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Chapter 7
Chemical Reactions
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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
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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)
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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.”
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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!!)
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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
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Practice
NaBrMgClMgBrNaCl ________ 22
Na
Cl
Mg
Br
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Types of Reactions
1. Combustion2. Synthesis3. Decomposition4. Single replacement5. Double replacement
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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)
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Combustion Examples
• Complete combustion of propane (C3H8)
• Combustion of aluminum
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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
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Synthesis Examples• Synthesis of barium chloride
• Combination of copper and sulfur
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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
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Decomposition Examples• Decomposition of sulfur trioxide gas
• Decomposition of ammonium nitrate to produce dinitrogen monoxide and water
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Decomposition vsDissociation
• Decomposition- atoms with NEW properties are created
• Dissociation- ions without new properties are created
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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
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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
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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)
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D.R. Examples• Solutions of magnesium nitrate and aluminum
phosphate are mixed
• Solutions of sodium chloride and aluminum nitrate are mixed
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Net Ionic Equations• Eliminate spectator ions• Solutions of sodium chloride and silver nitrate
are mixed
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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)
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Neutralization Examples
• Hydrochloric acid and sodium hydroxide are mixed
• Sulfuric acid and calcium hydroxide are mixed