reactions in aqueous solution - wikispacesin+aqueous... · reactions in aqueous solution learning...

Reactions in Aqueous Solution

Learning the basic types of reactions we will examine throughout the year.

1Friday, October 15, 2010

Aqueous Solutions• Aqueous means related to water

• Aqueous solutions are substances dissolved in water.

• Ionic compounds will separate in water to form cations, +, and anions, –.

• Charged substances allow electricity to move through solution. Charged substances dissolved in water are called electrolytes.

• Molecules will dissolve in water but not conduct.

Recall: Solutions = homogeneous mixtures

Solutions = solute dissolved in a solvent

Not all ionic compounds dissolve.

If a substance doesn’t dissolve it is a nonelectrolyte


Types of Reactions• Precipitate Reactions: When two solutions form an ionic

compound that will not dissolve.

• Acid Base reaction: When two solutions neutralize the acid or base characteristics of each other.

• Reduction-Oxidation Reactions: Where electron movement is a driving force in a reaction; often this is observed by the movement of oxygen from one substance to another.

So you will have to memorize what ions form solids

You will also have to learn which solids form acids and which form bases, and how a’s & b’s form gases

Reactions where you must balance mass & charge.


Oh No!• You must be ready and willing to begin

memorizing all this now. Don’t put it off!!!

• This is a chapter full of important and vital information, but we can’t slow down because most of the AP test is on the material covered in the last 3 months.


Ionic Compounds & Solubility

• Ions are held together by electrostatic attractions: + <-> –

• Ions dissolve by having water surround the ions.

• If the force of attraction between the ions is stronger than the attraction between the water molecules and ions then the compound will not dissolve.

• (increased rates of dissolving: smaller pieces, agitation(stirring), heat)


Solubility RulesIons that form weak bonds

& form strong bonds• Follow rules in this order:

1. Alkali metals and ammonia, NH4+, will always dissolve

2. Nitrate, perchlorate, chlorate, acetate will always dissolve

3. Hg22+ , Ag+ , Pb2+ will not dissolve (unless with above)

4. Halogens will dissolve (except above).

5. Most Alkaline Earth Metals: Ca2+, Ba2+, Sr2+ are insoluble

6. Sulfates are soluble

7. Carbonates, chromates, oxalates, phosphates, sulfides are insoluble.

8. Oxides and hydroxides and other ions are insoluble.

Li+, Na+, K+, Rb+, Cs+

NO3–, ClO4–, ClO3–, CH3CO2–

Cl–, Br–, I– (F– is sometimes an exception)

SO42–

CO32–, CrO42–, C2O42–, PO43–, S2– (COPS)

HAPpy

O2–, OH–


Practice

cation anion cation anion

Fe3+ NO3– Na+ MnO4

–

Co2+ CO32– NH4

+ PO43–

Ba2+ Br– Ca2+ SO42–


Practicereactants precipitate other

Na2O + CaCl2 CaO NaClSr(NO3)2 + K2SO4 SrSO4 KNO3

AgCH3CO2 + FeCl3 AgCl Fe(CH3CO2)3

K2C2O4 + Co(ClO4)2 CoC2O4 KClO4

➔ +

➔ +

➔ +

➔ +

2 2

3 3 2


Spectator Ions• The ions that don’t form a solid (or gas or liquid) are

called spectator ions.

• Spectator ions are still in solution and don’t really form a second compound. They are unchanged.

Na2O + CaCl2 CaO NaCl

Sr(NO3)2 + K2SO4 SrSO4 KNO3

Rb

Rb Rb

RbClO3

ClO3 ClO3

ClO3NH4

NH4 NH4

NH4➔ +

➔ +

CH3CO2

CH3CO2 CH3CO2

CH3CO2


Acid Base Reactions• Acids are compounds that donate an H+

(hydrogen ion or proton)—much more later.

• Bases are compounds that accept an H+

• The simplest reaction is H+ + OH– ➔ H2O

• Hydronium ion, H3O+, is equivalent to H+ since in water: H+ + H2O ➔ H3O+ . H+ is very reactive & cannot remain unbound in water.


Neutralization Rxn• Acids (obviously) react with bases to form water

and a salt. This is a neutralization reaction.

Acid Base Water SaltHCl NaOH H2O NaCl

HNO3 Ca(OH)2 H2O Ca(NO3)2

H2SO4 Al(OH)3 H2O Al2(SO4)3

+ ➔ +

+ ➔ +

+ ➔ +

2 2H OH3 2 6H OH


Spectator Ions• Water is a liquid and so H+ and OH– have formed a new

compound and are not spectator ions.

