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 Bases Strong 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 Acids multiple acidic H atoms H2SO4  H+ + HSO4- HSO4-  H+ + SO42- Not all H’s are acidic: CH3CO2H

Bases Strong bases are hydroxide salts For now, the only important weak base is NH3.

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

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 equal acidic: [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: 100 102 log(102) = 2 10 101 log(101) = 1 1 100 log(100) = 0 0.1 10–1 log(10–1) = –1 0.01 10–2 log(10–2) = –2 pH = –log [H3O+] pH if [H3O+] = 10–5? 10–9? Acidic or basic? pH if [H3O+] = 0.000057 M?

Finding [H3O+] from pH [H3O+] = 10-pH What is [H3O+] if pH = 8.9?

pH: Quantitative Measure of Acidity Acidity is related to concentration of H+ (or H3O+) pH = -log[H3O+]