The Nature of Chemistry

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The Nature of ChemistryThe Nature of Chemistry

Chapter 1Chapter 1

Robert Boyle 1627-1691. Originally defined the conceptof chemical “element.”

Geber (Abu Musa Jabir ibn Hayyan, حیان ابن 815-721) جابر .“Father of Arabic chemistry.”

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The Study of ChemistryThe Study of ChemistryThe The AtomicAtomic and and MolecularMolecular Perspective of Chemistry Perspective of Chemistry

• Matter is the physical material of the universe.• Matter is made up of relatively few (ca. 100) elements. • Elements are the building blocks of matter.• On the nano (ultramicroscopic) level, matter consists of atoms. An atom is a “nano-basketball” -- nano = 10 -9.• Atoms usually are found in the combined state, commonly molecules. • Molecules may consist of the same type of atoms or different types of atoms.

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CAUTION!!Not all compounds are made up of molecules.Many compounds, for example, are composed of ionic lattices.For this chapter, however, we will confine discussions of compounds to the concept of molecules, which are the combinations of nonmetallic elements.

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The Study of ChemistryThe Study of Chemistry

The Molecular Perspective of ChemistryThe Molecular Perspective of Chemistry• In these models, red represents oxygen, white

represents hydrogen, and gray represents carbon.

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Classification of MatterClassification of MatterPurePure SubstancesSubstances and and MixturesMixtures

A pure substance cannot be separated into simpler substances by physical means. A pure substance has definite and constant chemical and physical properties (i.e., ignition temp., melting point, magnetic susceptibility, spectral patterns) A pure element is a pure substance that consists only of one kind of atom. Examples of elements: U, P4, Cl2, C60

A pure compound is a pure substance that consists of more than one kind of atom. Examples of compounds: HCl, K2SO4 , C2H6O.

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Molecules are chemical combinations of two or more atoms which can all be the same,

e.g., Br2 , H2 , S8 , O3 (these are elements)

or different,

e.g., H2O, C2H5Br (these are compounds) (Elements which have a different number of atoms in their molecule are called allotropes.Allotropes have different physical and chemical properties, e.g., O2 and O3)

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Classification of MatterClassification of Matter

Individual elements or compounds are all pure substances.

For example, individual samples of C, Fe, O2, N2, H2O NaCl, K2SO4 are all pure substances.

If different samples are mixed together, this is called a mixture. Mixtures do not have constant compositions.

The composition of a pure substance is constant.(e.g., water is always 88.9% oxygen and 11.1% hydrogen)

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Examples of mixtures:

• air (which contains variable amounts of oxygen, nitrogen, carbon dioxide, water vapor, pollutants, etc.

• milk (which contains variable amounts of galactose, minerals, fats, proteins, etc.)

• carbonated drinks (which contain variable amounts of carbon dioxide, sugars, flavorings, etc.)

• “tap” water (which contains dissolved minerals)


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Pure Substances and MixturesPure Substances and Mixtures


pure substances

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If a pure substance can be decomposed (chemically)

into something simpler, then the substance is a compound.

Classification of MatterClassification of MatterPure Substances and MixturesPure Substances and MixturesIf matter is not uniform throughout, then it is a heterogeneous mixture.

If matter is uniform throughout, it is homogeneous.

If homogeneous matter can be separated by physical means, then the matter is a mixture.

If homogeneous matter cannot be separated by physical means, then the matter is a pure substance.

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• There are 115 elements known.• Each element is given a unique chemical symbol (one

or two letters).• Elements are building blocks of matter.• The earth’s crust consists of 5 main elements. • The human body consists mostly of 3 main elements

(O, C, and H).

ElementsElements

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ElementsElements


The next five elements are:Na 2%, K 2%, Mg 2%, H 1%, Ti 0.5%.

The next six elements are:N 3%, Ca 1.5%, P 1%, K,S,Na 0.75%

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Elements in the Human Body –Elements in the Human Body – including trace elementsincluding trace elements

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Classification of MatterClassification of MatterUse your Periodic Table to refer to all elements and their chemical symbols. Bring your Periodic Table

to each class!

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The Periodic TableThe Periodic Table

Bring your Periodic Tableto each class!

