ch1001 2010(language2) notes
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CH1001CH1001
CHEMISTRY: A Central ScienceCHEMISTRY: A Central Science
Department of Chemistry,School of Pharmacy & Molecular Sciences
22 The languageThe language
of chemistryof chemistry
MeasurementMeasurement
• Measurements
– always have a unit
– always an associated uncertainty
• Unit
– specific standard quantity of a particular property
– used to measure all other quantities of property
• The seven SI base units
Measurement Unit Symbollength metre m
mass kilogram kg
time second s
temperature kelvin K
amount of substance mole mol
electric current ampere A
luminous intensity candela Cd
MeasurementMeasurement
• The SI units for ANY physical quantity can
be built from the seven base units
– For example
• area = length ! width
• length and width are measured in metres
• the unit of area is square metre, m2
• Units undergo the same kinds of
mathematical operations as the numbers to
which they are attached
MeasurementMeasurement
EXAMPLES:
1. Volume = length x breadth x depth
The units will be metre x metre x metre = cubic metre,designated m3
MeasurementMeasurement
2. Speed (velocity) = distance/time
The units will therefore be metre/second, designated m/s or m s-1
3. Density = mass/volume
The units will be kilogram/cubic metre, designated kg/m3 or kg m-3
• Use prefixes that divide or multiply the unit by aparticular power of ten
Prefix Symbol Factor
mega M 106
kilo k 103
deci d 10-1
centi c 10-2
milli m 10-3
micro µ 10-6
nano n 10-9
pico p 10-12
• SI or derived units are sometimes inconvenient
MeasurementMeasurement
There is one non-SI unit that is in common
usage, particularly in an industrial and
environmental context - parts per million
(ppm)
! In solutions, it is a weight/volume measurement -
i.e. mg/litre (mg L-1)
! In solids, it is a weight/weight measurement -
i.e. mg/kg (mg kg-1)
MeasurementMeasurement
• Uncertainties and significant figures
– Every measurement has an associated
uncertainty
MeasurementMeasurement
24.3 ºC
297.45 K
24.32 ºC
297.47 K
– All figures up to and including the first
estimated figure are recorded
– These figures are called significant figures
• Uncertainties and significant figures
MeasurementMeasurement
24.3 ºC
297.45 K
24.32 ºC
297.47 K
– Scientific notation expresses numbers in
terms of powers of ten
• 24.3 ºC is 2.43 ! 101 ºC
• 247.45 ºC is 2.4745 ! 102 ºC
MeasurementMeasurement
• Uncertainties and significant figures
24.3 ºC
297.45 K
24.32 ºC
297.47 K
• Significant figures in calculations
– Multiplication or division of
measurements
MeasurementMeasurement
! Addition or subtraction of measurements
3.14 x 2.751
0.64= 13
NOT 13.49709375 !
3.247
+ 41.36
+ 125.2
169.8
NOT 169.807 !
• Precision
– Signifies how reproducible a
measurement is when made a number of
times
MeasurementMeasurement
MeasurementMeasurement
• Accuracy
– refers to how close the value is to the
correct value
Representations of moleculesRepresentations of molecules
• Chemical formulas
– Show the relative number of each type of atom
present in a substance
• H2O
• C30H34AuBClF3N6O2P2PtW
Note: a subscripted number refers ONLY to
the atom immediately preceding it.
– A molecular formula is a chemical formula that refers to a discrete molecule
– There is a standardised way of writing chemical formulas . . .
• Binary compounds– Element further to the left of the periodic table
appears first
• KCl, Al2S3
– Hydrogen is the exception to this rule
• LiH, NH3, H2O2, HCl
– If both elements are from the same group
the lowest element appears first• SiC, BrF3
– In ionic compounds the cation is written first
followed by the anion• NaBr, MgCl2
Representations of moleculesRepresentations of molecules
• Writing chemical formulas for
compounds containing more than two
elements requires some knowledge of
the bonding within the compound.
– Ionic compounds
– Covalent compounds
Representations of moleculesRepresentations of molecules
• Ionic compounds
– Cation followed by anion
– Total charge zero
Ca(NO3)2(s) " Ca2+(aq) + 2NO3-(aq)
Representations of moleculesRepresentations of molecules
• Covalent compounds
– Majority are carbon-based organic
compounds
– Chemical formula often written carbon
first, followed by hydrogen and then the
remaining elements in alphabetical
order, e.g. C2H6O, C4H9BrO
Representations of moleculesRepresentations of molecules
Representations of moleculesRepresentations of molecules
• Three-dimensional structures
PICTURING MOLECULESPICTURING MOLECULES
Water
Chemical formula - showing only relative number of atoms
Electron-dot and bond-line formulas - show a bond
between atoms as either a pair of dots or a line.
The bond-line formulas are often called structural formulas.
Ball-and-Stick models - show atoms as spheres and
bonds as sticks, with accurate angles and relative sizes,
but distances are exaggerated
Space-filling models - accurately scaled-up versions
of molecules, but do not show bonds
Electron-density models - show the ball-and-stick model
within the space-filling shape, and colour the regions of
high (red) and low (blue) electron charge
PICTURINGPICTURING
MOLECULESMOLECULES
• Structural formulas
– Attempt to show the way in which theatoms in the molecule are bondedtogether
– Chemical symbols still used for eachelement present
– Constituent atoms are placed in theorder in which they are bonded together
– Bonds between neighbouring atoms arerepresented as lines
Representations of moleculesRepresentations of molecules
• Structural formulas
– NH3
– Structural formulas do not necessarilyshow the correct geometry of acompound
Representations of moleculesRepresentations of molecules
HN
H
H
NH H
H
NH H
H
• •
NomenclatureNomenclature
• Nomenclature — the system for
naming of compounds
• International Union of Pure and Applied
Chemistry (IUPAC)
• Some compounds are better known by
their common unsystematic name rather
than their systematic IUPAC name
– The systematic name of water is oxidane !
1. The element closer to the left of the
periodic table appears first.
If both elements are from the same
group of the periodic table, the lower
one appears first.
NomenclatureNomenclature
• Naming inorganic compounds
• Nonmetallic binary compounds
– Guidelines for naming:
NO NO2 N2O5
Nitrogen monoxide Nitrogen dioxide Dinitrogen pentoxide
2. The element that appears first retains
its elemental name.
NomenclatureNomenclature
• Naming Inorganic compounds
• Nonmetallic binary compounds
– Guidelines for naming:
NO NO2 N2O5
Nitrogen monoxide Nitrogen dioxide Dinitrogen pentoxide
3. The second element begins with a
root derived from its elemental name
and ends with the suffix –ide.
NomenclatureNomenclature
• Naming Inorganic compounds
• Nonmetallic binary compounds
– Guidelines for naming:
NO NO2 N2O5
Nitrogen monoxide Nitrogen dioxide Dinitrogen pentoxide
4. When there is more than one atom of
a given element in the formula, the
name of the element usually contains
a prefix that specifies the number of
atoms present.
Nitrogen monoxide Nitrogen dioxide Dinitrogen pentoxide
NomenclatureNomenclature
• Naming Inorganic compounds
• Nonmetallic binary compounds
– Guidelines for naming:
NO NO2 N2O5
Nitrogen monoxide Nitrogen dioxide Dinitrogen pentoxide
• Binary compounds of hydrogen
– Hydrogen requires special consideration
– It may appear first or second in the
chemical formula of a compound,
therefore first or second in the name
• Lithium hydride LiH
• Hydrogen fluoride HF
• Hydrogen sulfide H2S
• Calcium hydride CaH2
NomenclatureNomenclature
• Ionic compounds
– Binary ionic compounds are written with
the cation first and the anion, which
takes the suffix –ide, last
• Potassium iodide KI
• Calcium fluoride CaF2
– Ionic compounds may contain
polyatomic ions
• Ammonium NH4+
• Nitrate NO3-
NomenclatureNomenclature
POLYATOMIC IONSPOLYATOMIC IONS
ANIONSOH- hydroxide
NO3- nitrate
CO32- carbonate
ClO4- perchlorate
SO42- sulfate
PO43- phosphate
CATIONS
NH4+ ammonium
MnO4- permanganate
Cr2O72- dichromate
HCO3- bicarbonate
O22- peroxide
C2H3O2- acetate
1. The name has a root taken from the
name of the central atom
• Carbonate CO32-
• Nitrite NO2-
• Ionic compounds
– Anions containing a central atom
surrounded by oxygen atoms are known
as oxoanions. Naming rules:
NomenclatureNomenclature
2. When an element forms 2 different
oxoanions, the one with fewer oxygen
atoms ends in –ite, and the other
ends in –ate
• Sulfite SO32-
• Sulfate SO42-
• Ionic compounds
– Anions containing a central atom
surrounded by oxygen atoms are known
as oxoanions. Naming rules:
NomenclatureNomenclature
3. Chlorine, bromine and iodine each
form 4 different oxoanions that are
distinguished by prefixes and suffixes
• Hypobromite BrO-
• Bromite BrO2-
• Bromate BrO3-
• Ionic compounds
– Anions containing a central atom
surrounded by oxygen atoms are known
as oxoanions. Naming rules:
NomenclatureNomenclature
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