www.cengage.com/chemistry/cracolice Mark S. Cracolice Edward I. Peters Mark S. Cracolice • The University of Montana Chapter 8 Chemical Reactions
Feb 23, 2016
www.cengage.com/chemistry/cracolice
Mark S. CracoliceEdward I. Peters
Mark S. Cracolice • The University of Montana
Chapter 8Chemical Reactions
Evidence of Chemical ChangeEvidence of Chemical Change
1. Color change2. Formation of a solid3. Formation of a gas4. Absorption or release of heat energy5. Emission of light energy
Evidence of Chemical ChangeColor change
Evidence of Chemical ChangeFormation of a solid
Evidence of Chemical ChangeFormation of a gas
Evidence of Chemical ChangeEvolution of heat and light
Evidence of Chemical ChangeEmission of light
Evidence of Chemical ChangeHeat transfer as evidence
of physical change
Many physical changes are accompanied by heat transfer, so be cautious in using this form of evidence of a chemical change.
Evolution of a Chemical EquationWhen solid potassium is added to liquid water, a reaction
occurs, producing hydrogen gas, potassium hydroxide solution, and heat:
Evolution of a Chemical EquationChemists have developed a standardized shorthand
method for describing chemical reactions, suchas the reaction of sodium and water, in writing.
State Symbols and Their MeaningsSymbol Meaning (s) solid (l) liquid (g) gas (aq) aqueous (dissolved in water)
Evolution of a Chemical EquationSolid potassium K(s)
plus +
liquid water H2O(l)
yields
hydrogen gas H2(g)
plus +
potassium hydroxide solution KOH(aq)
Evolution of a Chemical Equation
The equation K(s) + H2O(l) H2(g) + KOH(aq)
is said to be unbalanced because the number of atoms of each element is not the same before and after the chemical change.
An equation is balanced when the number of atoms of each element is the same before and after the chemical change
2 K(s) + 2 H2O(l) H2(g) + 2 KOH(aq)
Interpreting Chemical EquationsThe particulate-level interpretation of a chemical equation:
2 H2(g) + O2(g) 2 H2O(g)
Two molecules of hydrogen react with one oxygen moleculeto form two water molecules.
Interpreting Chemical EquationsThe particulate-level interpretation of a chemical equation:
Interpreting Chemical EquationsThe molar-level interpretation of a chemical equation:
2 H2(g) + O2(g) 2 H2O(g)
Two moles of hydrogen react with one mole of oxygento form two moles of water.
Writing Chemical Equations
1. Write the formulas of the given reactants to the left of an arrow and the formulas of the products to the right.
2. Balance the equation by adding coefficients, but do not change the qualitative description of the reaction by adding, removing, or altering any chemical formula.
Combination ReactionsCombination Reaction
Two or more substances combine to form a single product:
Combination ReactionsExample:Potassium combines with oxygen to form potassium oxide.
Solution:Write the qualitative description:K + O2 K2O
Balance the O:K + O2 2 K2O
Balance the K:4 K + O2 2 K2O
Combination Reactions
Reactants: Any combination of elements and/orcompounds
Reaction type: Combination
Equation type: A + X AX
Products: One compound
Decomposition ReactionsDecomposition Reaction
A compound breaks down into simpler substances:
Decomposition ReactionsExample:Water is decomposed into its elements.
Solution:Write the qualitative description:
H2O (l) H2 (g) + O2 (g) Balance the O:
2 H2O (l) 2 H2 (g) + O2 (g)
Decomposition ReactionsExample:Calcium carbonate (limestone) is decomposed into carbon
dioxide and calcium oxide (lime).
CaCO3(s) CaO (s) + CO2 (g)
Decomposition Reactions
Reactants: One compound
Reaction type: Decomposition
Equation type: AX A + X
Products: Any combination of elements andcompounds
Single-Replacement ReactionsSingle-Replacement Reaction
One element appears to replace another in a compound:
Single-Replacement ReactionsExample:Potassium is able to replace hydrogen in water.
2 K(s) + 2 HOH (l) H2 (g) + 2 KOH (aq)
Single-Replacement ReactionsExample:A copper strip is placed in a solution of silver nitrate.
Solution:Copper displaces silver from silver nitrate
Cu (s) + 2AgNO3 (aq) Cu(NO3)2 (aq) + 2 Ag (s)
The reverse reaction does not occur
Single-Replacement ReactionsExample:A magnesium strip is placed in a solution of hydrochloric acid.
Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)
Magnesium displaces hydrogen from acid
Single-Replacement ReactionsSummary:
Single-Replacement Reactions
Reactants: Element (A) plus a solution of an acidor an ionic compound (BX)
Reaction type: Single-replacement
Equation type: A + BX AX + B
Products: Element (B) must be less active than element A
Double-Replacement ReactionsDouble-Replacement Reaction
Ions of two reactants appear to change partners:
Double-Replacement ReactionsDouble-Replacement Reaction between two ionic
compounds.
When solutions of two ionic compounds are mixed, positive ions from one compound may combine with negative ions from the other compound to form a solid (precipitate) or a molecular compound (water, weak acid, weak base, gas)
Formation of a precipitateFormation of silver chloride precipitate from sodium chloride and
silver nitrate.
NaCl (aq) + AgNO3 (aq) NaNO3 (aq) + AgCl (s)
Formation of barium sulfate precipitate from barium chloride and sodium sulfate.
BaCl2 (aq) + Na2SO4 (aq) BaSO4 (s) + 2 NaCl (aq)
Neutralization Reactions: Formation of water
Neutralization reaction is a reaction between an acid and a base. One hydrogen ion from acid reacts with one hydroxide ion from a base to form one water molecule.
Neutralization of hydrochloric acid by sodium hydroxide.
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
Reaction between sulfuric acid and solid aluminum hydroxide.
3 H2SO4 (aq) + 2 Al(OH)3 (s) Al 2(SO4)3 (aq) + 3 H2O (l)
Formation of weak acid
Reaction of hydrochloric acid and sodium acetate to form acetic acid (weak acid)
HCl (aq) + NaCH3CO2 (aq) NaCl (aq) + HCH3CO2 (aq)
Reaction between hydrochloric acid and sodium sulfide to form hydrosulfuric acid (weak acid)
Na2S(aq) + 2 HCl (aq) 2 NaCl (aq) + H2S (aq)
Formation of a gas
Reaction between hydrochloric acid and sodium carbonate to form carbon dioxide gas
Na2CO3 (aq) + 2 HCl (aq) 2 NaCl (aq) + H2O(l) + CO2 (g)
Double-Replacement ReactionsSummary:
Double-Replacement Reactions
Reactants: Solutions of two ionic compounds,(AX + BY)
Reaction type: Double-replacement
Equation type: AX + BY AY + BX
Products: Two new compounds (AY + BX), one of which may be a solid, water, weak acid, weak base, or a gas
Summary
Homework
Homework: 9, 15, 19, 23, 27, 29, 31, 35, 39, 53, 59, 69, 76