Acid Base Water SaltHCl NaOH H2O NaCl

HNO3 Ca(OH)2 H2O Ca(NO3)2

H2SO4 Ba(OH)2 H2O BaSO4

+ ➔ +

+ ➔ +

+ ➔ +Solid


MemorizeStrong Acids

Weak Acids

Strong Bases

Weak Bases

HCl HF LiOH NH3

HBr HCH3CO2or

HC2H3O2

NaOH Na2CO3

HIHCH3CO2

or

HC2H3O2 KOH NaHCO3

HNO3 H3PO4 RbOH Al(OH)3H2SO4 H2CO3 Ba(OH)2 Fe(OH)3HClO4 H2C2O4 Ca(OH)2 Co(OH)2

Etc. sort of ? Etc.


Strong and Weak• Strong acids and bases are strong electrolytes; they

separate completely in water. They dissociate.

• Weak acids & bases only partially dissociate in water.

• Show partial dissociation by: HF ⇄ H+ + F–

• recall OH– form insoluble salts. So how does Ca(OH)2 become a strong base.

• If an alkaline metal hydroxide dissolves it stays separated and is a strong base. That is, not much is in solution but what is in solution is found as separate ions.


using ⇄HBr + NaOH ➔ HOH + NaBr

HF + KOH ⇄ H2O + KFthe weak acid wants to reform

HNO3 + Na2CO3 ⇄ H2O + CO2 + NaNO3➔a gas


Reactions that form Acids & Bases

• Acids are H+ and a nonmetal, bases are often OH– and a metal.

• Acids form when a nonmetal oxide reacts with water:SO3 + H2O ➔ H2SO4 ; CO2 + H2O ➔ H2CO3

• Bases form when a metal oxide reacts with water:Na2O + H2O ➔ 2NaOH ; CaO + H2O ➔ Ca(OH)2


Gas Forming Reactions

• Acids react with bases to form gases

metal carbonates ➔ carbon dioxideHCl + NaHCO3 ➔ H2O + CO2(g) + NaCl

metal sulfide ➔ hydrogen sulfide2HCl + Na2S ➔ 2NaCl + H2S(g)

metal sulfite ➔ sulfur dioxide2HCl + Na2SO3 ➔ 2NaCl + SO2(g) + H2O


Net Ionic Equations• Net Ionic Equations are chemical equations that show

only the formation of new substances. All spectator ions are removed.

NaOH + NH4Cl ➔H2O + NH3 + NaCl(g)(l)(aq)(aq) (aq)

OH–(aq)+ NH4+(aq) ➔ H2O(l) + NH3(g)

NaOH +Al(ClO4)3 ➔NaClO4 + Al(OH)3(aq)(aq)(aq) (s)

OH–(aq) +Al3+(aq) ➔ Al(OH)3(s)


Gas Forming Reactions

• Bases react with ammonium to form gasesammonium salt ➔ ammoniaNaOH + NH4Cl ➔ H2O + NH3(g) + NaCl

Ca(OH)2 + (NH4)2CO3 ➔ 2H2O(l) + NH3(g) + CaCO3(s)


Redox Reactions: an introduction

• Redox reactions occur when a chemical reaction transfers electrons from one substance to another.

• There is a series of rules needed to determine the charge of each atom, but first let’s look at some decomposition and synthesis reactions that are almost always redox reactions.


Synthesis of anIonic Compound

Na + Cl2 ➔ NaCl

Na + Cl2 ➔ { Na+ + Cl– }

Na ➔ Na+ + 1e– & Cl2 + 2e– ➔ 2Cl–

balance the movement of electrons

2 Na ➔ 2Na+ + 2•1e– & Cl2 + 2e– ➔ 2Cl– oxidation = loss of e– reduction = gain of e–

LeO (OiL) GeR (RiG)


Synthesis of aCovalent Compound

C + O2 ➔ CO2

C + O2 ➔ { C4+ + O2– : sort of }

C ➔ C4+ + 4e– & O2 + 4e– ➔ 2O2–


C ➔ C4+ + 4e– & O2 + 4e– ➔ 2O2–

oxidation = loss of e– reduction = gain of e–

LeO (OiL) GeR (RiG)But this isn’t going to always work; e.g., CO.


Decomposition of anIonic Compound

Cr2O3 ➔ Cr + O2

{ 2 Cr3+ + 3 O2– } ➔ Cr + O2

Cr3+ + 3e– ➔ Cr & 2O2– ➔ O2 + 4e–


4 Cr3+ + 4•3e– ➔ 4Cr & 6O2– ➔ 3O2 + 3•4e–

reduction = gain of e– oxidation = loss of e– GeR (RiG) LeO (OiL)


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