Compounds and their formulas

Transcription

1 Compounds and their formulas Molecular Compounds A molecular compound is a substance made up of discreet entities called molecules. A molecule consists of a small number of non-metal atoms held together by covalent bonds, or sharing electrons. We can represent the molecule by its chemical formula. The chemical formula is a comprised of the symbols of the elements present and the relative number of each element. An empirical formula is a representation that shows only the lowest whole number ratio of the elements present. The molecular formula is based on the actual molecule. For example the empirical formula of a sugar is C 2 O. The molecular formula is some whole number multiple of this (i.e., C 5 10 O 5 or C 6 12 O 6 ). A structural formula is a formula that indicates how the atoms are attached to each other. This formula uses lines to denote the bonds between atoms. A condensed structural formula is a simplified version of the structural formula. For example, an alcohol is represented as C 3 C 2 O. In this formula, the hydrogens that follow the carbons are attached to those carbons, and the oxygen is attached to the second carbon. A structural formula for this looks like: C C O We need to remember that all of these molecules, tiny as they may be, take up space. They also have a three-dimensional shape which these formulas do not indicate. We can represent these aspects of molecules by building models of them. There are several different kinds of models such as ball-and-stick and space filling models. We will look at these later in this course. Ionic Compounds Compounds of metals and nonmetals are usually ionic compounds. These compounds are comprised of positive and negative ions. Positive ions are called cations and negative ions are anions. There is not an actual bond between the ions. The attraction of positive and negative charges holds these compounds together. The chemical formulas do not show the actual number of atoms in each molecule because there are no molecules. The formulas show the ratio of the ions

2 in the compound. For example, in Sodium Chloride, NaCl, there is one Sodium ion for every Chloride ion. This is called the formula unit of the compound. Molecular mass: The sum of the atomic masses of all the atoms in a molecule of a substance. Formula mass: The sum of the atomic masses of all the atoms in a formula unit of a substance. For molecules: Molecular weight and formula weight are the same. For ionic compounds: Only the term formula weight applies. Molar Mass: The mass of one mole of substance. The molar mass in g/mol is numerically equivalent to the molecular or formula mass in amu. Examples: 1. ow many formula units are in g of Lead(II) Oxide? The formula for Lead(II) Oxide is PbO which results in a molar mass of g ( g g) g PbO 1mol PbO g PbO formula units PbO 1mol PbO = formula units PbO molecules of water has what mass in grams? Water has the formula 2 O so it has a molar mass of g ( g g). 22 1mole 2O g 2O molecules 2O = 1.02 g 2O molecules O 1mole O Remember that one mole of the elements of ydrogen, Nitrogen, Oxygen, Fluorine, Chlorine, Bromine, Iodine, Phosphorus and Sulfur are not in the form of atoms. The elements exist as molecules: N 2 O 2 F 2 Cl 2 Br 2 I 2 P 4 S 8

3 Mass percentages: The percentage by mass of all the elements in a compound. mass of A in the sample mass % ofa = 100% mass of the sample The sum of the percentages for a compound must equal 100%. Determining chemical formulas Empirical formula: a formula for a compound that shows the lowest whole number ratio of the elements in the compound. Empirical formula for water: 2 O Empirical formula for hydrogen peroxide: O ( 2 O 2 ) Empirical formula for glucose: C 2 O (C 6 12 O 6 ) Empirical formula for ribose: C 2 O (C 5 10 O 5 ) Determining the empirical formula from mass percentages 1. Assume g of compound. This converts the percentages directly to masses of the elements. 2. Convert the mass of each element to moles of that element. 3. Divide each of the moles by the smallest. 4. If you do not get a series of integers from step 3. Multiply each of the values by a factor to get everything to integers (±0.1). This factor will usually be about 2 or 3, but it may be larger. Determining molecular formulas: 1. Find the empirical formula. 2. Find the empirical mass (the mass of one empirical formula unit). 3. Divide the molar mass by the empirical mass. This should be an integer. 4. Multiply this integer by all the subscripts in the formula.

4 For compounds containing the elements C,, and O, the percentages can be determined by combustion. Combustion itself gives carbon dioxide, which gives the percentage of C, and water, which gives the percentage of. Percentage of O is determined by difference. Example: Therefore the empirical formula is C 3 6 O. If the molar mass of this compound is g mol -1, what is the molecular formula? Molar mass Empirical mass g mol = g mol -1-1 = 2 So the molecular formula is twice the empirical formula, C 6 12 O 2.

