Chapter 4: Types of Chemical Reactions

Goals:

• To be able to predict chemical reactivity.

• To know how to synthesize specific compounds.

Types of Reactions

• Acid-Base: proton-transfer• Oxidation-Reduction: electron-transfer• Precipitation: formation of insoluble salts• Gas Forming• Organic:

– Substitution– Addition– Elimination

Reactions in Aqueous Solution

Unless mentioned, all reactions studied this and next week occur in aqueous solution.

Electrolytes

Strong Electrolytes: solute breaks apart to give ions in solution.

NaCl Na+ + Cl-

Weak Electrolytes: solute partially breaks apart to give ions.

CH3CO2H CH3CO2- + H+ happens less than 5%

Nonelectrolytes: no ions formed.

CH3CH2OH

Brønsted-Lowery Acid-Base Definitions

• An acid is a substance that donates a proton (H+) to a base

• A base is a substance that accepts a proton (H+) from an acid

Brønsted-Lowery Definitions• acid: donates a proton (H+) to a base

• base: accepts a proton (H+) from an acid

• Acid-base reactions can be reversible:reactants products or products reactants

Brønsted-Lowery Definitions• An acid is a substance that donates a proton (H+)

to a base

• A base is a substance that accepts a proton (H+) from an acid

• Acid-base reactions can be reversible:reactants products or products reactants

Important Acids and BasesStrong Acids:

HCl hydrochloric

HBr hydrobromic

HI hydroiodic

HNO3 nitric

H2SO4 sulfuric

HClO4 perchloric

Weak Acid:

CH3CO2H acetic

Any other acids are WEAK

Strong Bases:

LiOH lithium hydroxide

NaOH sodium hydroxide

KOH potassium hydroxide

Ca(OH)2 calcium hydroxide

Ba(OH)2 barium hydroxide

Weak Base:

NH3 ammonia

STRONG acids in water: 100% of acid molecules form ions:

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

H3O+ is

hydronium ion

WEAK acids in water, ~5% or less of acid molecules form ions (acetic, H3PO4, H2CO3)

Polyprotic Acidsmultiple acidic H atoms

H2SO4 H+ + HSO4-

HSO4- H+ + SO4

2-

Not all H’s are acidic:

CH3CO2H

If H3PO4 reacts as an acid, which of the following can it not make?

• 1. H4PO4+

• 2. H2PO4-

• 3. HPO42-

• 4. PO43-

If C2O42- reacts in an acid-base

reaction, which of the following can it not make?

• 1. H2C2O4

• 2. HC2O4-

• 3. 2 CO2

Acid-Base Reactions

Strong Acid + Strong Base

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) acid base “salt” water

Acid-Base Reactions

Diprotic Acids or Bases

H2SO4(aq) + NaOH(aq)

H2SO4(aq) + Ba(OH)2(aq)

HCl(aq) + Ba(OH)2(aq)

Acid-Base Reactions

Strong Acid + Weak Base

HCl(aq) + NH3(aq) NH4Cl(aq)

Acid-Base Reactions

Weak Acid + Strong Base

HCN(aq) + NaOH(aq) NaCN(aq) + H2O(l) acid base “salt” water

Net Ionic Equations

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

What really happens:

H+(aq) + OH-(aq) H2O(l)

Sodium ion and chloride ion are “spectator ions”

Reactions involving weak bases

HCl(aq) + NH3(aq) NH4+(aq) + Cl-(aq)

Net-Ionic Equation:

NH3(aq) + H+(aq) NH4+(aq)

CH3CO2H(aq) + NaOH(aq)

• 1. CH3CO2H2+(aq) + NaO(aq)

• 2. CH3CO2-(aq) + H2O(l) + Na+(aq)

• 3. CH4(g) + CO2(g) + H2O(l)

HCN(aq) + NH3(aq)

• 1. NH4+(aq) + CN-(aq)

• 2. H2CN+(aq) + NH2-(aq)

• 3. C2N2(s) + 3 H2(g)

Solution Concentration: Molarity

• Molarity = moles solute per liter of solution

• 0.30 mol NH3 dissolved in 0.500 L

Concentration =

• Written like: [NH3] = 0.60 M

pH Scale• In pure water, a few molecules

ionize to form H3O+ and OH–

H2O + H2O OH– + H3O+

• In acidic and basic solutions, these concentrations are not equalacidic: [H3O+] > [OH–]basic: [OH–] > [H3O+]neutral: [H3O+] = [OH–]

pH Scale• Measure how much H3O+ is in a

solution using pH• pH < 7.0 = acidic• pH > 7.0 = basic• pH = 7.0 = neutral• Measure of H3O+ and OH–

concentration (moles per liter) in a solution

• As acidity increases, pH decreases

pH Scale• The pH scale is logarithmic:The pH scale is logarithmic:

100100 101022 log(10log(1022) = 2) = 21010 101011 log(10log(1011) = 1) = 111 101000 log(10log(1000) = 0) = 00.10.1 1010–1–1 log(10log(10–1–1) = –1) = –10.010.01 1010–2–2 log(10log(10–2–2) = –2) = –2

• pHpH = –log [H= –log [H33OO++]]

• pH if [HpH if [H33OO++] = 10] = 10–5–5? 10? 10–9–9? ?

Acidic or basic?Acidic or basic?

• pH if [HpH if [H33OO++] = 0.000057 M?] = 0.000057 M?

Finding [H[H33OO++] from pH] from pH

[H[H33OO++] = 10] = 10-pH-pH

What is [HWhat is [H33OO++] if pH = 8.9?] if pH = 8.9?

pH: Quantitative Measure of Acidity

• Acidity is related to concentration of H+ (or H3O+)

• pH = -log[H3O+]