Chemistry pH and pOH—Acids and Bases
Useful Acids • Artists often use hydrofluoric acid to etch designs on glass. • Hydrochloric acid (Chlorine*) is used to keep the pH of your pool healthy
Acids and Bases have distinctive properties Acids Bases Bitter taste Slippery feel (think about soap) Aqueous solutions of bases are electrolytes (conduct electricity) Bases cause certain dyes to change color (indicators) • Tart, sour taste • Aqueous solutions of acids are electrolytes (conduct electricity) • Acids cause certain dyes to change color (indicators) • Many metals (i.e. Zn, Mg) react with aqueous solutions of acids to produce hydrogen gas (H2)
Arrhenius Acids and Bases • Chemists have known the properties of acids and bases for many years. However, they were not able to propose a theory to explain their behavior. • In 1887, the Swedish chemist Svante Arrhenius proposed a new way of defining and thinking about acids and bases.
Acids and Bases According to Arrhenius: Acids Bases Compounds that ionize to yield hydroxide ions (OH-) in aqueous solutions. • Hydrogen-containing compounds that ionize to yield hydrogen ions (H+) in aqueous solutions. Sodium Hydroxide NaOH Red=OH- White=Na+ Hydrochloric Acid HCl Green=Cl- Red=H+
What causes a substance to become an acid? • Only a hydrogen that is bonded to a very electronegative element (O, F, Cl) can be released as a ion. (very polar bonds!)
Hydronium Ions • When a hydrogen ion (H+) ionizes from the element it was attached to, it is released into the aqueous solution. • However, it isn’t really floating freely. It becomes attached to another water molecule and forms a Hydronium Ion (H3O+) http://preparatorychemistry.com/acids_flash.htm
Arrhenius Bases • Bases dissociate to form hydroxide ions (OH-) in aqueous solutions. • Some bases (such as NaOH or KOH) are very caustic. A caustic substance can burn or eat away materials which it contacts. Drain cleaners usually contain a strong base such as NaOHto eat away the clog.
Arrhenius Bases • KOH and NaOH are strong bases. Potassium (K) and Sodium (Na) are Group IA elements (alkali metals) and react violently with water. • The products of these reactions are aqueous solutions of a hydroxide and H2 gas. Ex: • 2 Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) http://www.youtube.com/watch?v=FjRD89Ukfv8&noredirect=1
BrØnsted-Lowry Acids and Bases • The Arrhenius definition of acids and bases does not include all substances that have acidic or basic properties. (ex: NH3 is a base!) • In 1923, the Danish chemist Johannes BrØnsted and the English chemist Thomas Lowry (working independently) developed a different definition of acids and bases: • Acids=Hydrogen-ion donor • Bases=Hydrogen-ion acceptor
Conjugate Acids and Bases Conjugate acid Conjugate base The ion or molecule that remains after an acid loses a hydrogen ion. • The ion or molecule formed when a base gains a hydrogen ion. • Ex: • NH3(aq) + H2O(l)↔NH4+(aq) + OH-(aq) • NH4+ = conjugate acid of base NH3 • OH- = conjugate base of the acid H2O Remember: an acid is the hydrogen-ion donor; a base is the hydrogen-ion acceptor. http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/animations/NH3eqtg.html
Hydrogen Ions and Acidity • Water molecules are highly polar and are in constant motion. On occasion, the collisions between water molecules are energetic enough for a reaction to occur. • When this happens, a H+ ion can leave one H2O and be transferred to another. http://www.youtube.com/watch?v=IjsgLU8C5X4
Keq of Water is Kw • The self-ionization of water occurs to a very small extent. In pure water at 25oC, the concentration of H+ ions is only 1X1o-7 M. • That also means the concentration of OH- is also 1 X 10-7 M. • Together, the Keq of water (Kw) is 1 X 1o-14 M • Kw = [H+][OH-] = [1X10-7][1X10-7] = 10-14 [H2O] 
pH • pH is the Negative log of the concentration of H+ ions. • If the concentration of H+ ions is 1 X 10-7, the negative log of this is 7. • So, the pH is 7 NEGATIVE LOG 10-2 = - -2 = 2 10-3 = - - 3 = 3 10-4 = - - 4= 4 10-5 = - - 5= 5
pH and pOH • If the concentration of H+ is 1 X 10-7, the concentration of OH- is also 1 X 10-7 • But, if the concentration of H+ changes, so will the concentration of OH-. • What will the concentration of OH- be if the H+ concentration is 1 X 10-6? What will be the pH?