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Predicting Whether a Reaction Will Occur
“Forces” that drive a reaction:• Formation of a solid• Formation of water• Transfer of electrons• Formation of a gas
When chemicals (dissolved in water) are mixed and one of these four things can occur, the reaction will generally happen.
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Dissociation
• Ionic compounds– Metal + nonmetal (Type I & II)– Metal + polyatomic anion– Polyatomic cation + anion
• Dissociation: When ionic compounds dissolve in water the anions and cations are separated from each other.
• We know that ionic compounds dissociate when they dissolve in water because the solution conducts electricity.
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Dissociation (cont.)
• Potassium chloride dissociates in water into potassium cations and chloride anions.
KCl(aq) = K+ (aq) + Cl- (aq)
• Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions.
CuSO4(aq) = Cu+2(aq) + SO42-(aq)
K+ Cl-K Cl
Cu+2 SO42-Cu SO4
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Precipitation Reactions
• In all precipitation reactions, the ions of one substance are exchanged with the ions of another substance when their aqueous solutions are mixed.
• At least one of the products formed is insoluble in water.
KI(aq) + AgNO3(aq) KNO3(aq) + AgIs
K+
I-
Ag+
NO3-
K+
NO3-
Ag I
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Process for Predicting the Products of a Precipitation Reaction
• Determine what ions each aqueous reactant has.• Exchange ions.
– (+) ion from one reactant with (-) ion from other• Balance charges of combined ions to get formula
of each product.• Balance the equation.
– Count atoms.• Determine solubility of each product in water.
– Use solubility rules.– If product is insoluble or slightly soluble, it will
precipitate.
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Solubility Rules
• Most compounds that contain NO3- ions are
soluble.
• Most compounds that contain Na+, K+, or NH4
+ ions are soluble.
• Most compounds that contain Cl- ions are soluble, except AgCl, PbCl2, and Hg2Cl2
• Most compounds that contain SO42- ions are
soluble, except BaSO4, PbSO4, CaSO4
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Solubility Rules (cont.)
• Most compounds that contain OH- ions are slightly soluble (will precipitate), except NaOH and KOH, which are soluble, and Ba(OH)2, Ca(OH)2, which are moderately soluble.
• Most compounds that contain S2-, CO32-, or
PO43- ions are slightly soluble (will
precipitate).
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Predict what will happen when the following solutions are mixed:
• AgNO3(aq) and NH4Cl(aq)
• Na3PO4(aq) and Co(NO2)2(aq)
• K2CrO4(aq) and Pb(C2H3O2)2(aq)
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Describing Reactions in Aqueous Solutions
• Molecular equations: equations that show the complete formulas of all reactants and products
Example:
• Ionic equations: All substances that are strong electrolytes are represented as ions. Solids are not written as separate ions.
Example:
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Describing Reactions in Aqueous Solutions (cont.)
• Spectator ions: ions that do not participate in the reaction
Example:
• Net ionic equation: Only the components directly involved the reaction are written.
Example:
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Write molecular, complete ionic, and net ionic equations for the following reaction:
• nickel(II) sulfate and barium chloride
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Write molecular, complete ionic, and net ionic equations for the following reaction:
• iron(III) nitrate and sodium hydroxide
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Reactions that Form Water: Acids and Bases
• When acids dissociate in water they release H+ ions and their anions.
• When bases dissociate in water they release or form OH- ions and their cations.
HBrH2O
H+(aq) Br-(aq)+
Mg(OH)2H2O
+Mg2+(aq) 2OH-(aq)
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Acid-Base Reactions
• In the reaction of an acid with a base, the H+ from the acid combines with the OH- from the base to make water.
• The cation from the base combines with the anion from the acid to make the salt.
acid + base salt + waterH2SO4(aq) + Ca(OH)2(aq) CaSO4(aq) + 2 H2O(l)
• The net ionic equation for a strong acid-strong base reaction is always:
H+ (aq) + OH- (aq) H2O(l)
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Erucic Acid (a weak acid from marsh marigold)
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Complete the following acid-base reactions:
HCl(aq) + RbOH(aq) →
HNO3(aq) + LiOH(aq) →
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Oxidation-Reduction Reactions
• Oxidation-reduction reactions: reactions that involve a transfer of one or more electrons
• The substance that loses electrons in the reaction is oxidized. The substance that gains electrons in the reaction is reduced.
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Reactions of Metals withNonmetals (Oxidation-Reduction)
• The metal loses electrons and becomes a cation (oxidation)
• The nonmetal gains electrons and becomes an anion (reduction)
• In the reaction, electrons are transferred from the metal to the nonmetal.
2K(s) + F2(g) → 2KF(s)
metal nonmetal
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F
F
1e-
1e-
K
K
K+1
K+1
F-1
F-1
2K(s) + F2(g) 2KF(s)
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Characteristics of Oxidation-Reduction Reactions
• Metal + nonmetal ionic compound (solid)
• In the ionic compound the metal is now a cation and the nonmetal is an anion.
• Two nonmetals can undergo an oxidation-reduction reaction. Look for O2 as a reactant or product. In this case the products are not ionic.
• Examples:
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Ways to Classify Reactions
• Precipitation reactions: reactions that involve solid formation.Example:
• Double displacement reactions: ion exchange reactions AB + CD → AD + CBExample:
• Acid-base reactions: reactions that involve water formation
Example:
• Both precipitation reactions and acid-base reactions involve compounds exchanging ions.
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Ways to Classify Reactions (cont.)
• Oxidation-reduction reactions: reactions that involve electron transferExample:
Also classified as a single displacement reaction:A + BC → B + AC
• Combustion reactions: reactions in which O2(g) is reacted with a carbon compound– Release a lot of energy– Subclass of oxidation-reduction reactions– Example:
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Other Ways to Classify Reactions
• Synthesis reactions: reactions in which chemicals combine to make one product– Metal + nonmetal reactions can be classified as
synthesis reactions.Example:
– Reactions of metals or monmetals with O2 can be classified as synthesis reactions.Example:
• These two types of synthesis reactions are also subclasses of oxidation-reduction reactions.
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Other Ways to Classify Reactions
• Decomposition reactions: reactions in which one reactant breaks down into simpler compounds or elements
• Generally initiated by addition of electric current or heat
Example:
• Opposite of a synthesis reaction
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Summary of Classes of Reactions
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Classify the following reactions in as many ways as possible:
C(s) + O2(g) → CO2(g)
Zn(s) + 2HBr(aq) → H2(g) + ZnBr(aq)
2KClO3(s) → 2KCl(s) + 3O2(g)
H2SO4(aq) + Ba(OH)2(aq) → 2H2O(l) + BaSO4(s)