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Chemistry 5

Chapter-3

Chemical Compounds

23 September 2002

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Some Cool Chemistry Research Prof. X. Zhuang & Group

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Xiaowei Zhuang

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Cool Chemistry Research, cont. Prof. Xiaowei Zhuang & Group

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Molecular Compounds

Molecular compounds are made up of discrete units

called molecules. Each molecule consists of covalentlybonded elements.

Chemical Formula: describes a molecule in terms of 1. elements present

2. relative number of atoms of each element

cyclohexane: C6H12

benzene: C6H6

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Molecular Compounds, cont’d

Empirical Formula: The most basic representation

of a compound. It shows the types of elements andrelative amounts (like chemical formula) but withamounts reduced to lowest whole number ratio.

acetylene: C2H2 EF = CH

benzene: C6H6 EF = CH

empirical formula does not provide a lot of information about a compound, except composition.

What about how elements are

bonded to one another?

Molecular Formula: Provides the elements andrelative amounts for a molecule in a molecular compound.

The molecular formula will be an integer multipleof the empirical formula.

acetylene: EF = CH MF = C2H2

benzene: EF = CH MF = C6H6

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Molecular Compounds, cont’d

Structural Formula: Provides types of elementsand relative amounts, and moreover, order in whichatoms are bonded and the types of bonds used.

An alternative is the condensed structural formula,which shows connectivity but not types of bonds.

1-propanol C3H8O(propyl alcohol) CH3CH2CH2OH

H C

H

H

C

H

H

C

H

H

OH

H C

H

H

C

H

OH

C

H

H

H

2-propanol C3H8O

(isopropyl alcohol) CH3CHOHCH3

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Ionic Compounds

Ionic compounds are made up of positive and negative ions joined together by electrostatic

forces. Typically, ionic compounds consist of ametallic element, which loses an electron, and anonmetallic element, which gains an electron.

Cations: the positively charged element that hasgiven up an electron; e.g., Na+ (in NaCl).

Anions: the negatively charged element (or groupof elements) that has taken up an electron; e.g., Cl-

(in NaCl).

Formula unit: The smallest neutral group ions withthe same ratio of atoms (ions)as in the chemical

formula.The formula unit is not a molecule.

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Molecular vs. Ionic Compounds

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Moles & Chemical Compunds

Molecular and formula masses:The molecular and formula masses– the masses of these units in atomic mass units– are obtained fromweighted average atomic masses

C6H6: molecular mass = 6(atomic mass C) +6(atomic mass H)= 6(12.01) + 6(1.008)

=

FeSO4: formula mass = atomic mass Fe + atomic mass S +4(atomic mass O)

= 55.847 + 32.066 + 4(15.999)

=

A mole is a mole? A mole of compound is the amount containing

Avogadro’s number (6.02 x 1023 )molecules or

formula units.

The molar mass is the mass of 1-mole of compound

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Chemical Composition

formula percent compositionExample: H2O

1. Calculate total mass = 2 x mass(H) + 1x mass(O)

= 2x1.008 + 15.999

%H = 2 x mass(H)

total mass

%O = 1 x mass(O)

total mass

percent composition

formula

See example 3-6 in text (typical)

combustion analysis: common analyticmethod for determining chemicalcomposition.

The chemical formula of a compound provides

quantitative information about composition, etc.

x100%

x100%

CxHyOz + xs O2 xCO2 + y/2(H2O)

Measure amount CO2 and H2O produced in combustionreaction provides empirical formula for C and H.

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