Chemical Reactions 1. Nature of Chemical Reactions 2. Reaction Types 3. Balancing Chemical Reactions

The Nature of Chemical Rxns Chemical rxns change substances Chemical rxns change substances Production of gas & change of color are signs of a chem. rxn Production of gas & change of color are signs of a chem. rxn Chemical rxns rearrange the atoms Chemical rxns rearrange the atoms

The Nature of Chemical Rxns Energy is involved in chem. rxns. Energy is involved in chem. rxns. Energy must be added to BREAK bonds. Energy must be added to BREAK bonds. Energy is released when bonds are FORMED. Energy is released when bonds are FORMED. Energy is conserved in a chem. rxn. Energy is conserved in a chem. rxn. remember, energy cannot be created or destroyed remember, energy cannot be created or destroyed Stored energy- chemical energy Stored energy- chemical energy Rxns that release energy- EXOTHERMIC Rxns that release energy- EXOTHERMIC Rxns that absorb energy- ENDOTHERMIC Rxns that absorb energy- ENDOTHERMIC

Chemical Equations Chemical equations describe chemical reactions. Chemical equations describe chemical reactions. You read the equation You read the equation 2CH 4 + 4O 2 → 2CO 2 + 4H 2 O 2CH 4 + 4O 2 → 2CO 2 + 4H 2 O as Methane reacts with oxygen to form carbon dioxide and water. as Methane reacts with oxygen to form carbon dioxide and water. Chemical equations Chemical equations ReactantsProducts

Types of Reactions Classifying Reactions Classifying Reactions Synthesis rxns. combine substances. Synthesis rxns. combine substances. General Synthesis (combination) rxn formula General Synthesis (combination) rxn formula A + B → AB ex. ex. 2Na + Cl 2 → 2NaCl It’s always 2 substances that react to form 1 new substance.

Types of Reactions Decomposition rxns - Break substances apart. Decomposition rxns - Break substances apart. General rxn formula: General rxn formula: AB → A + B Ex. 2H 2 O → 2H 2 + O 2

Types of Reactions Combustion Rxns use O 2 as a reactant and always produce CO 2 & H 2 O. In combustion rxns, the products depend on the amount of O 2. Combustion Rxns use O 2 as a reactant and always produce CO 2 & H 2 O. In combustion rxns, the products depend on the amount of O 2. General Combustion rxn formula: General Combustion rxn formula: C n H 2n+2 + O 2 → CO 2 + H 2 O C n H 2n+2 + O 2 → CO 2 + H 2 O Ex. 2CH 4 + 4O 2 → 2CO 2 + 4H 2 O

Types of Reactions Single Replacement rxn - a rxn in which one element takes the place of another element in a compound Single Replacement rxn - a rxn in which one element takes the place of another element in a compound General equation : General equation : AX + B → BX + A Ex. Ex. 3CuCl 2 + 2Al → 2AlCl 3 + 3Cu

Types of Reactions Double-Replacement rxns - a rxn in which a substance is formed from the exchange of 2 atoms or ions between 2 compounds. Double-Replacement rxns - a rxn in which a substance is formed from the exchange of 2 atoms or ions between 2 compounds. General formula General formula AX + BY → AY + BX Ex. Ex. Pb(NO 3 ) 2 + K 2 CrO 4 → PbCrO 4 + 2KNO 3 Chemical Reactions Chemical Reactions

Balancing Chemical Equations Law of Conservation of Mass- Matter cannot be created or destroyed. Law of Conservation of Mass- Matter cannot be created or destroyed. For mass to remain constant before and after a chemical reaction, the # of atoms of each element must be the same before and after a chemical reaction. For mass to remain constant before and after a chemical reaction, the # of atoms of each element must be the same before and after a chemical reaction.

Balancing Chemical Equations To balance a chemical equation, use COEFFICIENTS- whole numbers written before formula. To balance a chemical equation, use COEFFICIENTS- whole numbers written before formula. Ex. 2H 2 O → 2H 2 + O 2 Ex. 2H 2 O → 2H 2 + O 2

Balancing Chemical Equations Rules 1. Write a formula equation with the correct symbols and formulas. 1. Write a formula equation with the correct symbols and formulas. 2.Count the # of atoms of each element on each side of the arrow. 2.Count the # of atoms of each element on each side of the arrow. 3.Balance atoms by using coefficients 3.Balance atoms by using coefficients 4.Check your work by counting atoms of each element. 4.Check your work by counting atoms of each element.