Presentation on theme: "Bases Sodium hydroxide Preferred IUPAC name Sodium hydroxide Systematic name Sodium oxidanide Other names Caustic soda Lye Graphics Source: Wikipedia."— Presentation transcript:
Bases Sodium hydroxide Preferred IUPAC name Sodium hydroxide Systematic name Sodium oxidanide Other names Caustic soda Lye Graphics Source: Wikipedia
Acid/Base Definitions Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors Lewis Acid Model Acids are electron pair acceptors Bases are electron pair donors
Dissociation of Strong Bases Strong bases are metallic hydroxides Group I hydroxides (NaOH, KOH) are very soluble Group II hydroxides (Ca, Ba, Mg, Sr) are less soluble pH of strong bases is calculated directly from the concentration of the base in solution MOH(s) M + (aq) + OH - (aq)
Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH 3 NH 2 + H 2 O CH 3 NH OH - K b = 4.38 x 10 -4
K b for Some Common Weak Bases
Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H 2 O BH + + OH - (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #1: Write the equation for the reaction NH 3 + H 2 O NH OH -
A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #2: ICE it! x +x x xx NH 3 + H 2 O NH OH -
A Weak Base Equilibrium Problem Step #3: Set up the law of mass action xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O NH OH -
A Weak Base Equilibrium Problem Step #4: Solve for x, which is also [OH - ] xxx E [OH - ] = 3.0 x M NH 3 + H 2 O NH OH - What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ?
A Weak Base Equilibrium Problem Step #5: Convert [OH - ] to pH xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O NH OH -
Soft cover book Pg
Text Pg. 713 do question Then you can practice for hw 16.76
For conjugate acid base pairs only K a x K b = K w pK a = -logK a pK b = -logK b pK a + pK b = 14 Now do question on Pg. 713
Hydrolysis of Salts Salt of strong acid and strong base – neutral Salt of weak acid and strong base – basic Salt of strong base and weak acid – acidic In general Ka>Kb acidic Kb>Ka basic Soft cover book Pg. 387
Text Pg. 697 Text pg. 698 sample and 16.19
Reactions of Anions with Water Anions are bases. As such, they can react with water in a hydrolysis reaction to form OH − and the conjugate acid: X − (aq) + H 2 O(l)HX(aq) + OH − (aq)
Reactions of Cations with Water Cations with acidic protons (like NH 4 + ) will lower the pH of a solution. Most metal cations that are hydrated in solution also lower the pH of the solution.
Reactions of Cations with Water Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water. This makes the O-H bond more polar and the water more acidic. Greater charge and smaller size make a cation more acidic.
Effect of Cations and Anions 1.An anion that is the conjugate base of a strong acid will not affect the pH. 2.An anion that is the conjugate base of a weak acid will increase the pH. 3.A cation that is the conjugate acid of a weak base will decrease the pH.
Effect of Cations and Anions 4.Cations of the strong Arrhenius bases will not affect the pH. 5.Other metal ions will cause a decrease in pH. 6.When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the K a and K b values.
Factors Affecting Acid Strength The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound. Acidity increases from left to right across a row and from top to bottom down a group.
Factors Affecting Acid Strength In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.
Factors Affecting Acid Strength For a series of oxyacids, acidity increases with the number of oxygens.
Factors Affecting Acid Strength Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.
Lewis Acids Lewis acids are defined as electron-pair acceptors. Atoms with an empty valence orbital can be Lewis acids.
Lewis Bases Lewis bases are defined as electron-pair donors. Anything that could be a Brønsted–Lowry base is a Lewis base. Lewis bases can interact with things other than protons, however.