Unit 6 Introduction to Chemistry Chapter 19 Chemical Reactions

19A – Compounds and Chemical Formulas

Chemical Formulas A shorthand way to tell the identities and the amounts of elements in a compound Is unique to each compound Empirical formula The smallest whole number ratio of its components Covalent compounds are sometimes written like empirical Ionic compounds are always written empirical

Oxidation Numbers The number of electrons that a bonded atom or ion would have to gain or lose to return to its neutral state May be positive or negative Same as the charge of the ion How do we know? Look at the periodic table!

Oxidation Number Rules The oxidation number of pure elements is 0 The oxidation number of a monatomic ion is equivalent to the charge of the ion Elements in certain families regularly have the same oxidation numbers The sum of the oxidation numbers of all the atoms in a compound must equal zero because compounds are not electrically charged

Compare to the Periodic Table

Writing Chemical Formulas Write the two elements next to each other. Always write the less electronegative element first. Usually the most metallic. Use table 19-1 (until you have this memorized) to find the oxidation numbers of the two elements. Write these numbers above the symbols The quantities of the two elements must be adjusted so that the sum of the oxidation numbers equals zero. NOT IN BOOK – CRISS CROSS METHOD!!!

Polyatomic Ions Two or more atoms that act as a single unit Oxidation number rule 5: the sum of the oxidation numbers of all atoms in a polyatomic ion is equal to its charge

Naming Compounds Binary Compounds Binary Ionic Compounds Composed of only two elements Binary Ionic Compounds Metal name first without change Nonmetal next, change ending to “ide” What about elements with two different charges? Stock system Roman numeral in parentheses comes after the name of the metal, indicating its oxidation number There is another way to name, the classical method which you will be introduced to in Chemistry

Naming Compounds Binary Covalent Compounds Uses prefixes to tell how many of each element is involved Mono, di, tri, tetra, penta, hexa Mono is never used for the first element in the compound Some compounds also have common names; however, those names do not tell anything about the elements involved

Naming Compounds Polyatomic Ion Compounds Similar to binary ionic compounds First name the metal Second name the full anion ion name Ending will be ate, or ite unless the last element is single, then it would be ide

19B – Chemical Changes

Evidences for Chemical Changes A chemical change takes place when substances react to form new substances In a chemical reaction atoms are rearranged into different substances A solid separates into a liquid mixture of chemicals A gas is produced by means other than evaporation A substance’s color changes; such a change is usually permanent The temperature of a substance changes Light, sound, or another type of energy is produced Sometimes physical changes may produce one or more of these signs. If a chemical reaction has truly taken place, the change cannot be undone by physical processes.

Chemical Equations Describes what happens during a chemical reaction Can be word equations or formula equations Symbols in a formula equation show how the atoms of each element are both rearranged and conserved during the reaction The left side is called the reactants On the right of the arrow are the products Just like in math, a chemical equation must be balanced To balance equations coefficients are placed in front of the formulas Let’s do some examples…

Special Symbols in Chemical Equations Physical states Aqueous – aq Solid – s Liquid – l Gas – g Arrows for precipitates A product that settles out of solution Greek delta for heat, or catalyst Speeds up a reaction, but is neither a reactant nor a product Facet, page 476

19C – Types of Chemical Reactions

Types of Chemical Reactions There are six categories of reactions, although your book only discusses four main ones Composition Decomposition Single-Replacement Double-Replacement

Composition Also called synthesis or combination Combine two or more substances into a single, more complex compound X + Y XY Distinguishing feature of this reaction is a single product

Decomposition Break apart or decompose a substance Opposite of composition reactions Usually energy is necessary Distinguishing feature of this reaction is a single reactant XY X + Y

Single-Replacement One element is replaced by another element Distinguishing feature of this reaction is the presence of both an element and a compound before and after the reaction The element with the greater activity replaces a similar element of lower activity XY + Z ZY + X

Double-Replacement Two compounds swap cations or anions with each other Distinguishing feature of this reaction is the presence of two reactants and two products whose ions exchange paces. A precipitate is often formed WX + YZ WZ + YX

Chemical Thermodynamics Fifth type of reaction: Combustion Burning Involves oxygen Carbon dioxide and water are products Exothermic reactions Give off large quantities of heat, release thermal energy Bond making Endothermic reactions Take in energy, absorb thermal energy Bond breaking Most decomposition reactions