Molecules and Compounds

Transcription

1 Molecules and Compounds ed. Brad Collins Some images Copyright The McGraw-Hill Companies, Inc.

2 A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds Chemical Bond H 2 H 2 O NH 3 CH 4 A diatomic molecule contains only two atoms H 2, N 2, O 2, Br 2, HCl, CO A polyatomic molecule contains more than two atoms O 3, H 2 O, NH 3, CH 4 3.1

3 Molecules vs. Compounds Molecules are composed of 2 or more atoms Molecules have a definite arrangement of atoms Molecules are held together by chemical bonds Compounds are composed of 2 or more elements Compounds have a definite ratio of elements, but may or may not have a definite arrangement of those elements Compounds may be held together by chemical bonds or by other attractive forces. Examples of molecules: H 2, O 3 (ozone), NH 3, H 2 O, C 6 H 12 O 6 Examples of compounds: H 2 O, CH 4, Al 2 O 3, NaCl 3.1

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5 Molecular mass (or molecular weight) is the sum of the average atomic masses (in amu) in a molecule. SO 2 1S 2O SO amu + 2 x amu amu For any molecule molecular mass (amu) = molar mass (grams) 1 molecule SO 2 = amu 1 mole SO 2 = g SO 2 3.1

6 Do You Understand Molecular Mass? How many H atoms are in 72.5 g of C 3 H 8 O? 1 mol C 3 H 8 O = (3 x 12) + (8 x 1) + 16 = 60 g C 3 H 8 O 1 mol C 3 H 8 O molecules = 8 mol H atoms 1 mol H = x atoms H 72.5 g C 3 H 8 O 1 mol C 3H 8 O x 60 g C 3 H 8 O x 8 mol H atoms 1 mol C 3 H 8 O x H atoms x = 1 mol H atoms 5.82 x atoms H 3.1

7 Organic vs. Inorganic Compounds Organic compounds are carbon-containing substances Do not include carbon-oxides, e.g., carbon dioxide (CO2), carbon monoxide (CO) or carbonate (CO3 2 ) million known organic compounds Carbon can concatenate (C C C C C C C C C) Inorganic compounds are substances based on all the other elements. ~500,000 known inorganic compounds 3.1

8 Ionic Compounds Compounds composed of ions Ionic compounds don t exist as molecules. Exist as 3-dimensional networks of ions: crystal lattice A monatomic ion contains only one atom Na +, Cl, Ca 2+, O 2, Al 3+, N 3 A polyatomic ion contains more than one atom OH, CN, NH 4 +, NO 3 3.2

9 Examples of Monatomic Ions 3.2

10 Do You Understand Ions? How many protons and electrons are in Al? 13 protons, 10 (13 3) electrons For cations subtract the charge from the number of protons How many protons and electrons are in Se? protons, 36 (34 + 2) electrons For anions add the charge to the number of protons

11 ionic compounds consist of a combination of cations and anions the formula is always the same as the empirical formula the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl 3.2

12 Formulas of Ionic Compounds 2 x +3 = +6 3 x -2 = -6 Al 2 O 3 Al 3+ O 2-1 x +2 = +2 2 x -1 = -2 CaBr 2 Ca 2+ Br 1 x +2 = +2 1 x -2 = -2 Na 2 CO 3 Na + CO

13 A molecular formula shows the exact number of atoms of each element in the smallest unit of a molecule. An empirical formula shows the simplest whole-number ratio of the atoms in a substance molecular H 2 O C 6 H 12 O 6 empirical H 2 O CH 2 O O 3 O N 2 H 4 NH 2 Formulas for ionic compounds always reflect their empirical formulas. 3.2

14 Hydrated Compounds Definition: Ionic compounds (usually) that have a specific number of water molecules trapped in their crystal lattice. CuSO 4 5 H 2 O copper sulfate pentahydrate Anhydrous (anhydride) - a hydrate with the water molecules driven off (usually upon heating) CuSO 4 copper sulfate Molecular mass of hydrates: Include the mass of water given in the formula. 3.2

15 Covalent Compounds An atomic force microscopy image of a graphene molecule A ball-and-stick model of graphene Image credit: IBM Research 3.3

16 Covalent Compounds Electrons are shared between 2 atoms in a covalent bond. Do not give up or take electrons - no ions formed Usually between 2 non-metals, or a non-metal and metaloid Formulas for covalent compounds are molecular formulas. Examples: NH3 H2O CO2 ammonia water (dihydrogen oxide) carbon dioxide 3.3

17 Naming Compounds Organic Compounds: Contain carbon (except oxides, sulfides, cyanide) Inorganic Compounds: Everything else, divided into 4 classes Ionic compounds Molecular compounds Acids and Bases Hydrates 3.4

