Acids and Bases

Acids & Bases These were introduced in Chapter 4 Arrhenius: Acid = any substance that produces H + in soution. Base = any substance that produces OH - in solution. AcidBase Sour TasteBitter Taste Turn Blue Litmus to RedTurn Red Litmus to Blue Attack Most MetalsSlippery, Soapy Feel Neutralize BasesNeutralize Acids

Bronsted-Lowery Theory Definition of acids and bases focused on the transfer of a single proton. Acid = any substance that donates a proton (H + ). Base = any substance that accepts a proton (H + ). Expands the number of acids and bases. Not restricted to aqueous solutions.

Hydronium Ion In water, HCl dissociates completely to: HCl (aq)  H + (aq) + Cl - (aq) In essence, this shows that a free proton is present. In reality, this does not ever happen! HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl - (aq)

Bronsted-Lowery Theory As mentioned, acid-base reactions do not have to be aqueous. NH 3(g) + HCl (aq)  NH4 + (s) + Cl - (s)

Acid-Base Equilibrium In any acid-base equilibrium reaction, both the forward and the reverse reactions involve the transfer of a proton. NH 3(aq) + H 2 O (l)  NH 4+(aq) + OH - (aq) HF (aq) + H 2 O (l)  H 3 O + (aq) + OH - (aq)

Acid-Base Equilibrium Any two substances on either side of the double arrow that differ by a proton are called a conjugate acid-base pair. Thus, every acid has a conjugate base made by removing the proton and every base has a conjugate acid made by adding a proton. What did you notice about the role of water in the previous two reactions?

Strong Acids Strong acids completely ionize, leaving no un- dissociated molecules in solution. Only six acids are considered “strong.” The conjugate base of a strong acid has no base properties. Acid strengths sheet shows the order of strengths. Weaker the acid, the stronger the conjugate base. HCl (aq) + H 2 O (aq)  H 3 O + (aq) + OH - (aq)

Strong Bases Strong Bases completely ionize, due to the fact that they are ionic. Only the group 1A and 2A metal hydroxides are considered to be “strong”. All other hydroxides are insoluble in water. NaOH (aq)  Na + (aq) + OH - (aq)

Auto-Ionization of Water On rare occasion, two water molecules react with each other.

Auto-Ionization of Water Because water is a liquid, this is a heterogeneous equilibrium. K w = At 25 o C, the value of K w = 1.0 E-14. This is called the ion-product constant. Allows us to calculate an unknown [H 3 O + ] or [OH - ].

pH Scale Uses a logarithm in base 10, which is different than the natural logarithm. The letter “p” always represents the negative logarithm in chemistry. pH = -log[H + ] pOH = -log[OH - ] pH + pOH = 14

pH Scale A word about significant figures. Suppose that the [H3O+] = 2.4 x M pH = -log(2.4 E-3) = … 2.4 x M pH = 2.62 Red numbers are the significant digits. Blue numbers are exact numbers.

Acidic or Basic?  When the [H 3 O + ] > [OH - ], the solution is acidic.  When the [OH - ] > [H 3 O + ], the solution is basic.  What about pH?

pH of Strong Acids and Bases Since these dissociate 100%, these are relatively easy. 0.10M HCl would produce ____M H 3 O M NaOH would produce ____M OH M Ca(OH)2 would produce ____M OH - Can also have gram and volume amounts. Can also do partial neutralization.

Weak Acids A weak acid only partially ionizes. Too many to list, but formula generally begins with an “H”. Appendix D1 lists many weak acids. Equilibrium constant is called K a. Will require an ICE table to find the pH.

Weak Acids General set-up is: HA + H 2 O  H 3 O + + A - I C E Appendix D1 used to find the K a values. Problem will look like a quadratic problem, but …

Weak Acids Problems may include: Finding a pH from starting concentration. Finding a Ka value given pH and starting concentration. Finding a percent ionization.

Polyprotic Acids A polyprotic acid contains two or more acidic protons per molecule. Some common ones include: H 3 PO 4 and H 2 CO 3. One is also a strong acid: H 2 SO 4. All ionize in successive steps and have multiple K a values. Intuitively, each H + is more and more difficult to remove.

Weak Bases A weak base reacts with water, which donates a proton to the N atom. N atom has lone pair to bond the proton. Table D2 lists some examples of the weak bases. The equilibrium constant is called K b. Will require an ICE table for most problems.

Weak Bases Generally, the set-up is: B + H 2 O  BH + + OH - I C E Appendix D2 has K b values. Will look like a quadratic, but…

Weak Bases Problems may include: Finding a pH from starting concentration. Finding a Ka value given pH and starting concentration. Finding a percent ionization.

Conjugate Pairs NH3 is a base and NH4+ is its conjugate acid. NH 3 + H 2 O  NH OH - K b = 1.8 x NH H 2 O  H 3 O + + NH 3 K a = ??? What if we need the K a for NH 4 + ?

Conjugate Pairs Like a pH = -log[H + ], there is also a pK a and a pK b. For any conjugate pair, the pK a + pK b = __. Can also convert a pK a back to a K a.

Salts Solutions of NaF are always basic and solutions of NH 4 Cl are always acidic. On the other hand, a solution of NaCl is neutral. Why??? Predicting and calculating a pH of various salts based on the hydrolysis of certain ions.

Salts 1. Salts containing the cation of a _________ and an anion of a _________ will be neutral. 2. Salts containing the cation of a _________ and an anion of a _________ will be __________. 3. Salts containing the cation of a _________ and an anion of a __________ will be _________. 4. Salts containing the cation of a _________ and an anion of a _________ can be ____________. 5. Transition metal and Al ions of high charge will be ____________.

Structure and Strength Acid strength can be predicted based on two factors. Factor #1: Degree of polarity of the H-X bond. This depends on E.N. difference. Rule: Factor #2: Bond Strength Ch. 8: HI = 299kJ, HBr = 366kJ, HCl = 431kJ, and HF = 567kJ Rule:

Structure and Strength Binary Acids – hydrogen is bonded to only one other element. Bond strength (factor #2) is most important. Which group 7A acid is the strongest? Weakest? General Rule =

Structure and Strength Oxy-acids are very common. General structure is: H – O – X For all of these, factor #1 is dominant. 1. For oxy-acids where the number of oxygen atoms differs and X is the same. 2. For oxy-acids where the number of oxygen atoms are identical, but the X atom varies.

Structure and Strength Carboxylic acids are organic acids containing the –COOH group. Acidic due to stability of –COO - group. Increasing strength by adding electronegative atoms. CH 3 COOH vs. CF 3 COOH