Chemistry Lecture ’93 B



Chemistry Lecture ’93 B. Rife CHS

Text: Modern Chemistry; Holt, Rinehart & Winston 1993

Chemical Formulas and Chemical Compounds Chapter 7

Homework:

1 Section Reviews (pg 212,216,223,228)

2 Reviewing Concepts:

(1,2,6,8,9,10,11,12,13,14,15,19,20,21,23) (pg 229-230)

3 Problems (all) (pg 230-231)

4 Chapter/Section Review (Handout)

Exam Date _

7.1 Chemical names and Formulas

7.1A Explain the significance of a chemical formula. Significance of a Chemical Formula

CHEMICAL FORMULA IS A SHORTHAND METHOD OF REPRESENTING THE

COMPOSITION OF A SUBSTANCE BY USING CHEMICAL SYMBOLS AND

NUMERICAL SUBSCRIPTS.

CHEMICAL FORMULAS INDICATE THE RELATIVE NUMBER OF ATOMS FOR EACH

ELEMENT PRESENT IN A COMPOUND.

A COMPOUND IS A SUBSTANCE IN WHICH TWO OR MORE ELEMENTS ARE CHEMICALLY

COMBINED. THE FORMULA H2O REPRESENTS: ONE MOLECULE OF H2O

ONE MOLE OF H2O ONE MOLAR MASS (18 g) OF H2O

STRUCTURAL FORMULA REPRESENTS THE ARRANGEMENT OF ATOMS IN A

COMPOUND.

7.1B Determine the formula of an ionic compound between any two given ions ( )

MONATOMIC IONS ARE IONS FORMED FROM A SINGLE ATOM

CATIONS (METALS) HAVE LOST ELECTRONS POSITIVE IONS ARE NAMED BY THE ELEMENT NAME FOLLOWED BY “ION”.

ANIONS (NONMETALS) HAVE GAINED ELECTRONS

NEGATIVE IONS ARE NAMED BY DROPPING THE ENDING OF THE ELEMENT NAME AND ADDING THE SUFFIX “IDE”

NOTE COMMON MONATOMIC IONS TABLE 7-3

BINARY COMPOUNDS ARE COMPOUNDS COMPOSED OF TWO DIFFERENT ELEMENTS.

ION CHARGE METHOD OF WRITING FORMULAS

1. RECOGNIZE POSITIVE AND NEGATIVE IONS NAMED IN COMPOUND.

WRITE THE SYMBOLS FOR THE IONS SIDE BY SIDE, WITH THE POSITIVE ION FOR

MOST METALLIC ELEMENT FIRST.

2. ADJUST THE NUMBER OF EACH ION SO THAT THE TOTAL POSITIVE AND TOTAL NEGATIVE CHARGES EQUAL IN MAGNITUDE. CROSSOVER THE CHARGE VALUES

TO GIVE SUBSCRIPTS.

3. CHECK THE SUBSCRIPTS SO THAT CHARGE IS BALANCED AND WRITE THE

FORMULA CHARGES OF IONS ARE NOT USUALLY INCLUDED IN CHEMICAL

FORMULAS OF IONIC COMPOUNDS.

THE SUBSCRIPT 1 IS UNDERSTOOD WHEN NO SUBSCRIPT IS PRESENT AND IS NOT WRITTEN

EXAMPLE: MONATOMIC IONS

SODIUM CHLORIDE Na+ & Cl- NaCl

MAGNESIUM BROMIDE Mg2+ & Br-- MgBr2

ALUMINUM SULFIDE Al3+ & S2- Al2S3

POLYATOMIC IONS

LEAD(II) SULFATE Pb2+ & SO42- PbSO4

MAGNESIUM HYDROXIDE Mg2+ & OH- Mg(OH)2

AMMONIUM SULFATE NH4+ & SO42- (NH4)2SO4

A ROMAN NUMERAL USED IN THE NAME OF A METALLIC ION DOES NOT APPEAR IN THE FORMULA OF A COMPOUND CONTAINING THE ION.

IT IS POSSIBLE TO WRITE THE FORMULA FOR A COMPOUND AND THEN LEARN THAT SUCH A COMPOUND DOES NOT EXIST.

7.1C Explain the two systems for distinguishing different ionic compounds of the same two elements. ( )

NOMENCLATURE - IS A TERM THAT REFERS TO METHODS OF NAMING CHEMICAL COMPOUNDS.

THE “OLD SYSTEM” DIFFERENT NAMES ARE NEEDED FOR POSITIVE IONS (TRANSITION METALS) OF TWO DIFFERENT CHARGES.

THE SUFFIX -OUS IS GIVEN TO THE ION WITH THE LOWER CHARGE

THE SUFFIX -IC IS GIVEN TO THE ION WITH THE HIGHER CHARGE

THE STOCK SYSTEM

COMPOUNDS OF METALS THAT FORM MORE THAN ONE ION (HAVE MORE THAN ONE OXIDATION STATE) ARE NAMED BY INCLUDING THE A ROMAN NUMERAL

REPRESENTING THE CHARGE.

IN BOTH SYSTEMS MONATOMIC ANIONS ARE NAMED WITH THE ROOT NAME OF THE

ANION AND THE THE SUFFIX -IDE.

A POLYATOMIC ION IS A GROUP OF ATOMS BONDED TOGETHER COVALENTLY (SHARED) BUT POSSESSING AN OVERALL CHARGE.

NOTE COMMON POLYATOMIC IONS TABLE 7-4

OXYANIONS - ARE POLYATOMIC IONS THAT CONTAIN OXYGEN

THE ION WITH THE LARGER NUMBER OF OXYGEN ATOMS IS NAMED WITH

THE -ATE SUFFIX. (NITRATE NO3, SULFATE SO4)

THE ION WITH THE SMALLER NUMBER OF OXYGEN ATOMS IS NAMED WITH

THE -ITE SUFFIX. (NITRITE NO2, SULFITE SO3)

7.1D Name an ionic compound, given its formula ( )

PSEUDOBINARY COMPOUNDS - CONTAIN MORE THAN TWO ELEMENTS. IN THESE COMPOUNDS ONE OR MORE OF THE IONS CONSIST OF MORE THAN ONE ELEMENT BUT BEHAVE AS IF THEY WERE SIMPLE IONS.

THE MOST COMMON EXAMPLES OF SUCH ANIONS ARE THE HYDROXIDE ION OH-

AND THE CYANIDE ION, CN-.

THE AMMONIUM ION NH4+, IS THE MOST COMMON CATION THAT BEHAVES LIKE A

SIMPLE METAL CATION.

IN NAMING COMPOUNDS CONTAINING POLYATOMIC IONS, THE NAME OF THE

POLYATOMIC ION IS NOT CHANGED.

Fe2O3 IRON (III) OXIDE

HCN HYDROGEN CYANIDE

NaOH SODIUM HYDROXIDE

NH4NO3 AMMONIUM NITRATE

7.1E Using prefixes, name a binary molecular compound from its formula. ( )

7.1F Write the formula of a binary molecular compound, given its name. ( )

THE “OLD SYSTEM” OF NAMING BINARY MOLECULAR COMPOUNDS

THE GENERAL RULE IS TO NAME THE LESS ELECTRONEGATIVE ELEMENT FIRST AND

THE MORE ELECTRONEGATIVE ELEMENT SECOND.

THE FIRST WORD OF A BINARY MOLECULAR COMPOUND IS MADE UP OF

(A) THE PREFIX OF THE NUMBER OF ATOMS OF THE FIRST ELEMENT AND

(B) THE NAME OF THE FIRST ELEMENT.

IF ONLY ONE ATOM OF THE FIRST ELEMENT APPEARS IN THE FORMULA THEN NO

PREFIX (MON) IS USED.

THE SECOND WORD OF A BINARY MOLECULAR COMPOUND IS MADE UP OF

(A) THE PREFIX OF THE NUMBER OF ATOMS OF THE SECOND ELEMENT AND

(B) THE ROOT OF THE NAME OF THE SECOND ELEMENT AND

(C) THE SUFFIX -IDE WHICH MEANS THAT ONLY THE TWO ELEMENTS NAMED

ARE PRESENT.

