Unit 4 Chemical Reactions & Chemical Formulas
High School Physical Science: Unit 4 - Chemical Reactions
1. Introduction to Chemical Reactions
A chemical reaction is a process where one or more substances are transformed into one or more new substances.
The chemical and physical properties of the new substances differ from those of the original materials.
Often referred to as chemical change.
2. Signs of Chemical Reactions
Unexpected change of color
Change in odor
Gas formation: Observed as bubbles or fizzing
Energy change: Can be exothermic (heating up) or endothermic (cooling off)
Solid formation: This is referred to as a precipitate.
3. Understanding Bonds in Reactions
In chemical reactions, bonds must break for new substances to form.
Molecules are in constant motion and can break bonds when they collide with sufficient energy.
After breaking, atoms rearrange to form new bonds, resulting in new substances.
4. Chemical Formulas
Chemical symbols represent individual elements (found on the periodic table).
A chemical formula is shorthand that uses these symbols and numbers to represent a substance.
Subscript: A number written below and to the right of a chemical symbol.
5. Naming Binary Covalent Compounds
Binary covalent compounds consist of two different nonmetals. Example: ClF3 (chlorine trifluoride)
Rule 1: Lower group number element is written first.
Rule 2: If both elements are in the same group, the one with the higher period number is named first.
Rule 3: The second element is named as an anion, by adding the suffix -ide.
Rule 4: Greek prefixes indicate the number of atoms (e.g., mono-, di-, tri-, etc.).
6. Writing Formulas for Covalent Compounds
If the name of the covalent compound is known, it helps in writing the chemical formula.
7. Naming Binary Ionic Compounds
Naming Ionic Compounds:
The metal is named first by its elemental name.
The nonmetal is named by its elemental name with an -ide ending.
8. Writing Formulas for Ionic Compounds
Ionic compounds consist of a metal and a nonmetal.
Use the crisscross method to determine the formula:
Identify the charges of the elements.
Cross the charges to formulate the compound.
9. Charges of Elements
Metals can have multiple charges (e.g., Iron can be Fe+2 or Fe+3).
Roman numerals are used in naming to indicate the charge of the metal.
10. Balancing Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction.
Reactants: Starting materials, Products: Substances formed.
It is crucial that equations are balanced: the number of atoms must be the same for both reactants and products.
The law of conservation of matter states that matter cannot be created or destroyed.
Use coefficients to balance equations.
11. Types of Chemical Reactions
Four major types:
Synthesis reactions: Two substances form one compound.
Decomposition reactions: A single compound breaks down into simpler substances.
Single-displacement reactions: An element replaces another in a compound.
Double-displacement reactions: Ions from two compounds exchange places, often resulting in a gas or precipitate.
12. Practice Problems and Additional Resources
Practice problems available in the unit to reinforce learning.
Utilize Quizlet links provided for further practice with naming and writing chemical formulas.