18 Chemical Nomenclature Ionic Compounds often a metal + nonmetal (binary compounds) anion (nonmetal), add ide to element name BaCl 2 K 2 O MgCl 2 K 3 N barium chloride potassium oxide magnesium chloride potassium nitride 3.4

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20 Naming Polyatomic Ions: 8 -ates + 5 Names for all the polyatomic ions and the acids derived from them can be derived from the names of 12 core ions: 8 -ates ates + 5 carbonate CO 2 3 chlorate ClO 3 chromate CrO 2 4 dichromate Cr 2 O 2 7 nitrate NO 3 phosphate PO 3 4 manganate MnO 3 sulfate SO 2 4 cyanide CN thiocyanate SCN hydroxide OH peroxide O2 ammonium NH 4 + per- -ate hypo- -ite Hydrogen in front of an -ate ion is named hydrogen -ate, except sulfate, which is bisulfate.

21 Naming Ionic Compounds With Polyatomic Ions sometimes a metal + polyatomic ion Use the metal and the polyatomic ion naming rules Ba(ClO 2 ) 2 KNO 3 Mg(OH) 2 KClO 4 barium chlorite potassium nitrate magnesium hydroxide potassium perchlorate 3.4

22 Naming Ionic Compounds Transition metal ionic compounds have variable charges Iron can be +2 or +3 Smaller charge is sometimes named as an ic ion higher charge as an ous ion So in ferric chloride (FeCl2) iron ion is Fe 2+ Modern method is to indicate charge on the metal with Roman numerals So FeCl2 is now named iron(ii) chloride FeCl 2 2 Cl - are -2 so Fe is +2 iron(ii) chloride FeCl 3 3 Cl - are -3 so Fe is +3 iron(iii) chloride Cr 2 S 3 3 S -2 are -6 so Cr is +3 (6/2) chromium(iii) sulfide 3.4

23 Covalent compounds Between 2 (or more) nonmetals or nonmetals + metalloids Naming: Some have common names: H 2 O, NH 3, CH 4, C 60 Otherwise: Element further left in periodic table is 1 st Element closest to bottom of group is 1 st Last element ends in ide (for inorganics) If more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom 3.4

24 Covalent Compounds HI NF 3 SO 2 N 2 Cl 4 hydrogen iodide nitrogen trifluoride sulfur dioxide dinitrogen tetrachloride NO 2 nitrogen dioxide TOXIC! N 2 O dinitrogen monoxide Laughing Gas 3.4

25 Exceptions: Molecular Compounds B 2 H 6 CH 4 SiH 4 NH 3 PH 3 H 2 O H 2 S diborane methane silane ammonia phosphine water hydrogen sulfide 3.4

26 Naming Compounds Compound Ionic Metal or NH4 + plus mono or polyatomic anion Molecular Binary compounds of nonmetals or nonmetals and metalloids Yes Alkali metal ions Alkaline earth metal ions Ag +, Al 3+, Cd 2+, Zn 2+ Name metal ion first, then anion If monatomic anion, add ide to the end of the root of the element name If polyatomic ion, use 8 -ates + 4 rules Cation has only one charge No Transition metal cations Name metal ion first Specify charge on cation with a Roman numberal If monatomic anion, add ide to the end of the root of the element name If polyatomic ion, use 8 -ates + 4 rules Name element furthest left or furthest down in a group first. Add ide to the end of the root of the second element s name Use prefixes to designate the numbers of each element present The mono prefix is usually omitted 3.4

27 Practice Name the following compounds: SiCl4 P4O10 What are the formulas for the following compounds? carbon dioxide disilicon hexabromide 3.4

28 Acids and Bases An acid can be defined as a substance that yields hydrogen ions (H + ) when dissolved in water. HCl Pure substance, hydrogen chloride Dissolved in water (H + Cl - ), hydrochloric acid Binary acids consist of hydrogen and one other atom: Examples: HF, HCl, HBr, HI, H2S 3.4

29 Naming Binary Acids -ides are -ic -y 3.4

30 An oxoacid is an acid that contains hydrogen, oxygen, and another element. HNO 3 H 2 CO 3 nitric acid carbonic acid H 2 SO 4 sulfuric acid HNO 3 -ates are -ic -y and -ites -ous 3.4

31 Naming Oxo-acids Per ate Add H + Per ic acid ate Add H + ic acid Representative -ic acid ite Add H + ous acid hypo ite Add H + hypo ous 3.4

32 Worked Examples

33 Acids and Bases A base can be defined as a substance that yields hydroxide ions (OH ) when dissolved in water. NaOH KOH Ba(OH) 2 sodium hydroxide potassium hydroxide barium hydroxide Name the cation, then add hydroxide Don t use prefixes with hydroxide 3.4

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