PREFIXES: (TABLE 7-5)

MON = 1 DI = 2 TRI = 3 TETRA = 4 PENTA = 5

ROOTS: H HYDRO

III IV V VI VII

B BOR C CARB N NITR O OX F FLUOR

Si SILIC P PHOSPH S SULF Cl CHLOR

As ARSEN Se SELEN Br BROM

EXAMPLES:

CO CARBON MONOXIDE CO2 CARBON DIOXIDE

SbCl3 ANTIMONY TRICHLORIDE CCl4 CARBON TETRACHLORIDE

As2S5 DIARSENIC PENTASULFIDE

7.1G List the names and formulas of the common laboratory acids. ( )

SOME MOLECULAR COMPOUNDS CONTAINING H ATOMS DISSOCIATE IN WATER TO FORM H+ IONS. THESE ARE CALLED ACIDS.

BINARY ACIDS - CONTAIN HYDROGEN AND ONE OF THE HALOGENS

IF THE ANION DOES NOT CONTAIN OXYGEN, THE ACID IS NAMED WITH THE PREFIX HYDRO- AND THE SUFFIX -IC.

HCl (g) HYDROGEN CHLORIDE HF (g) HYDROGEN FLUORIDE

HCl (aq) HYDROCHLORIC ACID HF (aq) HYDROFLUORIC ACID

OXYACIDS - CONTAIN HYDROGEN, OXYGEN, AND A THIRD ELEMENT

WHEN THE ANION CONTAINS OXYGEN, THE ACID NAME FORMED IS FROM AN

ANION WITH -ATE SUFFIX IS THE ROOT NAME AND THE SUFFIX -IC (acids)

FROM AN ANION WITH -ITE SUFFIX IS THE ROOT NAME AND

THE SUFFIX -OUS (acids)

ClO3- CHLORATE ION HClO3 CHLORIC ACID

ClO2- CHLORITE ION HClO2 CHLOROUS ACID

AN IONIC COMPOUND COMPOSED OF A CATION (FROM A BASE) AND THE ANION

FROM AN ACID IS REFERRED TO AS A SALT.

SOME SALTS CONTAIN ANIONS IN WHICH ONE OR MORE HYDROGEN ATOMS FROM THE

ACID ARE RETAINED SUCH ANIONS ARE NAMED BY ADDING THE WORD

HYDROGEN OR THE PREFIX BI-

HCO3- HYDROGEN CARBONATE BICARBONATE

7.2 Oxidation Numbers

OXIDATION NUMBERS REPRESENT APPARENT CHARGE ON AN ATOM OR ATOMS AND

ARE USED TO WRITE BALANCED CHEMICAL FORMULAS.

7.2A List the rules for assigning oxidation numbers ( )

1. AN UNCOMBINED ELEMENT (MOLECULAR) HAS AN OXIDATION NUMBER OF ZERO.

2. A MONATOMIC ION HAS AN OXIDATION NUMBER EQUAL TO THE CHARGE OF THE

ION.

3. FLUORINE ALWAYS HAS AN OXIDATION NUMBER OF -1.

4. OXYGEN HAS AN OXIDATION NUMBER OF -2 IN MOST COMPOUNDS EXCEPT IN

PEROXIDES SUCH AS H2O2 WHERE IT IS -1.

5. HYDROGEN HAS AN OXIDATION NUMBER OF +1 IN ALL COMPOUNDS WITH MORE

ELECTRONEGATIVE ELEMENTS, AND AN OXIDATION NUMBER OF -1 IN

COMPOUNDS WITH METALS KNOWN AS HYDRIDES.

6. THE MORE ELECTRONEGATIVE ELEMENT IN A BINARY COMPOUND IS ASSIGNED

THE NUMBER EQUAL TO THE CHARGE IT WOULD HAVE IF IT WERE AN ION.

7. THE ALGEBRAIC SUM OF THE OXIDATION NUMBERS OF ALL ATOMS IN A NEUTRAL

COMPOUND IS ZERO.

8. THE ALGEBRAIC SUM OF THE OXIDATION NUMBERS OF ALL ATOMS IN A

POLYATOMIC ION IS EQUAL TO THE CHARGE OF THE ION.

