What is the major difference between a molecular formula and formula unit?

Transcription

1 Lecture 5 Molecular Substances versus Ionic Substances Molecular Substance Ionic Substance What are some examples of molecular substances? What are some examples of ionic substances? What is the major difference between a molecular formula and formula unit? How does this molecular picture relate to the macromolecular properties (bp, mp) of ions and molecules? Chemical Nomenclature While substances can be described by their chemical or molecular formulas, it is also important to be able to give them formal names. This systematic naming of chemical compounds is known as the chemical nomenclature. Naming Compounds In this course, we will focus only on the nomenclature of ionic compounds and binary molecular compounds. 1. Ionic Compounds 2. Molecular Compounds (Binary)

2 1. Nomeclature of Ionic Compounds Recall, ionic compounds consist of a lattice of cations and anions. cations positively charged ions anions negatively charged ions Na + + Cl cation + anion In naming an ionic compound, we write the Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + (cation part) followed by (anion part) Either as a symbol, or as a name, NaCl sodium chloride The challenge is learning the prefixes and suffixes associated with the name. sodium > sodium chlorine > chloride (ide) Cation Part For simple cations (alkali and alkaline earth metals), its simply the name of the neutral atom or molecule. Examples: Monatomic ions Na + Sodium ion + NH 4 Li + Lithium ion CH 3 COO Mg 2+ Magnesium Ca 2+ Calcium 2 CO 3 Polyatomic ions Ammonium Acetate Carbonate

3 For transition elements (elements in the middle of the periodic table), however, the cation can have different charges. The most common is +2 and +3. Example Fe +2, Mn +2, Co +2 FeCl 2, MnCl 2, CoCl 2 Fe +3, Mn +3, Co +3 FeCl 3, MnCl 3, CoCl 3 Some transition elements favor +1 and +2 charges, however. Ni +, Cu + NiCl, CuCl 2 Ni +2, Cu +2 NiCl 2, CuCl 2 Stock system In order to distinguish these charges, the Stock system adds a Roman numeral after the name of the monatomic cation. Example Fe +2 > iron(ii) > read as irontwo Fe +3 > iron(iii) > read as ironthree Older system (ous vs ic) An older system for naming cation is to use the Latin. Here one adds either an ous or ic. ous denotes the lower charge ic denotes the higher charge Iron Manganese Copper Fe +2 ferrous Mn +2 manganous Cu + cuprous Fe +3 ferric Mn +3 manganic Cu +2 cupric

4 Anion Part The nomenclature of the anion is much more difficult. In all cases a suffix is required, and in some cases a prefix. Monatomic anions Naming monatomic anions (anions containing one element) is quite simple. Take the root and add an ide. Monatomic Anions Element Anion Symbol Chlorine Chloride Cl Bromine Bromide Br Oxygen Oxide O 2 Polyatomic anions Polyatomic anions are trickier. We will only discuss one class of polyatomic anion known as oxoanions which consists of anion formed from one or more oxygens coordinated to another element. Examples Polyatomic Anions Carbonate 2 CO 3 Perchlorate ClO 4 Chlorate ClO 3 Sulfate 2 SO 4 Chlorite ClO 2 Sulfite 2 SO 3 Hypochlorite ClO For oxoanions, the number of oxygens can vary. We use different suffixes to denote the relative numbers of oxygens. In general, we use the ate suffix. CO 3 2 carbonate Polyatomic Anions with two different forms When there are two forms of an oxoanion, we use the terms ite and ate.

5 ite denotes the lower number of oxygens ate denotes the higher number of oxygens NO 2 NO 3 nitrite nitrate 3 PO 3 3 PO 4 phosphite phosphate 2 SO 3 2 SO 4 sulfite sulfate 3 AsO 3 3 AsO 4 arsenite arsenate 2 SeO 3 2 SeO 4 selenite selenate What would you predict for antimony (Sb) and tellurium? Polyatomic Anions with four different forms In some cases there are four different forms of a given type of oxoanion. To denote these we add the prefixes hypo and per. hypo( )ite denotes the lowest # of oxygens ite denotes a lower # of oxygens ate denotes a higher # of oxygens per( )ate denotes the highest number of oxygens. ClO ClO 2 ClO 3 ClO 4 hypochlorite chlorite chlorate perchlorate How do you know how many different oxoanion forms there are?

