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Prep Chem

04/25/09

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Compounds


�Before You Begin:

To master this material you need to know:

  • the difference between an element and a compound from the matter section
  • how to write atomic symbols from the atomic theory section
  • how to determine the common charges for the representative elements from the periodic law section

 

Compounds are two or more elements combined chemically to form a new substance.

Ionic Compounds

Description:

Ionic compounds are composed of positively and negatively charged ions and are held together by electrostatic attractions. Ionic compounds do not contain molecules. Instead, because opposite charges attract one another, the ions stack themselves in crystals such that positive ions are surrounded by negative ions, and vice versa.

Identification:

Students can identify a compound as being ionic from either the name or the formula. Metals form positive ions relatively easily, and non-metals tend to form negative ions. Hydrogen, H+1, and ammonium ion, NH4+1, are the only common positive ions that do not contain a metal. To identify a compound as being ionic, scan the name or the formula and identify the elements. If the compound contains a metal (or the ammonium ion) and at least one non-metal, it is almost always an ionic compound. Compounds that contain the hydrogen ion are acids, which we will discuss separately.

Nomenclature:

To name ionic compounds, name the cation then the anion. Monatomic cations have the same name as the element from which they form. For example, Ca+2 is calcium ion. The names of the simple monatomic anions are similar to the names of their element but end in an –ide suffix. S-2 is sulphide ion. The name of the ionic compound containing Ca2+ and S-2 is calcium (II) sulfide. See the periodic table section for common charges of monatomic ions.

Some metals can form more than one ion. The names of these ions include a Roman numeral in parenthesis to indicate the charge. For example, Cu+1 is copper (I) ion and Cu+2 is copper (II) ion. The name of the ionic compound containing Cu+1 and O-2 is copper (I) oxide. There is an older style of nomenclature which uses the Latin form of the metal name and a suffix, the higher charged ion has an –ic ending and the lower charged ion has an –ous ending. This means that Cu+1 is also called cuprous ion. This leads to some difficulties (not just learning the Latin), because the higher/lower charges might be +2/+1 or +2/+3 or some other combination. The Roman numeral designation always represents the charge of the ion (except for the polyatomic mercury ion Hg2+2 which is mercury (I)—the exceptions are annoying, aren’t they?). The metals that only form one type of ion do not need a Roman numeral in their names, but, when in doubt, put one in. It is not wrong, merely redundant.  

An ion may have more than one atom joined together by bonds to form a single charged unit. These are called polyatomic ions. While there are certain rules governing the names of polyatomic anions, there are so many exceptions that the beginning student is better off just memorizing them. You will see subscripts in the formulas for many polyatomic ions. These represent the number of atoms of the element immediately preceding the subscript:  NO31- is nitrate ion and it has one nitrogen atom and three oxygen atoms bonded together to form an ion with a negative one charge. Note that you must learn the formulae, charges, and names of the common ions. Some of the most common are listed in the table below.

The name of an ionic compound is the name of the cation and anion. For example, CuSO3 is copper (II) sulfite because, for the compound to be neutral, the copper ion must have a charge of 2+ to match the charge of the sulfite ion, 2-. Cu2S is copper(I) sulphide because, for the compound to be neutral, the copper ion must have a charge of 1+ for two of them to balance the charge of the sulphide ion, 2-. Ca(NO3)2 is calcium nitrate; a Roman numeral is not needed because calcium has only one possible ion. Note that the nitrate is enclosed in parenthesis and given a subscript of 2. This indicates that there are two nitrate ions needed for every one calcium ion, for the compound to be neutral.

 

Common Polyatomic Ions

Charge

Formula

Name

+1

NH4+1

Ammonium

+2

Hg2+2

Mercury (I)

-1

CN-1

OH-1

C2H3O2-1

NO3-1

NO2-1

MnO4-1

ClO-1

ClO2-1

ClO3-1

ClO4-1

HCO3-1

H2PO4-1

Cyanide

Hydroxide

Acetate

Nitrate

Nitrite

Permanganate

Hypochlorite

Chlorite

Chlorate

Perchlorate

Hydrogen carbonate

Dihydrogen phosphate

 

-2

CO3-2

CrO4-2

Cr2O7-2

SO4-2

SO3-2

HPO4-2

Carbonate

Chromate

Dichromate

Sulphate

Sulfite

Hydrogen phosphate

 

-3

PO4-3

Phosphate

 

 

Formulae:

To write the formula of an ionic compound from its name, identify the ions, write their symbols, and use subscripts to make the compound neutral. Iron (III) sulfate contains the iron (III) ion and the sulfate ion. The symbol for iron (III) is Fe3+, and the symbol for sulfate is SO42-. These charges do not balance in a one to one ratio, so the compound must have multiples of the ions in order to be neutral. If there are two iron (III) ions for every three sulfate ions, the charges will be six positive and six negative, which balance. This gives the ionic formula Fe2(SO4)3. The parentheses are needed to make it clear that this compound has three parts sulfate ion rather than 43 parts oxygen.

 


4Concept Check:  What is the name of the compound with the formula Cu3(PO4)2? What is the formula for potassium dichromate?

Answer: Cu3(PO4)2 is copper (II) phosphate. The formula for potassium dichromate is K2Cr2O7.


 

Molecular Compounds

Description:

Molecular compounds are composed of molecules in which atoms share electrons via bonds. The chemistry of molecular compounds is varied and complicated, far more so than ionic compounds. For this reason (plus some tradition and a pinch of inertia), most of the content of a general chemistry course involves ionic compounds.

Identification:

Most molecular compounds contain two or more non-metals (but not the ammonium ion). While there are exceptions, the absence of metals signals the beginning chemistry student that the compound is molecular. While many molecular compounds contain hydrogen, if the formula of a compound has hydrogen written first, assume it is an acid rather than a molecular compound.

Nomenclature:

The only type of molecular compound a beginning student is expected to name is the binary molecular compound. These contain two non-metallic elements (but not carbon with hydrogen; hydrocarbons have their own special nomenclature). Many binary molecular compounds have common names. For example, H2O is water and NH3 is ammonia. You just have to memorize these. There is also an older system of naming binary compounds that contain oxygen. For example, N2O is nitrous oxide and NO is nitric oxide. You may need to memorize a list of these compounds early in the course, but that depends on your instructor.

You will be expected to know how to binary molecular compounds. Unlike ionic compounds, the elements in a molecular can combine with a variety of ratios. We use prefixes to indicate the number of atoms of each element.

Prefixes for Molecular Compounds
number prefix number prefix

1

mono-

2

di-

3

tri-

4

tetra-

5

penta-

6

hexa-

7

hepta-

8

octa-

9

nona-

10

deca-

 

To name the compound using the formula, name the first element then the second element giving it an –ide ending. For example, the compound PCl3 is phosphorus trichloride, and the compound SO2 is sulfur dioxide.

Formulae:

A student can write the formula of a binary molecular compound from its name by translating the prefixes into subscripts. Given the name, write the symbol for the first element followed by the symbol for the second element. Translate the prefixes into subscripts. For example, SeCl2 is selenium dichloride and PF5 is phosphorus pentafluoride.

!Warning! It is very common for beginning chemistry students to mix up the system for naming ionic compounds with the system for naming binary molecular compounds. Some students write the formula for binary molecular compound then attempt to make the charges balance. They then get frustrated because both elements seem to have negative charges and they can’t get the compound neutral! Another common mistake is to become fond of the prefixes used for binary molecular compounds and use that system with ionic compounds. Some of the polyatomic ions have names that seem similar to these prefixes. Chromate ion is CrO4-2 and dichromate ion is Cr2O72-. Sn(CrO4)2 is tin(IV) chromate NOT tin dichromate. There are two tin dichromates: tin(II) dichromate is SnCr2O7, and tin(IV) dichromate is Sn(Cr2O7)2.

 


4Concept Check: What is the name of the compound that has the formula XeF6? What is the formula for sulfur dioxide?

Answer: XeF6 is xenon hexafluoride. Sulfur dioxide has the formula SO2.


 

Acids

Description:

Acids are substances that react with water to form an aqueous hydronium ion, H+1(aq). These compounds have characteristics in common with both ionic and molecular compounds. They are studied extensively in General Chemistry, in part, because they are useful compounds and, in part, because they are very good examples of equilibria, the topic studied for about a third of the year of a typical General Chemistry course.

Identification:

There is more than one way to define an acid. Unfortunately, compounds that fit one definition might not be considered acids by another definition. In the broadest and most general sense, acids are compounds similar to ionic compounds but with H+1 as their positive ions. This will be written first in the formula. Acid names end with the word ‘acid’ making them easy to identify from the name.

Nomenclature:

There are two categories of acids: binary acids and oxyacids.

Binary Acids: these compounds contain hydrogen and one other non-metal. To name a binary acid, identify the other element. Use that element’s name as the root in the format ‘hydro____ic acid.’ For example, HBr is hydrobromic acid and H2Se is hydroselenic acid.

Oxyacids: these compounds contain hydrogen, oxygen, and at least one other element. The non-hydrogen part of the compound will be an anion, usually from the list in the ionic nomenclature section above. To name an oxyacid, identify the name of the anion. If the ending of that ion is –ate, change it to –ic. If the ending of that ion is ite, change it to –ous. End the name with the word ‘acid.’ For example, H2SO4 is sulfuric acid, and HNO2 is nitrous acid. Note that these names do not include the prefix ‘hydro’ like the binary acids.

Formulae:

When given the name of an acid, you must first identify the negative ion. If the acid name ends in ‘-ic’ the negative ion is a simple, monatomic ion and you can determine its charge from its location on the periodic table. If the acid name ends in –ic, it contains a negative ion whose name ends in –ate. If the acid name ends in –ous, it contains a negative ion whose name ends in –ite. Write H+1 as the positive ion followed by the negative ion. Use subscripts to make the compound neutral. For example, hydrochloric acid is HCl, chromic acid is H2CrO4, and hypoclorous acid is HClO. Note that this last example contains the hypochlorite ion rather than having a misspelled binary acid prefix.


4Concept Check: What is the name of the acid with the formula H3PO4? What is the formula for acetic acid?

Answer: Phosphoric acid is the name of the acid with the formula H3PO4. Acetic acid has a formula of HC2H3O2.


 

 

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