7.2B Give the oxidation number for each element in the formula of a chemical compound ( )

+1 -2 +1 +2 +5 -2 +1 +7 -2

NaOH Cu(NO3)2 KMnO4

7.2C Name binary compounds, using oxidation numbers and the Stock system. ( )

THE STOCK SYSTEM IS AN ALTERNATIVE TO THE PREFIX SYSTEM FOR NAMING BINARY MOLECULAR COMPOUNDS

N2O LAUGHING GAS DINITROGEN MONOXIDE

NITROGEN (I) OXIDE

7.3 Using Chemical Formulas

7.3A Calculate the formula mass or molar mass of any given compound. ( )

FORMULA MASS - IS THE SUM OF THE ATOMIC MASSES OF THE ATOMS IN A FORMULA

UNIT (IONIC COMPOUND)

MOLECULAR MASS - THE MASS OF ONE MOLECULE OF A MOLECULAR SUBSTANCE

(COVALENT), IN ATOMIC MASS UNITS.

MOLECULAR MASS IS THE SUM OF THE ATOMIC MASSES OF THE ATOMS IN THE

MOLECULE.

MOLECULAR WEIGHT OF CANE SUGAR C12H22O11

12 x C = 12 x 12 amu = 144 amu

22 x H = 22 x 1 amu = 22 amu

11 x O = 11 x 16 amu = 176 amu

__________________________________

MOLECULAR WEIGHT = 342 amu

Ca3(PO4)2 = 3(40.08) + 2(30.97) + 2(4)(16.0) = 310.2 u

7.3B Use molar mass to convert between mass in grams and amount in moles of a chemical compound ( )

MOLES X MOLAR MASS (g) = MASS IN GRAMS MOLES

MASS IN GRAMS X MOLES = MOLES MOLAR MASS

7.3C Give the number of molecules, formula units, or ions in a given molar amount of a chemical

compound.( )

6Ba3(AsO4)2

STATE THE NUMBER OF MOLECULES IN THE ABOVE FORMULA.

STATE THE NUMBER OF IONS IN THE ABOVE FORMULA.

STATE THE NUMBER OF ATOMS IN THE ABOVE FORMULA.

7.3D Calculate the percent composition of a given chemical compound. ( )

THE PERCENT COMPOSITION OF A COMPOUND IS THE PERCENT BY MASS OF EACH ELEMENT IN THE COMPOUND. THE PERCENT IS THE SAME, NO MATTER WHAT THE SIZE OF THE SAMPLE (LAW OF CONSTANT OR DEFINITE COMPOSITION.

PERCENTAGE COMPOSITION MAY BE USED IN THE IDENTIFICATION OF SOME

COMPOUNDS.

ATOMIC WT. x # OF ATOMS OF THE ELEMENT

MOLAR MASS OF THE COMPOUND x 100% COMPOUND = = % ELEMENT

TOTAL PERCENT OF ALL ELEMENTS IN A COMPOUND EQUALS 100% (CHECK)

EXAMPLE:

HYDRATE - A SOLID COMPOUND CONTAINING A DEFINITE PERCENTAGE OF

BOUND WATER.

SODIUM CARBONATE DECAHYDRATE Na2CO3 . 10H2O

FORMULA WEIGHT Na2CO3 . 10H2O

2 x Na = 2 x 23 = 46 amu

1 x C = 1 x 12 = 12

3 x O = 3 x 16 = 48

20 x H = 20 x 1 = 20

10 x O = 10 x 16 = 160

__________________________

FORMULA WEIGHT = 286 amu

PERCENT OF Na2

46 Na2

286 Na2CO3 . 10H2O X 100% Na2CO3 . 10H2O = 16.8% Na

PERCENT OF H2O

180 H2O

286 Na2CO3 . 10H2O X 100% Na2CO3 . 10H2O = 62.9% H2O

7.4 Determining Chemical Formulas

7.4A Define simplest formula, and explain how the term applies to ionic and molecular compounds. ( )

EMPIRICAL (SIMPLEST) FORMULA - THE SMALLEST WHOLE-NUMBER RATIO OF

ATOMS PRESENT IN A COMPOUND. (IONIC COMPOUNDS)

LAW OF CONSTANT COMPOSITION - DIFFERENT SAMPLES OF ANY PURE COMPOUND

CONTAIN THE SAME ELEMENTS IN THE SAME PROPORTIONS BY MASS.

7.4B Find a simplest formula from either percent or mass composition. ( )

FROM PERCENTAGE COMPOSITION

A. CONVERT PERCENT INTO GRAMS.

ONE HUNDRED PERCENT EQUALS ONE HUNDRED GRAMS.

