copyright © houghton mifflin company. all rights reserved. 7 | 1 predicting whether a reaction will...

Copyright © Houghton Mifflin Company. All rights reserved. 7 | 1

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.


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.


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


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


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.


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


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).


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)


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:


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:


Write molecular, complete ionic, and net ionic equations for the following reaction:

• nickel(II) sulfate and barium chloride


Write molecular, complete ionic, and net ionic equations for the following reaction:

• iron(III) nitrate and sodium hydroxide


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)


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)


Erucic Acid (a weak acid from marsh marigold)


Complete the following acid-base reactions:

HCl(aq) + RbOH(aq) →

HNO3(aq) + LiOH(aq) →


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.


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


F

F

1e-

1e-

K

K

K+1

K+1

F-1

F-1

2K(s) + F2(g) 2KF(s)


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:


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.


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:


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.


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


Summary of Classes of Reactions


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)

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