Chemical Formulas Review.doc

Chemical Formulas Review: Nomenclature and Formula Writing

Naming Simple Compounds

There are four naming systems you should familiarize yourself with to succeed on the SAT II Chemistry exam. The trick is recognizing which naming system to use. Here are the guidelines:

·         If the compound starts with H, it is an acid. Use the naming acids rules.

·         If the compound starts with C and contains quite a few H’s and perhaps some O’s, it is organic. Use the naming organic compounds rules.

·         If the compound starts with a metal, it is most likely ionic. Use thenaming binary ionic compounds rules.

·         If the compound starts with a nonmetal other than H or C, use thenaming binary molecular compounds rules.

It is also essential that you memorize some common polyatomic ions. Polyatomic ions behave as a unit. If you need more than one of them, enclose them in parentheses when you write formulas. You need to know their names, formulas, and charges. If you learn the nine that follow, you can get many others from applying two simple patterns.

·         Pattern 1: The -ates “ate” one more oxygen than the -ites and their charge doesn’t change as a result! For instance, if you know nitrate is NO3-,then nitrite is NO2-.If you know phosphate is PO43-,then you know phosphite is PO33-.You can also use the prefixes hypo- and per- with the chlorate series. Perchlorate, ClO4-,was really “hyper and ate yet another oxygen” when compared to chlorate, ClO3-.Hypochlorite is a double whammy: it is -ite and therefore “ate” one less oxygen than chlorate and it is hypo-, which means “below,” so it “ate” even one less oxygen than plain chlorite, so its formula is ClO-. You can also substitute the other halogens for Cl and make additional sets of the series.

·         Pattern 2: The -ates with charges less than negative 1 (that is, ions with charges of -2, -3, etc.) can have an H added to them to form new polyatomic ions. For each H added, the charge is increased by a +1. For instance, CO32-can have an H added and become HCO3-.HCO3-is called either the bicarbonate ionor the hydrogen carbonate ion. Since phosphate is -3, it can add one or two hydrogens to make two new polyatomic ions, HPO42-and H2PO4-.These are named hydrogen phosphate and dihydrogen phosphate, respectively. If you keep adding hydrogen ions until you reach neutral, you’ve made an acid! That means you need to see the naming acids rules.

·         Pattern 3: The following periodic table will also come in handy. Notice there are simple patterns for determining the most common oxidation states of the elements based on their family’s position in the periodic table. Notice the 1A family is +1, while the 2A family is +2; then skip across to the 3A family and see that aluminum is +3. Working backward from the halogens, or 7A family, the oxidation states are most commonly -1, while the 6A family is -2, and the 5A family is -3. The 4A family is “wishy-washy”: they can be several oxidation states, with the most common being +4.

Naming Acids

How do you know it’s an acid? The compound’s formula begins with an H, and water doesn’t count! Naming acids is extremely easy if you know your polyatomic ions. There are three rules to follow:

·         H + element: When the acid has only an element following the H, use the prefix hydro-, followed by the element’s root name and an -ic ending. HCl is hydrochloric acid; H2S is hydrosulfuric acid. When you see an acid name beginning with hydro-, think: Caution, element approaching! HCN is an exception since it is a polyatomic ion without oxygen, so it is named hydrocyanic acid.

·         H + -ate polyatomic ion: If the acid has an -ate polyatomic ion after the H, that makes it an -ic acid. H2SO4 is sulfuric acid.

·         H + -ite polyatomic ion: When the acid has an -ite polyatomic ion after the H, that makes it an -ous acid. H2SO3 is sulfurous acid.

Acids have enough H+ added to the anion to make the compound neutral. Supply either the acid’s name or its formula to complete the table below: