Ch.4 Chemical Rxns and Solution

Stoichiometry

4.1 Water

I. H2O Structure• A. Polar Covalent bonds between O and H

• B. Unequally shared e-s, more by O

• C. Oxygen gets slightly negative charge, Hydrogen slightly positive

• D. “Polar” molecule: negative end and positive end

II. Hydration• A. Process of water

separating ionsNaCl (s) Na+ (aq) + Cl- (aq)• B. Aqueous (aq): means

dissolved in water• C. Cations attracted to

Oxygen in water

• D. Anions attracted to Hydrogen in water• E. Dissolving: ions surrounded by water molecules or uncharged atoms surrounded by water

• F. Like dissolves like: polar molecules, ions, dissolved by water

• G. Non-polar (ex. Oil) dissolved by other non-polar molecules

4.2: I. Strong/ Weak Electrolytes• A. Solute: particles that are dissolved

• B. Solvent: dissolving agent

• C. Electrolytes: ionic compounds that break into ions allowing passage of electrons (electricity)

• E. Strong: conduct lots of electricity, fully break into ions

• F. Weak: not good conductors, don’t break apart easily

II. Arrhenius

• C. Strong bases (contain OH-), completely break into ions in solution

HCl (aq) + NaOH (aq) NaCl (s)+ H2O (l)100% 100%

• A. Defined “Acid”: produces protons (H+) when dissolved in water

• B. HCl, HNO3, H2SO4, HBr, HI, HClO4 : strong acids, completely break into ions (MEMORIZE THESE!)

III. Weak Electrolytes

HC2H3O2 (aq) H+(aq) + C2H3O2- (aq)

99% 1% 1%

Weak Acid

NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)

99% 99% 1% 1%Weak Base

• A. Non-electrolytes: do not become ions at all, ex. Ethanol, sucrose; but can still be dissolved

4.3: I. Composition of Solutions• A. Use molarity to measure solution concentration

Molarity (M) = moles of solute/L of solution

• B. Units are written as M or as moles/L

• C. Dilution: reducing concentration by adding water (or more solvent) to a solution

**Moles of solute does not change with dilution**

II. Dilution Calculation

• B. Molarity = moles/volume

• C. M1 x Volume1 = Moles = M2 x Volume2

• D. M1V1 = M2V2

1. Use MV=MV calculation to determine how many liters of a stock solution will give us the moles we need to make desired Molarity

2. Make up the remainder of the missing volume with distilled water

• A. To dilute a certain concentration solution to a lesser concentration

4.5: I. Precipitation Rxns• A. When two solutions are mixed, sometimes

insoluble products are made (“Precipitates”)Ex. KI (aq) + Pb(NO3)2 (aq) PbI2 (s) + KNO3 (aq)

• B. Also called “double replacement” rxns because two ions are switching places

• C. Need to be able to predict the products and their likely states

II. Determining Precipitates• A. If we mix KOH and Fe(NO3)3 what do we form?

Ex. KOH (aq) + Fe(NO3)3 (aq) _____ + _____

• B. A reddish precipitate is formed, how do we know which product it is?

• C. There are general solubility trends that we can use to reasonably predict

III. Solubility Rules• A. Always Soluble: Alkali metals (1st column),

NH4+, NO3

-, ClO3-, ClO4

-, C2H3O2-

• B. Mostly Soluble:Cl-, Br-, I- (except Ag+, Pb2+, Hg2

2+)• C. F- (except Ca2+, Ba2+, Sr2+, Pb2+, Mg2+)• D. SO4

2- (except Ca2+, Ba2+, Sr2+, Pb2+)• E. Mostly Insoluble: O2-, OH- ,CO3

2-, PO43-, S2-,

SO32-, C2O4

2-, CrO42- (except w/ soluble cation)

4.6: I. Reactions in Solutions• A. Balanced Molecular equation

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

• B. Complete Ionic equation: separate aqueous into ions

2K+ + CrO42- + Ba2+ + 2NO3

- BaCrO4(s) + 2K+ + 2NO3-

• C. Net Ionic equation: get rid of “Spectator Ions”, not participating in reaction

CrO42- (aq) + Ba2+ (aq) BaCrO4 (s)

4.7: I. Precipitation Rxn Stoichiometry

• 1. Start with net ionic equation

• 2. Since we’re dealing with solutions we are given Molarity of reactants instead of grams

• 3. Use Molarity to convert to moles

• 4. Other steps are the same

4.8: I. Acid – Base Rxns• A. Bronsted- Lowry defined acid as a proton donor

• B. Base: proton acceptor

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

• C. Net ionic: H+ (aq) + OH- (aq) H2O (l)

• D. A strong acid and a strong base neutralize each other to a salt and water

KOH (aq) + HC2H3O2 (aq) KC2H3O2 (aq) + H2O (l)

• F. Net Ionic:

OH- (aq) + HC2H3O2 (aq) C2H3O2- (aq) + H2O (l)

Since weak acid, doesn’t dissociate well in water (“aqueous”)

• G. Acid Base rxns often called “Neutralization Rxns” because of formation of water

E. Strong Base + Weak Acid:

Weak acid can’t lose all H+, but base is strong enough to rip off Proton from Acetic Acid

II. Titrations• A. Measured volume of known concentration

(“Titrant”) into unknown substance (“Analyte”) to react equally

• B. Acid base titration: exact amount of acid and base neutralize each other (“equivalence point”)

• C. Indicator changes color to signal end of reaction (“End point”)

• D. Oxidation/Reduction titration: use oxidizing agent (takes electrons) and reducing agent (loses electrons) in equal quantities to reach “End point”

4.9: I. Oxidation-Reduction Rxns• A. Also called REDOX• B. Involve electrons being transferred • C. To determine if electrons are being transferred in a

reaction, need to look at charges and see if and how they change

• D. For covalent (non-charged) compounds, we assign artificial charges called “Oxidation #’s”

II. Oxidation # Rules• 1. Atoms by themselves or attached to identical

atoms always 0

• 2. An ion charge = its oxidation #

• 3. Oxygen is -2 except in peroxides (-1)

• 4. Hydrogen is +1, except in metal hydrides (-1)

• 5. Sum of the ox. #’s of a neutral compound must equal zero

III. REDOX Rxn Information• A. Always have one element that is oxidized

and one element that is reduced

• B. Oxidation: Losing electrons

• C. Reduction: Gaining electrons• D. Oxidized molecule (“reducing agent”), reduced molecule (“oxidizing agent”)

LEO

4.10: I. Balancing REDOX Rxns: Half-Rxn Method for Acidic Solutions

• A. For hard to balance Acid-Base Rxns

1. Assign oxidation #’s to complete ionic eqn.

2. Make half-rxns of just the molecules before and after that are being oxidized or reduced

3. Balance all elements in half rxns except H, O

4. Balance O using H2O5. Balance H using H+

6. Balance charges using electrons7. Combine half-rxns and cancel out repeats

• 1. Balance like an acidic solution• 2. To both sides of rxn, add OH- equal to number

of H+

• 3. Make equal H+ and OH- into water and cancel other waters out

Half-Rxn Method for Basic Solutions

II. Balancing: