Chem-To-Go Lesson 38 Unit 10

 Both acids and bases ionize or dissociate in water  Acids: taste sour, conduct electricity, cause certain indicators to change color, turn blue litmus paper red, and react with metals to form H 2 gas  Bases: taste bitter, feel slippery, conduct electricity, and cause certain indicators to change color, turn red litmus paper blue PROPERTIES

 Acids and bases dissociate in water.  Dissociation reactions show this phenomenon in a chemical equation.  EX. HCl H + + Cl -  EX. Ca(OH) 2 2OH - + Ca 2+  Notice the arrow. Acid/base dissociations are reversible reactions.  Some reactions are MORE reversible than others. PROPERTY: DISSOCIATION IN WATER

 Strong and weak do not mean the same as concentrated and dilute. HCl H + + Cl -  Strong  Strong = completely dissociate when dissolving in water; the forward dissociation reaction is favored  Weak  Weak = partially dissociate when dissolving in water; the dissociation reaction reaches equilibrium (the forward and reverse reactions are equally likely to occur) STRONG AND WEAK

Strong Acids  Only a few acids dissociate completely and remain dissociated.  Halides: HCl, HBr, HI  Ex. HCl + H 2 O  H + + Cl -  Oxyacids: H 2 SO 4, HClO 4, HNO 3  Ex. HNO 3 + H 2 O  H + + NO 3 -  We’ll use strong acids later during our unit in calculations. Weak Acids  Most acids dissociate during the forward reaction, but the reverse reaction rebuilds the original acid quickly.  HC 2 H 3 O 2 H + + C 2 H 3 O 2 -  The reaction reaches equilibrium, meaning the ions often rebuild the acid. STRONG ACIDS VS. WEAK ACIDS NOTE: SAME IDEAS APPLY TO STRONG VS. WEAK BASES.

 Arrhenius definition for acid: compound that produces hydrogen ions (H + ) when dissolved in water.  Let’s write the dissociation reaction for the HCl example. HCl + H 2 O  H + + Cl - Or HCl + H 2 O  H 3 O + + Cl - H 3 O + = Hydronium ion FIRST ACID DEFINITION

 Arrhenius definition of base: a compound that produces hydroxide ions (OH-) when dissolved in water.  Let’s write the dissociation reaction for the NaOH example. NaOH + H 2 O  Na + + OH - FIRST BASE DEFINITION

 Acids are hydrogen-containing compounds that ionize to yield hydrogen ions in aqueous solution...  Bases are compounds that ionize to yield hydroxide ions in aqueous solution...  BUT, NH 3 is a base! Arrhenius’ theory doesn’t hold up in every case, so... ARRHENIUS ACIDS & BASES

 An acid is a hydrogen-ion donor, and a base is a hydrogen-ion acceptor.  Example: NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH - (aq)  Analyze the compounds as they react. What happens during the reaction?  NH 3 accepts an H + to become NH 4 +  H 2 O donates an H + to become OH - BRONSTED-LOWRY ACIDS AND BASES

 Previous Example: NH 3 (aq) + H 2 O(l)  NH 4 + (aq) + OH - (aq)  According to the Bronsted-Lowry theory:  An acid has a conjugate base.  Likewise, a base has a conjugate acid.  We refer to them as conjugate acid-base pairs. The pair differs only by a SINGLE hydrogen.  Example: NH 3 and NH 4 +, H 2 O and OH - ACID-BASE PAIRS

 H 2 SO 4 + H 2 O  H 3 O + + HSO 4 - 1)Label the acid, base, conjugate acid, and conjugate base. 2)Write conjugate acid-base pairs. APPLICATION OF THE BRONSTED-LOWRY THEORY