Chapter 7

Chemical Reactions

Understanding Chemical ReactionsA chemical reaction occurs when:

•A change in energy occurs• Exothermic –gives off energy• Endothermic – absorbs energy

•The color changes•A new gas is created•A precipitate, gas or water is formed from 2 solutions

Parts of a Chemical Equation

Reactants Product

Coefficient

)(2)(2)(2 22 lgg OHOH

Subscript Physical state

Symbols- used to specify properties of reactants and products or conditions of the reaction (know table 11.1, page 323)

Word Equations

• CuSO4(aq) + Ag(s) Ag2SO4(aq) + Cu(s)

• “aqueous copper (II) sulfate and solid silver react to produce aqueous silver sulfate and solid copper.”

Balancing EquationsWhy?•Law of Conservation of Mass (LCM)- mass is neither created nor destroyed in chemical reactions (Antione Lavosier, 1700’s

•Coefficients- added so reactions obey the LCM (DON’T touch subscripts!!)

Procedure for Balancing

• Write formulas for reactants and product on respective sides of the equation

• Perform initial “atom inventory” on each side of the equation

• Add coefficients to make inventories match • Update and repeat as needed• Verify answer

Practice

NaBrMgClMgBrNaCl ________ 22

Na

Cl

Mg

Br

Types of Reactions

1. Combustion2. Synthesis3. Decomposition4. Single replacement5. Double replacement

Combustion• Reaction with oxygen that releases energy (heat or light)• Hydrocarbons (contain only C and H)

• Complete combustion always produces CO2(g) and H2O(g)

• Note: incomplete combustion may result in C(s) or CO(g)

Complete Combustion (Bunsen Flame) CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Incomplete Combustion (Luminous Flame) CH4(g) + O2(g) C(g) + 2H2O(g)

Combustion Examples

• Complete combustion of propane (C3H8)

• Combustion of aluminum

Synthesis (or Combination) • Two or more substances combine to form a “larger”

compound. • Group 1A & 2A metals- use common charges to

predict ionic compound formed

• Transition metals or 2 nonmetals- many products are possible

Synthesis Examples• Synthesis of barium chloride

• Combination of copper and sulfur

Decomposition• More complex molecule breaks down into simpler

substances

• Binary compounds- decompose into constituent elements

• Ternary or more complex compounds- decompose

into either constituent elements or into a combination of compounds

Decomposition Examples• Decomposition of sulfur trioxide gas

• Decomposition of ammonium nitrate to produce dinitrogen monoxide and water

Decomposition vsDissociation

• Decomposition- atoms with NEW properties are created

• Dissociation- ions without new properties are created

Single Replacement

• Possible reaction between an element and an aqueous compound

• Metals ONLY replace metals and nonmetals ONLY replace nonmetals

• Activity series used to predict whether replacement will occur

S.R. Examples• Lithium is combined with calcium nitrate

• Silver is placed into a solution of magnesium carbonate

• Chlorine gas is bubbled through a solution of aluminum bromide

Double Replacement

• Possible exchange of partners by two compounds

• Opposites attract- two positive ions WILL NOT combine (two negative ions won’t either)

• Solubility Rules used to predict if double replacement will occur (a precipitate, a gas, or water must be a product)

D.R. Examples• Solutions of magnesium nitrate and aluminum

phosphate are mixed

• Solutions of sodium chloride and aluminum nitrate are mixed

Net Ionic Equations• Eliminate spectator ions• Solutions of sodium chloride and silver nitrate

are mixed

Double Replacement to Form Water• Acids and bases react in a double replacement

reaction and produce water• Acid- produces H+ ions, pH < 7• Base- produces OH- ions, pH > 7

• Neutralization- producing neutral water (pH = 7)

Neutralization Examples

• Hydrochloric acid and sodium hydroxide are mixed

• Sulfuric acid and calcium hydroxide are mixed