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Chemistry 5

Chapter-3 Chemical Compounds

23 September 2002

Some Cool Chemistry Research Prof. X. Zhuang & Group

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 covalently bonded elements. Chemical Formula: describes a molecule in terms of 1. elements present 2. relative number of atoms of each element

cyclohexane: C6H12

benzene: C6H6

Molecular Compounds, cont’d ƒ Empirical Formula: The most basic representation of a compound. It shows the types of elements and relative amounts (like chemical formula) but with amounts 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. ƒ Molecular Formula: Provides the elements and relative amounts for a molecule in a molecular compound. The molecular formula will be an integer multiple of the empirical formula. acetylene: benzene:

EF = CH EF = CH

MF = C2H2 MF = C6H6

What about how elements are bonded to one another?

Molecular Compounds, cont’d ƒ Structural Formula: Provides types of elements and relative amounts, and moreover, order in which atoms 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 (propyl alcohol)

C3H8O CH3CH2CH2OH H

2-propanol (isopropyl alcohol)

H

H

H

C

C

C

H

H

H

OH

C3H8O CH3CHOHCH3

H

H

OH

H

C

C

C

H

H

H

H

Ionic Compounds Ionic compounds are made up of positive and negative ions joined together by electrostatic forces. Typically, ionic compounds consist of a metallic element, which loses an electron, and a nonmetallic element, which gains an electron. ƒ Cations: the positively charged element that has given up an electron; e.g., Na+ (in NaCl). ƒ Anions: the negatively charged element (or group of elements) that has taken up an electron; e.g., Cl(in NaCl). ƒ Formula unit: The smallest neutral group ions with the same ratio of atoms (ions)as in the chemical formula. The formula unit is not a molecule.

Molecular vs. Ionic Compounds

Moles & Chemical Compunds ƒ Molecular and formula masses: The molecular and formula masses– the masses of these units in atomic mass units– are obtained from weighted 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) =

ƒ 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

Chemical Composition The chemical formula of a compound provides quantitative information about composition, etc.

ƒ formula Æ percent composition Example: H2O 1. Calculate total mass = 2 x mass(H) + 1x mass(O) = 2x1.008 + 15.999 %H = 2 x mass(H) total mass x100% %O = 1 x mass(O) x100% total mass

ƒ percent composition Æ formula See example 3-6 in text (typical)

ƒ combustion analysis: common analytic method for determining chemical composition. CxHyOz + xs O2 Æ xCO2 + y/2(H2O) Measure amount CO2 and H2O produced in combustion reaction provides empirical formula for C and H.