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. CH 3 CO 2 H CH 3 CO H + happens less than 5% Nonelectrolytes: no ions formed. CH 3 CH 2 OH Brnsted-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 Brnsted-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 Brnsted-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: HClhydrochloric HBrhydrobromic HIhydroiodic HNO 3 nitric H 2 SO 4 sulfuric HClO 4 perchloric Weak Acid: CH 3 CO 2 Hacetic Any other acids are WEAK Strong Bases: LiOHlithium hydroxide NaOHsodium hydroxide KOH potassium hydroxide Ca(OH) 2 calcium hydroxide Weak Base: NH 3 ammonia STRONG acids in water: 100% of acid molecules form ions: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq) H 3 O + is hydronium ion WEAK acids in water, ~5% or less of acid molecules form ions (acetic, H 3 PO 4, H 2 CO 3 ) Polyprotic Acids multiple acidic H atoms H 2 SO 4 H + + HSO 4 - HSO 4 - H + + SO 4 2- Not all Hs are acidic: CH 3 CO 2 H If H 3 PO 4 reacts as an acid, which of the following can it not make? 1. H 4 PO H 2 PO HPO PO 4 3- If C 2 O 4 2- reacts in an acid-base reaction, which of the following can it not make? 1. H 2 C 2 O 4 2. HC 2 O CO 2 Acid-Base Reactions Strong Acid + Strong Base HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) acid base salt water Net Ionic Equations HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) What really happens: H + (aq) + OH - (aq) H 2 O(l) Sodium ion and chloride ion are spectator ions Reactions involving weak bases HCl(aq) + NH 3 (aq) NH 4 + (aq) + Cl - (aq) Net-Ionic Equation: NH 3 (aq) + H + (aq) NH 4 + (aq) CH 3 CO 2 H(aq) + NaOH(aq) 1. CH 3 CO 2 H 2 + (aq) + NaO(aq) 2. CH 3 CO 2 - (aq) + H 2 O(l) + Na + (aq) 3. CH 4 (g) + CO 2 (g) + H 2 O(l) HCN(aq) + NH 3 (aq) 1. NH 4 + (aq) + CN - (aq) 2. H 2 CN + (aq) + NH 2 - (aq) 3. C 2 N 2 (s) + 3 H 2 (g) Solution Concentration: Molarity Molarity = moles solute per liter of solution 0.30 mol NH 3 dissolved in L Concentration = Written like: [NH 3 ] = 0.60 M pH Scale In pure water, a few molecules ionize to form H 3 O + and OH H 2 O + H 2 O OH + H 3 O + In acidic and basic solutions, these concentrations are not equal acidic: [H 3 O + ] > [OH ] basic: [OH ] > [H 3 O + ] neutral: [H 3 O + ] = [OH ] pH Scale Measure how much H 3 O + is in a solution using pH pH < 7.0 = acidic pH > 7.0 = basic pH = 7.0 = neutral Measure of H 3 O + and OH concentration (moles per liter) in a solution As acidity increases, pH decreases pH Scale The pH scale is logarithmic: log(10 2 ) = log(10 1 ) = log(10 0 ) = 1 log(10 1 ) = 2 log(10 2 ) = 2The pH scale is logarithmic: log(10 2 ) = log(10 1 ) = log(10 0 ) = 1 log(10 1 ) = 2 log(10 2 ) = 2 pH= log [H 3 O + ]pH= log [H 3 O + ] pH if [H 3 O + ] = 10 5 ? 10 9 ? Acidic or basic?pH if [H 3 O + ] = 10 5 ? 10 9 ? Acidic or basic? pH if [H 3 O + ] = M?pH if [H 3 O + ] = M? [H 3 O + ] from pH Finding [H 3 O + ] from pH [H 3 O + ] = 10 -pH What is [H 3 O + ] if pH = 8.9? pH: Quantitative Measure of Acidity Acidity is related to concentration of H + (or H 3 O + ) pH = -log[H 3 O + ]