B. CONVERT GRAMS OF ELEMENT INTO MOLES OF ELEMENT.

DIVIDE BY GRAM-ATOMIC WEIGHT (MOLAR MASS)

C. REDUCE RATIO OF MOLES OF ELEMENTS

DIVIDE EACH NUMBER BY THE SMALLEST, MULTIPLY THE RESULTING

NUMBERS BY THE SMALLEST WHOLE NUMBER TO REDUCE FRACTIONS.

EXAMPLE:

ANALYSIS OF A SAMPLE OF A PURE COMPOUND REVEALS

THAT IT CONTAINS 50.1% SULFUR AND 49.9% OXYGEN BY MASS.

WHAT IS THE SIMPLEST FORMULA?

A: 50.1% S = 50.1g S AND 49.9% O = 49.9g O

B: 50.1g S (1 MOL S / 32.1g S) = 1.56 MOL S

49.9g O (1 MOL O / 16.0g O) = 3.12 MOL O

C: 1.56/1.56 = 1 S AND 3.12/1.56 = 2 O = SO2

FROM RELATIVE MASS DATA BEGIN WITH STEP B

7.4C Explain the relationship between the simplest formula and the molecular formula of a given compound. ( )

A MOLECULAR FORMULA IS A WHOLE NUMBER MULTIPLE OF AN EMPIRICAL FORMULA.

MOLECULAR (TRUE) FORMULA - FORMULA THAT INDICATES THE ACTUAL NUMBER OF ATOMS PRESENT IN A MOLECULE OF A MOLECULAR SUBSTANCE (COVALENT COMPOUNDS)

7.4D Find a molecular formula from a simplest formula ( )

IT IS NOT POSSIBLE TO DECIDE WHICH IS THE TRUE FORMULA UNLESS THE MOLECULAR WEIGHT OF THE SUBSTANCE HAS BEEN DETERMINED.

MOLECULAR FORMULA IS A WHOLE-NUMBER MULTIPLE (x) OF THE EMPIRICAL

FORMULA.

(EMPIRICAL FORMULA)x = MOLECULAR FORMULA

MOLECULAR WEIGHT IS A WHOLE-NUMBER MULTIPLE (x) OF THE EMPIRICAL FORMULA WEIGHT

(EMPIRICAL WEIGHT)x = MOLECULAR WEIGHT

EXAMPLE

IN A SAMPLE OF GLUCOSE WE FOUND 40.0% C, 6.72% H, 53.3% O, AND THE MOLECULAR WEIGHT TO BE 180. DETERMINE THE SIMPLEST FORMULA AND THE MOLECULAR FORMULA OF GLUCOSE.

SIMPLEST (EMPIRICAL) FORMULA

A & B 40.0 g C (1 mol C / 12.01 g C) = 3.33 mol C

6.72 g H (1 mol H / 1.008 g H) = 6.66 mol H

53.3 g O (1 mol O / 16.0 g O) = 3.33 mol O

C: 3.33/3.33 = 1 C 6.66/3.33 = 2 H 3.33/3.33 = 1 O

THUS THE EMPIRICAL FORMULA = CH2O

MOLECULAR FORMULA

FORMULA WEIGHT OF CH2O IS 30.02

180 / 30.02 = 6

THUS THE MOLECULAR FORMULA IS (CH2O)6 = C6H12O6

HYDRATES

HYDRATES ARE CRYSTALS, WHICH CONTAIN A SPECIFIC RATIO OF WATER

MOLECULES TO THE COMPOUND.

FINDING THE FORMULA OF A HYDRATE

GIVEN: 0.391 g Li2SiF 0.0903 g H2O

MOLECULAR MASS Li2SiF6

2(6.94) + 28.1 + 6(19.0) = 156 g Li2SiF6

0.391 g Li2SiF6 (1 mol Li2SiF6 /156 g Li2SiF6)

= 0.00251 mol Li2SiF6

MOLECULAR MASS H2O 2(1.0) + 16.0 = 18.0 g H2O

0.0903 g H2O (1 mol H2O /18.0 g H2O) = 0.00502 mol H2O

MOLECULAR RATIO: 0.00251 mol Li2SiF6 / 0.00251 mol = 1

0.00502 mol H2O / 0.00251 mol = 2

THUS Li2SiF6 . 2H2O

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