1. Acids and Bases Unit 12

2. Properties of an Acid  Sour taste  Turns litmus paper red (and responds uniquely to other indicators)  Reacts with:  Hydroxide bases to produce water and an ionic compound.  Pure metals (i.e. Zn & Mg) to produce H 2 (g)  Carbonates to produce CO 2 (g).

3. Properties of a Base  Bitter taste  Slippery, soapy feeling  Turns litmus paper blue (and responds uniquely to other indicators)

4. Arrhenius Definition of an Acid/Base  Acids produce hydrogen ions, or protons (H + ) in aqueous solution.  HA represents a generic acid. HA ⇄ H + (aq) + A - (aq) Example: HCl (g) ⇄ H + (aq) + Cl - (aq)  Bases produce hydroxide ions (OH – ) in aqueous solution. MOH represents a generic base. MOH ⇄ M + (aq) + OH - (aq) Example: NaOH ⇄ Na + (aq) + OH - (aq)  The Arrhenius definition is very limited – Not all bases contain OH - Example – NH 3 or ammonia is a common base. (1859 – 1927)

5. Neutralization of an Arrhenius Acid and Base  An Arrhenius acid and an Arrhenius base will react to form a salt and water.  Example: HCl (aq) + NaOH (aq) ⇄ NaCl (aq) + H 2 O (l)  The salt is formed from the positive ion of the base (Na + ) and the negative ion of the acid (Cl - ).  The water is formed from hydrogen ion of the acid (H + ) and the hydroxide ion of the base (OH - ).  If such a reaction results in a solution with no hydrogen or hydroxide ions, the products would be neither an acid nor a base. This type of reaction is called a neutralization reaction.

6. Bronsted-Lowry Definition  Arrhenius’ model was limited because it only applied to substances with hydrogen and hydroxide ions.  Bronsted and Lowry proposed a new definition for acid and bases:  Acids are proton donors (give off an H + )  Bases are proton acceptors What do we mean by a proton donor? Hydrogen!What is H + ?

7. Bronsted-Lowry Acid/Base  Below is the general reaction that occurs when an acid (HA) is dissolved in water. HA (aq) + H 2 O (l) ⇄ H 3 O + (aq) + A - (aq)  The acid (HA) is donating a proton to the base (H 2 O).  When the H 2 O accepts the proton it become H 3 O + which is known as the hydronium ion. AcidBase

8. Conjugate Acid/Base Pairs HA (aq) + H 2 O (l) ⇄ H 3 O + (aq) + A - (aq)  This is a reversible reaction. If we look at the reverse reaction, which substance would donate a proton?  Because the H 3 O + is a proton donor in the reverse reaction, it is considered the conjugate acid.  If we look at the reverse reaction, which substance would accept a proton?  Because the A - is a proton acceptor in the reverse reaction, it is considered the conjugate base.  Acid donates proton, becomes the conjugate base.  Base accepts proton, becomes the conjugate acid. AcidConjugate BaseBaseConjugate Acid

9. Is water an acid or a base?  Pure water is neutral!  Water can act as an acid or a base: H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq)  One water molecule acts as an acid by donating a proton; the other water molecule acts as a base by accepting the proton.  The reverse reaction dominates in normal conditions, indicated by the longer arrow.