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• Most elements interact to form compounds.• The proportions of elements in a compound is the

same irrespective of how the compound was formed.• Law of Constant Composition (or Law of Definite

Proportions):

CompoundsCompounds

–The composition of a pure compound is constant (always the same). For example, water is always 88.9% oxygen and 11.1% hydrogen (by mass). For reasons we shall later see, the volumes of hydrogen and oxygen obtained from water are always in a fixed 2:1 ratio.

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Properties of MatterProperties of Matter

When a substance undergoes a physical change, its physical appearance changes, but its chemical nature does not.

Physical and Chemical ChangesPhysical and Chemical Changes

Example: the melting of ice (physical change) results in a solid being converted into a liquid, but it is still water. Physical changes do not result in a change of composition.

When a substance changes its composition, it undergoes a chemical change

Example: when pure hydrogen and pure oxygen react completely, they form pure water. In the flask containing water, there is no oxygen or hydrogen left over.

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Units of MeasurementUnits of Measurement

m/sseconds

meterstime of units

distance of unitsvelocity of nits

U

SI UnitsSI Units

There are two types of units:a) fundamental (or base) units;b) derived units.

There are 7 base units in the SI system.Derived units are obtained from the 7 base SI units.Example:

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Units of MeasurementUnits of MeasurementPowers of ten are used for convenience with smaller or larger units in the SI system.

What is a GigaByte?

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Units of Measurement - Units of Measurement - TemperatureTemperature

There are three temperature scales:Kelvin Scale (used in science)

Same temperature increment as Celsius scale.Lowest temperature possible (absolute zero) is zero Kelvin. Absolute zero: 0 K = -273.15oC.

Celsius Scale (used in science)Also used in science.Water freezes at 0oC and boils at 100oC.To convert: K = oC + 273.15.

Fahrenheit Scale (used in US engineering and commerce)Water freezes at 32oF and boils at 212oF. To convert:

32-F9

5C 32C

5

9F

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A user-friendly way to view the Celsius Scale:0° - Cold! (coat)10° - Cool (sweat shirt)20° - Pleasant (long sleeves)25° - Room temperature (short sleeves)30° - Very warm (T-shirt)40° - Hot! (swimming pool!)


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Units of Measurement - Units of Measurement - VolumeVolume

The units for volume are given by length3

SI unit for volume is 1 m3.

1 mL = 1 cm3

Other volume units:1 L = 1 dm3

(REMEMBER THIS!)

=1000 cm3 = 1000 mL

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Units of Measurement - Units of Measurement - DensityDensity

• Used to characterize substances.• Defined as: density = mass /volume.• Units: g/cm3, also known as specific gravity.• Originally based on mass -- the density was defined as

the mass of 1.00 g of pure water.

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• Measurements that are close to the “correct” value are accurate.

• Measurements which are close to each other are precise.

• Measurements can be:

Uncertainty in MeasurementUncertainty in Measurement

Precision and Accuracy Precision and Accuracy (actually discussed in Chapter 2)(actually discussed in Chapter 2)

accurate and precise;precise but inaccurate;neither accurate nor precise.

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Uncertainty in MeasurementUncertainty in MeasurementPrecision and AccuracyPrecision and Accuracy

Example: Weigh yourself on your bathroom scales.

Your scales are cheap, and the graduations are every 5 pounds. The precision is bad.

Your scales are expensive, and the graduations are every 0.1 pound. The precision is good.

You know your correct weight from a high-quality scale at the doctor’s office, to within 0.1 pound. You weigh yourself on your bathroom scales and find the reading is off by 10 pounds. The accuracy is bad.

Same situation as above, and find the reading is within 0.1 pound. The accuracy is good.

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Dimensional AnalysisDimensional Analysis

• Conversion factors represent “1” (unity).• Conversion factors are simple ratios.

Method of calculation utilizing a knowledge of units.

Example: 1 L or 1000 mL are equivalent. 1000 mL 1 L

Example: 1 m or 100 cm are equivalent 100 cm 1 m

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Dimensional AnalysisDimensional Analysis

5.6 km

Example: Convert 5.6 km to m:

Volume conversion. Convert 1 cubic meter to mL.

1 m3 x (100 cm)3 x 1 mL = 1 x 106 mL (1 m)3 1 cm3

x 1000 m km

= 5600 m

The Nature of Chemistry

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