6 Trends in the Periodic Table NO 2 NO 3 Group VA Chalcogens Halogens nitrite nitrate 3 PO 3 3 PO 4 phosphite 2 SO 3 phosphate 2 SO 4 sulfite ClO hypochlorite sulfate ClO 2 chlorite ClO 3 chlorate ClO 4 perchlorate 3 AsO 3 3 AsO 4 arsenite 2 SeO 3 arsenate 2 SeO 4 selenite BrO hypobromite selenate BrO 2 bromite BrO 3 bromate BrO 4 perbromate 3 SbO 3 3 SbO 4 antimonite 2 TeO 3 antimonate 2 TeO 4 tellurite IO hypoiodite tellurate IO 2 iodite IO 3 iodate IO 4 periodate Structures of oxoanions Cl O O Cl O O O Cl O O O Cl O O Example 1. Name the following ionic compounds: Na 2 SO 4 NaClO 4 MnCl 2 (ous/ic) CuCl 2 (ous/ic)

7 Example 2. Provide the formula unit for the following ionic compounds: Sodium Selenate Potassium hypobromite Ferric Chloride Ferrous Chloride Copper(II) chloride Nomenclature of Acids and Bases Acids are a special class of compound formed from a proton and an anion. H + + anion > acid The anion associated with this acid is known as the base or conjugate base. Examples: Name Acid Base(anion) hydrochloric acid HCl Cl nitric acid HNO 3 NO 3 sulfuric acid H 2 SO 4 HSO 2 4 or SO 4 Acids with monoatomic anions Example HCl, HBr, H 2 O These are generally molecular compounds and will be discussed later. Acids with oxoanions An acid formed from a proton and an oxoanion is known as an oxoacid. H + + oxoanion > oxoacid

8 The nomenclature for naming oxoacids is to take the stem name and add an ic. If there is a form with a lower number of oxygens, that lower oxygen containing acid is given the suffix ous. HNO 3 HNO 2 nitric acid nitrous acid If there are additional oxoacid forms, the prefixes hypo and per are added. Note the similarity to the naming of anions. Acid Acid Name Anion Anion Name HClO hypochlorous ClO hypochlorite HClO 2 chlorous acid ClO 2 chlorite HClO 3 chloric acid ClO 3 chlorate HClO 4 perchloric acid ClO 4 perchlorate Hence the relationship between the name of the acid and anion is the suffix ous to ite and ic to ate. Oxoacid ous ic Oxoanion ite ate Nomenclature of Acid Anions Some acids can lose more than one proton. Anions which contain a labile proton are known as acid anions. The name of the acid anion is given by Example (prefix)hydrogen + anion name Sulfuric acid. H 2 SO 4, has one acid anion HSO 4 called hydrogen sulfate(or bisulfate). H 2 SO 4 > H + + HSO 4 HSO 4 > H + + SO 4 2 H 3 PO 4 phosphoric acid has two forms which are acid anions:

9 H 2 PO 4 dihydrogen phosphate HPO 4 2 monohydrogen phosphate Note that the prefix before the hydrogen indicates the number of hydrogens and that the name of the cation complexed with acid anions remains the same. Hence, the name of salts containing acid anions are of the form cation (prefix)hydrogen + anion name Na(H 2 PO 4 ) Na 2 (HPO 4 ) sodium dihydrogen phosphate sodium monohydrogen phosphate Molecular Compounds (Binary) Recall, molecular compounds are made up of discrete molecules. For simplicity, we will only discuss the nomenclature of molecular compounds which contain only two elements. These are known as binary molecular compounds. The nomenclature of binary molecular compounds is quite easy. (name element 1) (name element 2 with ide) Examples: HCl CO 2 Hydrogen chloride Carbon dioxide How do you know which element comes first? The elemental symbols in the formula of a binary molecular compound listed by group numbers. III A IV A V A H VI A VII A B Si,C Sb,As,P,N H Te,Se,S F,Cl,Br,I

10 The names of the elements in a binary molecular compound are listed in the same order. CCl 4 Carbon tetrachloride When different ratios of elements are possible, prefixes such as mono and di which indicate the exact number of each atom. Occasionally the mono is omitted. Number Prefix 1 mono 2 di 3 tri 4 tetra 5 penta 6 hexa 7 hepta 8 octa 9 nona 10 deca Name the following binary compounds: CO CO 2 NO 2 N 2 O 4 Name the compound formed from three fluorines and a boron. Name the compound formed from three chlorines and a phosphorus. Binary Compounds of Hydrogen and Nonmetals Some binary compounds of hydrogen and nonmetals form acids when dissolved in water. When dissolved in solution it dissociates into ions. HCl(g) > H + (aq) + Cl (aq)

11 To denote this property, when these compounds are dissolved in water their name is given the prefix hydro and suffix ic. Molecular compound Dissolved in water HCl Hydrogen chloride Hydrochloric acid HBr Hydrogen bromide Hydrobromic acid Hydrates Hydrates are compounds that contain water molecule weakly bound in the crystal lattice. Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + Cl Na + When we write the chemical formula of a hydrated compound we denote this with a nh 2 O where n is the number of water molecules. For CuSO 4 crystals grown from water, five molecules of water per CuSO 4 unit are in the crystal lattice. The chemical formula is denoted by CuSO 4 5H 2 O. The chemical name of a hydrated compound is denoted by the name of compound + prefixhydrate Here, the prefix indicates the number of water molecules present. Example CuSO 4 CuSO 4 5H 2 O copper sulfate copper sulfate pentahydrate

12 Examples FeCl 3 6H 2 O (Stock) HCl (gas) CuCl (ous/ic) HCl (in water) Magnesium phosphate NaClO sodium hydrogen phosphate NH 4 ClO 4