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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