1. Unit 6 Introduction to Chemistry
Chapter 19 Chemical Reactions

2. 19A – Compounds and Chemical Formulas

3. 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

4. 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!

5. 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

6. Compare to the Periodic Table

7. 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!!!

8. 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

9. 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

10. 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

11. 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

12. 19B – Chemical Changes

13. 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.

14. 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…

15. 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

16. 19C – Types of Chemical Reactions

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

18. 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

19. 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

20. 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

21. 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

22. 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