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Fondamenti di Chimica & Educazione Ambientale Lecture 3
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Compounds and Molecules Pure substances composed of two or more different
elements are called compounds.
Compounds have definite percent composition of their combining elements.
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Molecular compounds
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Standard color scheme
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H2O2 CH3CH2Cl P4O10
CH3CH(OH)CH3 HCO2H
Some molecules
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z Atoms of almost all elements can gain or lose electrons to form charged species called ions.
z Compounds composed of ions are known as ionic compounds.
u Metals tend to lose electrons to form positively charged ions called cations.
u Non-metals tend to gain electrons to form negatively charged ions called anions.
Ionic compounds
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Extended array of Na+ and Cl- ions
Sodium chloride
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Simplest formula unit is NaCl
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S8
Inorganic molecules
P4
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Halothane C2HBrClF3
M(C2HBrClF3) = 2MC + MH + MBr + MCl + 3MF
= (2 × 12.01) + 1.01 + 79.90 + 35.45 + (3 × 19.00) = 197.38 g/mol
Mole ratio nC/nhalothane
Mass ratio mC/mhalothane
Chemical Composition
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Calculating the Mass Percent Composition of a Compound
Calculate the molecular mass M(C2HBrClF3
) = 197.38 g/mol
For one mole of compound, formulate the mass ratio and convert to percent:
%C = (2×12.01)g
197.38g ×100% =12.17%
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%C =(2 ×12.01)g
197.38g×100% = 12.17%
%H =1.01g
197.38g×100% = 0.51%
%Br = 79.90g197.38g
×100% = 40.48%
%Cl = 35.45g197.38g
×100% = 17.96%
%F =(3×19.00)g
197.38g×100% = 28.88%
Calculating the Mass Percent Composition of a Compound
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1. Choose an arbitrary sample size (100g). 2. Convert masses to amounts in moles. 3. Write a formula. 4. Convert formula to small whole numbers. 5. Multiply all subscripts by a small whole number to
make the subscripts integral.
5 Step approach:
Empirical formula
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Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition.
Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H and 27.79% O. Its experimentally determined molecular mass is 230 u. What are the empirical and molecular formulas of dibutyl succinate?
Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H and 27.79% O. Its experimentally determined molecular mass is 230 u. What are the empirical and molecular formulas of dibutyl succinate?
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Step 1: Determine the mass of each element in a 100g sample.
C 62.58 g
Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition.
O 27.79 g H 9.63 g
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Step 2: Convert masses to amounts in moles.
Step 3: Write a tentative formula.
Step 4: Convert to small whole numbers.
C5.21H9.55O1.74
C2.99H5.49O
Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition.
�
nC = 62.58gC×1molC
12.011gC= 5.210molC
nH = 9.63gH×1molH
1.008gH= 9.55molH
nO = 27.79gO×1molO
15.999gO= 1.737molO
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Step 5: Convert to a small whole number ratio.
Multiply × 2 to get C5.98H10.98O2
The empirical formula is C6H11O2
Step 6: Determine the molecular formula.
Empirical formula mass is 115 u. Molecular formula mass is 230 u.
The molecular formula is C12H22O4
Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition.
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Combustion analysis
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Metals tend to lose electrons.
Na D Na+ + e-
Non-metals tend to gain electrons.
Cl + e- D Cl-
Reducing agents Oxidizing agents
We use the Oxidation State to keep track of the number of electrons that have been gained or lost by an element.
Oxidation States
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1. The oxidation state (OS) of an individual atom in a free element is 0. 2. The total of the OS in all atoms in:
i. Neutral species is 0. ii. Ionic species is equal to the charge on the ion.
3. In their compounds, the alkali metals and the alkaline earths have OS of +1 and +2 respectively.
4. In compounds the OS of fluorine is always –1 5. In compounds, the OS of hydrogen is usually +1 6. In compounds, the OS of oxygen is usually –2. 7. In binary (two-element) compounds with metals:
i. Halogens have OS of –1, ii. Group 16 have OS of –2 and iii. Group 15 have OS of –3.
Rules for Oxidation States
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Anions (- Q):Groups 15, 16, 17 (VA, VIA, VIIA) Cations (+ Q):Groups 1, 2, 3 (IA, IIA, IIIA)
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Assigning Oxidation States.
What is the oxidation state of the underlined element in each of the following? a) P4; b) Al2O3; c) MnO4
-; d) NaH
a) P4 is an element. P OS = 0 .
b) Al2O3: O is –2. O3 is –6. Since (+6)/2=(+3), Al OS = +3.
c) MnO4-: net OS = -1, O4 is –8. Mn OS = +7.
d) NaH: net OS = 0, rule 3 beats rule 5, Na OS = +1 and H OS = -1.
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Periodic Trends In Oxidation States
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Naming Compounds
Trivial names are used for common compounds. A systematic method of naming compounds is known as a system of nomenclature. Organic compounds Inorganic compounds
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Binary Compounds of Metals and Nonmetals
NaCl = sodium chloride
name is unchanged
“ide” ending electrically neutral
MgI2 = magnesium iodide
Al2O3 = aluminum oxide
Na2S = sodium sulfide
Inorganic Nomenclature
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Molecular compounds usually write the positive OS element first. HCl hydrogen chloride
mono 1 penta 5
di 2 hexa 6
tri 3 hepta 7
tetra 4 octa 8
Some pairs form more than one compound
Binary Compounds of Two Non-metals
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Naming Binary Compounds of the Non-metals
Named in order of increasing group number
N2O4 Dinitrogen tetroxide
Sulfur hexafluoride
N2O Dinitrogen oxide
SF6
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Emphasize the fact that a molecule is an acid by altering the name.
