CHEMICAL COMPOSITION AND REACTIONS Chapter 8 OUTLINE Oxidation Numbers and Formulas Nomenclature Writing Equations Types of Reactions
OXIDATION NUMBERS AND THEIR RULES Oxidation numbers represent the number of electrons that an atom in a compound must gain or lose to return to its neutral state Remember, if an atom has a positive oxidation number then it loses electrons. If it has a negative oxidation number then it gains electrons. Why are the charges opposite of what is going on? OXIDATION NUMBERS AND RULES Rule 1: The oxidation number of atoms in their natural forks and of atoms in pure elements is zero. Also, elements that are diatomic molecules have an oxidation number of zero because the electrons are equally shared Fe, Na, Ar, H 2 Rule 2: The oxidation number of a monoatomic ion is equal to the charge of the ion Br - and Mg 2+
OXIDATION NUMBERS AND THEIR RULES Rule 3: The sum of the oxidation numbers of all at the atoms in a compound must be zero Examples: Rule 4: The oxidation number follows a trend based on their location on the periodic table By using the rules of oxidation numbers then you can determine the oxidation number of elements in a compound. OXIDATION NUMBERS AND THEIR RULES Examples: Determine the oxidation number of each element in the following compounds: Na 2 O H 2 SO 4
OXIDATION NUMBERS AND THEIR RULES You can also use the oxidation rules to write the formulas of compounds as well Example: Write the formula for the ionic compounds of Barium and iodine Calcium and oxygen OXIDATION NUMBERS AND POLYATOMIC IONS Polyatomic ions are covalently bonded groups of atoms that carry a charge Rule 5: The oxidation numbers of all the atoms in a polyatomic ion add up to its charge OH -
POLYATOMIC IONIC COMPOUNDS Remember that polyatomic ions are groups of atoms that carry a charge Polyatomic ions can be generalized into different groupings The only positive polyatomic ions are ammonium (NH 4 + ) and mercurous (Hg 2 + ) Oxyanions are those that contain oxygen and one other element. When naming compounds with polyatomic ions, you DO NOT have to change any endings. Examples POLYATOMIC IONIC COMPOUNDS Name NH4BrO3 What is the formula for potassium dichromate?
NAMING METALS Metals that can have multiple oxidation states are written with a Roman numeral placed after its name this is called the Stock system/roman Numeral System Examples: Name Hg(BrO 3 ) 2 What is the formula for lead (II) phosphate? BINARY ACIDS Acids are covalent compounds that typically begin with hydrogen Acids that consist of hydrogen and one other nonmetal are called binary acids Binary acids typically contain the prefix hydro-, the root name for the nonmetal with an -ic ending and then acid Hydrochloric acid and hydrofluoric acid
TERNARY ACIDS Ternary acids contain three elements: hydrogen, oxygen, and another nonmetal Ternary acids generally contain polyatomic ions in the acid form To name ternary acids you must change the ending just like binary acids but ternary follows different rules -ate -ic -ite -ous WRITING EQUATIONS Chemical equations are used to represent chemical reactions and must do three things: Give the chemical names of all substances Express the correct chemical formula for each substance Account for all atoms that are involved in the reaction
PARTS OF AN EQUATION Reactants Products Ca(HCO3)2 + Ca(OH)2 H2O + CaCO3 BALANCING EQUATIONS All chemical reactions must follow the law of conservation of mass Remember the law states that, matter cannot be created or destroyed in chemical reactions In simpler terms, whatever you start with you have to end with and whatever you ended with had to come from what you started with
BALANCING EQUATIONS Look at the equation from the previous slide and count how many atoms you started with and how many you ended with You start with 16 but only end with 8 In order to make sure that the equation is balanced you have to change the coefficients that come before each compound until both sides equal the same number of atoms GUIDELINES FOR BALANCING EQUATIONS 1. Write formulas for all reactants and products. Make sure that all the formulas are correct. 2. Check to see if the equation is already balanced. 3. Adjust the coefficients until there are equal numbers of atoms on both sides of the arrow. You cannot change the subscripts ever 4. Check to be sure that coefficients are whole numbers in the simplest ratio possible
EXAMPLES NO + O 2 NO 2 KClO 3 KCl + O 2 Ca 2 Br 2 + NaCO 3 CaCO 3 + NaBr TYPES OF REACTIONS Chemical reactions can be divided into 4 major categories based on what is going on within the reaction Synthesis reactions-combine two or more substance into a single product Decomposition reaction-substances are broken down into two or more simpler substances Single replacement reaction-a reactive element replaces a less reactive element in a compound Double replacement reaction-two compounds switch partners
SYNTHESIS REACTIONS Synthesis reactions follow a general pattern of A + B AB Ex. Ca (s) + Cl2 (g) CaCl2 (s) DECOMPOSITION REACTION Decomposition reactions follow a general pattern of AB A + B Usually some sort of catalyst (heat or another compound) is required to break apart a compound into its individual parts
SINGLE REPLACEMENT REACTIONS Single replacement reactions follow the general pattern of A + BZ B + AZ In order for A to switch places with B it has to be more reactive than B (refer to the activity series I will give you) DOUBLE REPLACEMENT REACTIONS Double replacement reactions follow the general form of AY + BZ AZ + BY Double replacement reactions differ from single because each compound has something to give up whereas before one compound switched with a single element
DOUBLE REPLACEMENT REACTIONS We have already looked at balanced equations but we can break equations down further and look at them as ionic equations Ionic equations represent all the particles present before and after the reaction The only compounds that are able to be broken down into its parts are those that are in the aqueous state Within an ionic equation there are spectator ions that appear in the reactants and the products but do not react with other ions A net ionic equation eliminates these spectator ions and show only the ions that actually react