Chapter 8

Reactions in Aqueous Solutions

Will the reaction occur?

• Driving forces– Forming a solid (“precipitate)– Transfer of electrons– Forming water– Forming a gas

• Will the reaction occur? Yes, if one of the above driving forces happens.

Dissolving ions

• A strong electrolyte completely separates into ions when it is mixed with water.– Example: NaCl

Complete the equation of silver nitrate dissolving in water:

AgNO3(s)

H2ONa+(aq) + Cl-(aq)NaCl(S)

Precipitates

• If you mix NaCl(aq) and AgNO3 (aq) , what possible products could form?

NaCl(aq) and AgNO3 (aq) ?

The anions (Cl- and NO3-)change places.

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

• AgCl is a precipitate, so write an “(s)”• A precipitate is an insoluble solid.

Insoluble = it cannot dissolve in water.• What is the driving force in this reaction?

Double replacement (continued)• A double replacement reaction happens when a

precipitate forms.

• Reactants must be aqueous (dissolved in water)– Example: NaCl dissolved would be shown as NaCl(aq)

• If something is “slightly soluble” it will also make a precipitate, so slightly soluble is almost the same as insoluble. Solubility chart on page 245 will show you when a precipitate will form.

Example

– Example: KNO3

– Soluble or insoluble?

– Rule #2: K+ salts are usually soluble.

– Answer: Soluble (No precipitate)

Will this double replacement actually happen??

• Practice: Will this form a reaction?

1.Write the formulas of the possible products (Inside and outside)

2.Use the solubility chart to determine if any of the products are INSOLUBLE.

3. If one or both products are insoluble, a chemical reaction will happen.

(Reacts??) ________)( )(23)(42 aqaq ClOBaSOLi

• Examples: – KNO3 and BaCl2– KOH and Fe(NO3)3– Na2SO4 and Pb(NO3)2

– Annimation: http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/crm3s2_3.swf

Double Replacement

• You should be able to write three types of chemical equations for double replacement:– 1. Molecular equation– 2. Complete ionic equation– 3. Net ionic equation

Double Replacement

• The molecular equation shows the complete formulas of all reactants and products

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

• The complete ionic equation shows all strong electrolytes (aq) as ions.

Na+(aq) + Cl-(aq) + Ag+

(aq) + NO3-(aq)Na+

(aq) + NO3-(aq + AgCl(s)

Double Replacement

• The net ionic equation shows only the chemicals participating in the reaction.

• Steps:– Remove the “spectator ions”– Spectator ions are the same on the reactant

side and the product side (Na+ and NO3-)

Na+(aq) + Cl-(aq) + Ag+

(aq) + NO3-(aq)Na+

(aq) + NO3-(aq + AgCl(s)

Net ionic equation:

Cl-(aq) + Ag+(aq) AgCl(s)

8.2

Acid-Base Reactions and

Oxidation-Reduction Reactions

Arrhenius Acids and Bases

• An Arrhenius Acid is a substance that produces H+ ions when dissolved in water.

• A “strong acid” is an acid that is a strong electrolyte. It completely separates into H+ and the anion.

HCl H+ + Cl-

– Strong acids: HCl, HNO3 and H2SO4

Arrhenius Acids and Bases

• An Arrhenius Base produces OH ions when dissolved in water.

• A strong base completely separates into a metal cation and OH-

• NaOH Na+ + OH-

Acid-Base Reaction

• Write the molecular equation, complete ionic equation and net ionic equation for the reaction of hydrochloric acid and sodium hydroxide.

• Molecular equation

HCl + NaOH H2O + NaCl

• Complete ionic equation

H+ +Cl- + Na+ + OH- H2O(l) + Na+ + Cl-

• Net ionic equation

H+ +OH- H2O(l)

Acid-Base Reaction

• When an acid and base are mixed, the products are always water and a salt.

• How do you know a reaction happened? The solution will be hot.

• An acid-base reaction is the reaction of H+ and OH-

Oxidation Reduction

• In an oxidation-reduction reaction, electrons move from one chemical to another.

• Common reactions:– A metal and a nonmetal react

– Reactions between nonmetals where O2 is a reactant or product.

Oxidation Reduction

• Ca(s) + Cl2(g) CaCl2(s)

• Which chemical is losing electrons? How many electrons?– Ca loses two electrons

• Which chemical is gaining electrons? How many electrons?– Cl gains one electron

2 electrons

Oxidation Reduction

• This happens in two steps:Ca Ca2+ + 2e-Cl + e- Cl-

• Draw a picture of this reaction:

Mg(s) + CuCl2(s) Cu(s) + MgCl2(s)

Which chemical loses electrons? How many?

Which chemical gains electrons? How many?

8.3Types of reactions & “How do we

know IF it will happen?”

Classifying reactions• Looking at a chemical equation, be able to

classify it…• Based on driving force:

– 1. Precipitation reactions– 2. Oxidation-reduction reactions– 3. Acid-base reactions

• Types of oxidation-reduction reactions:– 1. Combustion reactions– 2. Synthesis reactions– 3. Decomposition reactions– 4. Single Replacement

Double Replacement

AX+BY → AY +BX

• Each anion changes places with the cation.

• Example: sodium sulfate + copper (II) chloride

Single Replacement:

A+BX → AX +B

• A = a metal or group 7 element.

• B = metal, group 7 element or hydrogen

• X = anion

• A replaces B in the BX compound.

• Example:

Zn(s) +2HCl(aq) H2(g) + ZnCl2(aq)

Synthesis Reactions

• Synthesis– 2 or more substances combine to form one, bigger molecule.

• General formula…

• Example: Combustion of Ca

• Ca + O2 CaO

ABBA

Decomposition Reactions

• Decomposition– one substance splits to form several smaller molecules/atoms.

• General formula…

• Example: Breaking water molecules.

• What equation will it have? (it’s a reaction that results in Hydrogen gas, H2, and Oxygen,O2)

• Usually requires energy input (heat, electricity, etc), because we are breaking bonds.

BAAB

Combustion Reactions

Combustion Reactions

• Combustion – a substance combines with oxygen. When they react they release energy, water vapor and carbon dioxide.

• Will need to balance these

• Ex: C2H5OH + O2 CO2 + H2O

Classifying reactions

• Practice: Classify this reaction in as many ways as possible

• 2Ca + O2 2CaO

• This is classified as a synthesis and combustion. It is also an oxidation-reduction reaction.