Chapter 8: Reactions and Aqueous Solutions

Predicting Whether a Reaction Will Occur

• There are several different “driving forces” that cause chemical reactions to take place– Formation of a solid– Formation of water– Transfer of Electrons– Formation of a gas

• When two chemicals come together if any of these changes can occur a chemical reaction will occur.

• You can predict whether a reaction will occur and the products that will form by considering these driving forces

Reactions that form a Solid

• Precipitation: the formation of a solid in a reaction

• Precipitate: the solid formed by the reaction

• Called a precipitation reaction

Reactions that form a Solid

Reactions that form a Solid

Reaction of yellow potassium chromate and colorless barium nitrate.

K2CrO4 (aq) + Ba(NO3)2 (aq)

?How do we predict the

products?

Reactions that form a Solid

• We must think about what happens in an aqueous solution…– What happens when an ionic compound dissolves in

water?

– Ba(NO3)2, a white solid, was dissolved in water

– Ba+ and NO3- are present

– K2CrO4 is dissolved in water

– K+ and CrO42- are present

• These are called Strong Electrolytes because each unit produces separated ions

Reactions that form a Solid

CrO42-

K+

K+

K2CrO4 (aq) Ba(NO3)2 (aq)

NO3-

NO3-

Ba2+

K2CrO4 (aq) + Ba(NO3)2 (aq) Products

Reactions that form a Solid

• We can express the reaction in the form of the ions present

2K+(aq) + CrO42-(aq) + Ba2+(aq) + 2NO3

- (aq) Products

• Thus the mixed solution contains four types of ions

• How can we decide what products will form?

Reactions that form a Solid

• We know that the reaction must form a solid with a net zero charge– So the product must contain cations and

anions

• Most ionic compounds contain only two types of ions– One type of cation and one type of anion

Reactions that form a Solid

• Possibilities

• So the solid could be one of these

• BaCrO4 is the solid

NO3- CrO4

2-

K+ KNO3 K2CrO4

Ba2+ Ba(NO3)2 BaCrO4

Using Solubility Rules

• Soluble: will readily dissolve in water

• Insoluble: will not dissolve in water

• Slightly soluble: a tiny amount will dissolve in water

Soluble Compounds

Insoluble Compounds

NO3-, Na+,

K+, NH4+

S2-, CO32-,

PO43-

Cl-, Br-, I-

(Ag+, Hg2+, Pb2+)

OH-

(Na+, K+, Ca2+)

SO42-

(Ba2+, Pb2+, Ca2+)

Predicting Products

• AgNO3 (aq) + KCl (aq) white solid

• Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)

• Possible ionic compounds:

– AgNO3, AgCl, KNO3, KCl

• Use solubility rules to determine which is the white solid

• AgCl is white solid

Describing Reactions in Aqueous Solutions

• Molecular Equation

– K2CrO4(aq) + Ba(NO3)2(aq) BaCrO4(s) + 2KNO3(aq)

• Complete Ionic Equation

– 2K+(aq) + CrO42-(aq) + Ba2+(aq) + 2NO3

- (aq) BaCrO4(s) + 2K+(aq) +

2NO3- (aq)

• Spectator Ions

– 2K+(aq) and 2NO3- (aq)

• Net Ionic Equation

– Ba2+(aq) + CrO42-(aq) BaCrO4(s)

Describing Reactions in Aqueous Solutions

• Pb(NO3)2(aq) + Na2SO4(aq) PbSO4(s) + 2NaNO3(aq)

– Pb2+(aq) + SO42-(aq) PbSO4 (s)

• NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

– Cl-(aq) + Ag+(aq) AgCl(s)

• 3KOH(aq) + Fe(NO3)3(aq) Fe(OH)3 (s) + 3KNO3(aq)

– Fe3+(aq) + 3OH-(aq) Fe(OH)3(s)

Reactions that Form Water

• Reactions between a strong acid and a strong base

• Strong acids are strong electrolytes– HCl HCl H+(aq) + Cl-(aq)

– HNO3 H+(aq) + NO3-(aq)

• Arrehenius acids produce H+ ions when dissolved in water

Reactions that Form Water

ClH

ClH

HCl

WaterCl- H+

Cl-

Cl-

H+

H+

Each HCl Molecule dissociates when it dissolves in water

Reactions that Form Water

• Arrhenius bases produce OH- ions when dissolved in water– NaOH(s) Na+(aq) + OH-(aq)– KOH(s) K+(aq) + OH-(aq)

• Strong bases are also strong electrolytes

Reactions that Form Water

• When acids and bases react they form water and a salt (an ionic compound)

• In solution acids form H+ and bases form OH-

– H+ + OH- H2O

– Net ionic equation for all acid-base rections

• HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

Write the Complete and Net Ionic Equations for the following and identify each as either

an acid-base or precipitation reaction• HNO3(aq) + KOH(aq) H2O(l) + KNO3(aq)

• 2AgNO3(aq) + Na2CrO4(aq) Ag2CrO4(s) + 2NaNO3(aq)

• HBr(aq) + NaOH(aq) H2O(l) + NaBr(aq)

• Ni(NO3)2(aq) + K2CO3(aq) NiCO3(s) + 2KNO3(aq)

Reactions of Metals with Nonmetals

• When a metal reacts with a non-metal an ionic compound is formed. The ions formed when the metal transfers electrons to the non-metal. The metal becomes a cation and the non-metal becomes an anion

• Therefore a metal-nonmetal reaction can always be assumed to be an oxidation-reduction reaction, which involves electron transfer

Reactions of Metals with Nonmetals

• Ionic Compounds: compounds formed in a reaction between a metal and a non-metal– Made up of positive and negative ions– 2Na(s) + Cl2(g) 2NaCl(s)

• Oxidation-Reduction Reactions-a reaction that involves the transfer of electrons– Na + Cl Na+ + Cl-

Electron-e-

Reactions of Metals with Nonmetals

• Oxidation: loss of electrons– Al Al3+ + 3e-

– Mg Mg2+ + 2e-

• Reduction: gain of electrons– Fe3+ + 3e- Fe– Cl + e- Cl-

Reactions of Metals with Nonmetals

• 2Cs(s) + F2(g) 2CsF(s)

– Ions Present in CsF? • Cs+ and F-

– Which is oxidized?• Cs• Cs Cs+ + e-

– Which is reduced?• F• F + e- F-

Reactions of Metals with Nonmetals

• 2Na(s) + Br2(l) 2NaBr(s)

– Ions Present: Na+ + Br -

– Oxidation: Na Na+ + e-

– Reduction: Br + e- Br -

• 2Ca(s) + O2(g) 2CaO(s)

– Ions Present: Ca2+ + O2-

– Oxidation: Ca Ca2+ + 2e-

– Reduction: O + 2e- O2-

Ways to Classify Reactions

• Reactions can be classified into one of six categories determined by the driving force of the reaction

• Double-displacement, acid-base, single-replacement, combustion, synthesis, decomposition

Ways to Classify Reactions

• Double-displacement Reactions– Undergo a double anion exchange– AB + CD AD + CB – Precipitation reactions

– K2CrO4(aq) + Ba(NO3)2(aq) BaCrO4(s) + 2KNO3(aq)

Ways to Classify Reactions

• Acid-Base Reactions– Involve H+ ions that ultimately form water in

the products

– HA + BOH H2O(l) + BA

– HCl(aq) + KOH(aq) H2O(l) + KCl(aq)

– H+ + OH- H2O

Ways to Classify Reactions

• Single Replacement Reactions– A reaction in which a single anion is

exchanged– A + BC B + AC

– Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)

Ways to Classify Reactions

• Combustion Reactions– Chemical reactions that involve oxygen and

produce energy (heat) so rapidly a flame results

– CH4(g) + O2(g) CO2(g) + 2H2O(g)

– Used to produce heat or electricity • C3H8(g) + 5O2 3CO2(g) + 4H2O(g)

• 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O(g)

• C(s) + O2(g) CO2(g)

Ways to Classify Reactions

• Synthesis Reactions– When a given compound is formed from

simpler materials– A + B AB

– 2H2(g) + O2(g) 2H2O(l)

– C(s) + O2(g) CO2(g)

– 2Na(s) + Cl2(g) 2NaCl

Ways to Classify Reactions

• Decomposition Reactions– A given compound is broken down into

simpler substances, usually accomplished by heating or adding electric current

– AB A + B

– 2H2O(l) 2H2(g) + O2(g)

– 2HgO(s) 2Hg(l) + O2(g)

Chemical Reactions

Ways to Classify Reactions

• S8(s) + 8O2(g) 8SO2

• 2Al(s) + 3Cl2(g) 2AlCl3• BaCl2(aq) + Na2SO4(aq) BaSO4(s) +

2NaCl

• 2AlN(s) 2Al(s) + N2(g)

• KOH(aq) + HCl(aq) H2O(l) + KCl(aq)

• 2C2H2(g) + 5O2(g) 4CO2(g) + H2O(l)