Only the Strong Survive

1. Only the Strong Survive A Glance at Strong and Weak Acids

2. What is Strong? • Acids are considered strong if there is virtually 100% dissociation • Each molecule separates into an H+1 ion and anion (Cl-1, NO3-1, etc…) • Weak acids have percent dissociations that are noticeably less than 100%

3. What makes one strong and another weak • The more “ionic” the bond between H and the neighboring atom the more likely dissociation • Very ionic bonding creates strong acids • Less ionic bonding creates weak acids

4. Gauging the “Ionic-ness”: What makes the bond more (or less) ionic • Check electronegativity for hydrogen and the “next neighbor” • Check atoms bonded to the next neighbor • Check atoms bonded to the next neighbor’s next neighbor • 3 atoms away from H

5. Ionic character increases… • as the next neighbor’s electronegativity increases (for polyatomic acids) • as the atoms attached to the next neighbor’s electronegativity increases • as the electronegativity of the “third neighbor” increases H 1 2 3

6. Some examples • Compare HOCl to HOClO2 • Electrons are drawn away from the H by the O and Cl in #1 • The extra O in #2 increase the apparent electronegativity of O Cl H O H Cl O O O

7. Some examples (cont.) • So, the bond between H and O is more ionic in the HOClO2 • It is a strong acid • The bond is less ionic in HOCl (but more so than HOH) so is a weak acid Cl H O H Cl O O O

8. How Can I Tell Quickly • Strong acids: HCl, HBr, HI but not HF • For polyatomic acids you must have two oxygens with double or coordinating covalent bonds on the “2nd atom” • Two oxygens more than hydrogens