5 Oxidation States A useful concept in describing compounds and their changes is the concept of the oxidation state. The oxidation state is just an indication of the number of electrons lost (oxidized) or gained (reduced) by an element in a compound. This is a hypothetical charge that the element would have if it were in a purely ionic compound. This means that if we have a purely ionic compound comprised of monatomic ions, the oxidation state is the same as the charge on the ion. If the compound contains polyatomic ions or is a molecular compound, the oxidation state is the charge the element would have if it were actually an ionic compound. We have some rules to help us to determine the oxidation state of atoms. These are : 1. The oxidation number of an atom in an element is zero. 2. The oxidation number of an atom in an ionic compound is equal to its charge. 3. The oxidation number of oxygen is -2. Exceptions: In peroxides it is -1 and in compounds with fluorine it is The oxidation number of hydrogen is +1 unless it is combined with at metal then it is The oxidation number of fluorine in its compounds is -1. The oxidation number of other halogens is -1 unless it is combined with oxygen or fluorine. 6. The sum of the oxidation numbers in a compound must equal zero. In polyatomic ions it must equal the charge on the ion. Because oxidation states are not really charges, we can have fractional values for the oxidation state.

6 Nomenclature Naming of compounds Ionic Compounds To name ionic compounds you just name the ions that are present. The name of the positive ion (cation) is first followed by the name of the negative ion (anion) Naming monatomic ions. 1. Metal ions are named the same as the metal, if there is only one ion. 2. If there is more than one charge on the ion, the ion is named with the metal name with the charge written after it in Roman numerals in parenthesis. An older system of naming metals that form more than one kind of ion involves the use of the prefixes ous and -ic. The stem is derived from the name of the metal if the symbol is not from the Latin name. If the symbol comes from the Latin name the stem is from the Latin name of the metal. Fe 2+ Iron(II) or Ferrous Cu + Copper(I) or Cuprous Fe 3+ Iron(III) or Ferric Cu 2+ Copper(II) or Cupric Au + Gold(I) or Aurous Sn 2+ Tin(II) or Stannous Au 3+ Gold(III) or Auric Sn 4+ Tin(IV) or Stannic Pb 2+ Lead(II) or Plumbous g 2+ 2 Mercury(I) or Mercurous Pb 4+ Lead(IV) or Plumbic g 2+ Mercury(II) or Mercuric 3. Non-metals: stem name + -ide. Ionic compounds can also contain polyatomic ions. These are groups of atoms that are bonded together that have a charge. A list of these ions is provided in your text and in class. This list must be memorized. You need to know the name, formula and charge of all of the polyatomic ions in the list. Among the polyatomic ions there are a couple of patterns. If there are two ions with similar names (e.g., sulfate and sulfite), the one whose name ends in ate has 1 more oxygen than the one whose name ends in ite. This is can be extended to other ions. If the ion has 1 more oxygen than the ate ion, it is given the prefix - - per- (e.g., perchlorate, ClO 4, chlorate, ClO3 ). If the ion has one less oxygen than the ite ion, it is given the prefix hypo- (e.g., hypochlorite, ClO - -, chlorite, ClO 2 ).

7 Also, if an oxygen atom is replaces with a sulfur atom the prefix thio- is added to the name (e.g., CNO -, cyanate, CNS -, thiocyanate). The periodic law can also be applied to these patterns. For example ClO 3 - is the chlorate ion and BrO 3 - is the bromate ion. To get the formula for an ionic compound between any two elements we need to know the charges on the ions. What is the charge on the ions? 1. Main-group metals: charge = group number. 2. Main-group metal with high atomic number can have more than one charge. Usually the other charge is the group number minus Most transition metals have more than one charge. Most have an ion with a charge of +2. These charges have to be determined from the formula. 4. Non-metals have a charge of the group number minus 8. The total positive charge must equal the total negative charge in the compound. This can be done by crossing over the charges. Rb + and O 2- cross over Rb 2 O Ca 2+ and PO 4 3- cross over Ca 3 (PO 4 ) 2

8 Naming binary molecular compounds Order of elements in the formula B Si C Sb As P N Te Se S I Br Cl O F (by convention) Rules for naming 1. Name elements in order given above. 2. First element named is given the name of the element with the appropriate prefix see number Name of second element: stem name + -ide. 4. Prefixes used for compounds containing more than one of the same type of element. Mono- not used unless it is needed to distinguish two compounds of the same two elements. Prefixes 1 mono- 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 7 hepta- 8 octa- 9 nona- 10 deca-

9 Exceptions to the above rule for binary molecular compounds Some compounds that have been known for hundreds or thousands of years do not follow this naming system. Others, such as organic compounds, follow a different system altogether. The compounds you are required to know the common names of are: 2 O Water N 3 Ammonia N 2 4 ydrazine P 3 Phosphine As 3 Arsine Naming acids Binary acids. ydro- stem name of anion -ic acid. Cl Br ydrochloric acid ydrobromic acid Oxoacids are acids that contain oxygen. Anion suffix -ite -ate acid suffix -ous -ic ydrates NO YDRO- PREFIX!!!! ydrates are ionic compounds with molecules of water associated with them. The prefixes used with molecular compounds are used to denote the number of water molecules present. CuSO 4 ½ 5 2 O (5 molecules of water for every Copper(II) Sulfate) This is named as the name of the ionic compound + prefix-hydrate: Copper(II) Sulfate Pentahydrate.