HCl hydrogen chloride hydrochloric acid
HF hydrogen fluoride hydrofluoric acid
Acids produce H+ when dissolved in water.
They are compounds that ionize in water.
Binary Acids
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Polyatomic ions are very common. Here are a few:
ammonium ion NH4+ acetate ion C2H3O2
-
carbonate ion CO32- hydrogen carbonate HCO3
-
hypochlorite ClO- phosphate PO43-
chlorite ClO2- hydrogen phosphate HPO4
2-
chlorate ClO3- sulfate SO4
2-
perchlorate ClO4- hydrogensulfate HSO4
-
Polyatomic Ions
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Gaining (anion) and Losing (cation) Electrons to attain Inert gas electronic
structure
Ionic Bonding:
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When an atom gains electrons it becomes an anion. Anion are larger than the neutral atom.
W h e n a n a t o m loses electrons it becomes a cation. Cations are smaller than the neutral atom.
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The attractive force between + and - charges is called a coulombic attraction
Force of attraction =
KQ1×Q2
d2
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Predict the ionic bond strength of the following ionic bonds.
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Consider a cation and an anion
Na Cl
NaCl
Mg Cl Cl
MgCl2
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How to write formulas for ionic compounds
1. Identify ions
2.“Cross” charges
3. Write formula 1 2
Charge is Conserved: The + = - !
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Naming Ionic Compounds
Cations are named as the metal + “ion”
Na+ = sodium ion
Multicharged ions
Stock System Metal (Charge in Roman Numerals)
“ion” Cu1+ Cu2+
Copper(I) ion Copper(II) ion
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MgBr2 Magnesium bromide
Fe2O3 Iron(III) oxide
Ca3(PO4)2 Calcium phosphate
Naming Ionic Compounds
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More Examples
Mg(NO3)2
FeO
CuBr2
NaHCO3
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Compounds and Molecules
Some compounds are composed of molecules.
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Sharing electrons to attain inert gas electron configurations. E
d
Covalent Bonding:
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Organic compounds abound in nature Fats, carbohydrates and proteins are foods.
Propane, gasoline, kerosene, oil.
Drugs and plastics
Carbon atoms form chains and rings and act as the framework of molecules.
Naming Organic Compounds
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Molecular Structure
The arrangement of atoms in a molecule
Isomers:
Same number of constituent atoms BUT
different arrangements.
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Visualizations of some hydrocarbons
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Functional Groups Carboxylic Acid
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Functional Groups Alcohol
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Isomerism
Structural
Stereo
Optical
Geometrical
Positional
Functional
Chain
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Chain Isomers
n-pentane
2-methylbutane
2,2-dimethylpropane
C5H12
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Positional Isomers
Propan-1-olo
Propan-2-olo
C3H7OH
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Positional Isomers
2-Methylphenol 4-Methylphenol 3-Methylphenol
C7H7OH
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Functional Isomers
Ethanol Methoxymethane
C2H6O
Alcohols have the hydroxyl group, –OH Ethers have the functional group R–O–R'
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Functional Isomers
Propanal
C3H6O
Aldehydes and ketones have the carbonyl group C=O. In aldehydes C is attached either to 1 or 2 H atoms; in ketones C is attached to two carbon atoms.
Propanone
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Isomerism
Structural
Stereo
Optical
Geometrical
Positional
Functional
Chain
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Geometrical Isomers
Geometrical isomerism is an example of stereo-isomerism. This occurs when substances have the same molecular formula, but a different arrangement of their atoms in space.
Geometrical isomerism can happen if: i) There is a C=C bond in the molecule ii) The molecule has a ring iii) The molecule has a >C=N bond
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Geometrical Isomers
C4H8
Cis-But-2-ene Trans-But-2-ene
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trans-Pt(NH3)2Cl2
cis-Pt(NH3)2Cl2
Geometrical Isomers
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Pt(NH3)Cl3
No isomers
Geometrical Isomers
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cis-Pt(NH3)Cl2Br trans-Pt(NH3)Cl2Br
Two isomers
Geometrical Isomers
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ClN
Cl
Pt
N
ClCl
ClN
Cl
Pt
Cl
NCl
trans Two isomers
cis
Geometrical Isomers
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Cl N
Cl
Pt
Cl
N N
Cl N
Cl
Pt
N
NCl
fac Two isomers
mer
Geometrical Isomers
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Optical Isomers
Optical isomerism is an example of stereo-isomerism. It occurs when substances have the same molecular and structural formula, but one cannot be superimposed on the other. The different forms are called enantioners.
optical isomerism
non-superimposable mirror images
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Optical Isomers
Optical isomers can occur when there is an asymmetric carbon atom; i.e. a C atom bonded to four different groups.
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Lecture 3 End