- If
not all I2 reacts, assume greater mass I2
consumed than actual
- Zn
dust vs Zn granules: small size slowing rxn? (ZnO decrease amt Zn to
react?) ŕ I’ll check
- If
Zn dust forms ZnO easier when in solution (likely), mass zinc left over
artificially higher by presence of ZnO
- Zn
not dried sufficiently
- Zn
consumedactual > Zn consumedcalc
- ZnI
lost in spattering
- Report: stress error analysis
- Go
over Zeff and shielding by electrons
- Positive
charge of ions is centered on nucleus
- Large
atoms ŕ
lots of protons & electrons ŕ
e-
cloud shields charge on nucleus
- Ionic
bonds: Large difference in electronegativity between atoms (usually metal
and nonmetal) ŕ form positive and
negative charged ions ŕ held together by
electrostatic forces
- Covalent
bonds: usually two nonmetals; sharing of electrons to achieve closed shell
(octet)
- If
difference in electronegativity ŕ
polarity
- See
fig. 2.16
- (3.1)
The mole: an amount of substance
that contains the same # of entities as there are atoms in exactly 12g of 12C
ŕ
Avagadro’s number, NA = 6.022x1023 mol-1
- ŕ
specifies # of objects in a fixed mass: can count by weight
- ŕ “atomic mass in amu is numerically the same
as the mass of one mole of atoms of the element expressed in grams”
- ŕ calculating molar mass M
- converting
grams to moles: mass (in g) =
#moles x M (in g/mol)
- moles
to grams: moles = mass (in g) x 1/M
- # of
entities (atoms) = #moles x NA (in mol-1)
- # of
moles = #atoms x 1/NA
- (3.4)
Chemical reactions and limiting reactant
- ŕ
stoichiometry (Greek stoicheion, element or part; + metron,
measure): study of quantitative aspects of chemical formulas and
reactions
- Reactions
like a recipe for a sandwich: if you have extra cheese but no extra
bread, you can’t make another sandwich
- Molar
ratios: coefficients tell you how much you need relative to each
component
- Limiting
reactants: when given amounts of both reactants, you can have less of one
reactant (limiting) than needed to react all the other reactant, which is
then said to be in excess
- When
molar ratio of reactants isn’t 1:1 ŕ
need to get both in terms of moles of product
- Problem:
3.10 p 112
- Theoretical
and percent yield
- ŕ yield of product expected based on limiting
reactant, assuming complete rxn
- Actual
yield always less than theoretical!
- Can
have side rxns, loss of product, incomplete rxns, etc
- Percent
yield = (actual yield) ¸ (theoretical yield) x 100
- Problem:
3.11 p 113
- Graphing
with Excel (quickie workshop)
- Save
ZnI_exp.xls in temp drive (can be executed by only one at a time)
- Enter
new data (% yield, rxn color, dust vs granules) into file on masu ŕ programs ŕMatterMotion
ŕ projects
- Data
should be entered w/o units to be able to use as values
- Use
X-Y scatter option
- Under
‘Series’ tab, select data column(s) you want for x and y axes
- Can
have > 1 series on same graph
- Label
axes and title
- Add
trendline by right-clicking on data point
- Under
‘Options’ can display equation/R2 on graph
- Can
change graph options (add data, etc) by right-clicking on chart
- Meet
Tues in West CAL again for chem. 2-4
Homework: [3.6],
3.12, [3.23], 3.25, 3.83