Topic 14: Acids & Bases : Topic 14: Acids & Bases
I. Definitions of Acids and Bases : I. Definitions of Acids and Bases Arrhenius
Acids produce H+ ions in water (e.g. HCl, CH3COOH, H2SO4)
Bases produce OH- ions in water (e.g. NaOH, NH3)
Bronsted-Lowry
Acids are proton donors (e.g. HCl gives away H+ to become Cl-)
Bases are proton acceptors (e.g. NH3 accepts H+ to become NH4+)
Lewis
Acids are electron pair acceptors (e.g. BF3)
Bases are electron pair donors (e.g. NH3)
Conjugate Acid-Base Pairs : Conjugate Acid-Base Pairs Conjugate acid-base pairs are related by a hydrogen ion on either side of the equation.
E.g. HCl ? H+ + Cl-
HCl is an acid; Cl- is the conjugate base (difference of one H+)
E.g. NH3 + H+ ? NH4+
NH3 is a base, NH4+ is the conjugate acid (difference of one H+)
What is the conjugate base of perchloric acid? Of the bicarbonate ion? What is the conjugate acid of the bicarbonate ion?
The conjugate base of a strong acid is always weak (Cl- is an extremely weak base). The conjugate base of a weak acid is always strong (CH3COO- is a strong base; it strongly attracts protons). Same goes for bases.
d. Amphoteric Substances : d. Amphoteric Substances These compounds can act as either acids (donate H+) or bases (accept H+)
E.g. HCO3- : can become H2CO3 or CO32-
Water and most di- and triprotic acids can make amphoteric substances
E.g. H2O can become H3O+ or OH-
Phosphoric acids yields two amphoteric substances: H2PO4- and HPO4-
e. Strength vs. Concentration : e. Strength vs. Concentration Percent Ionization
The greater the degree of ionization/dissociation, the stronger the acid
E.g. HCl dissociates entirely, CH3COOH doesn’t
The greater the number of moles in a solution, the higher the concentration
E.g. 1.9 M CH3COOH is more concentrated, but weaker, than 0.001 M HCl
It is possible to have a dilute strong acid and a concentrated weak acid
Danger is unrelated to strength and concentration!!!
HF, which is classified as a weak acid because it doesn’t dissociate, is still extremely corrosive (attacks glass)
ii. Prediction of acidic/basic strength : ii. Prediction of acidic/basic strength Oxides of elements on the left side of the periodic table are strong bases (the X-O bond is ionic, and breaks easily)
Na – O – H will break between Na and O
Oxides of elements on the right side of the periodic table are strong acids (the X-O bond is covalent and doesn’t break easily)
H – O – Cl will break between H and O
The more oxygen atoms on an oxyanion, the stronger the acid (H2SO4 vs. H2SO3) due to decreased electron density on the O-H bond
HClO4 > HClO3 > HClO2 > HClO
Do Examples 6 & 7
iii. Ka and Kb : iii. Ka and Kb The equilibrium constant for acids and bases expresses how strong the acid/base is (small K means weaker acid, since less dissociation
For HA (aq) + H2O (l) ? H3O+ (aq) + A- (aq)
Ka = [H3O+] [A-] / [HA]
This usually turns into [H3O+]2 / [HA]
For B (aq) + H2O (l) ? HB+ (aq) + OH- (aq)
Kb = [OH-] [HB+] / [B]
This usually turns into [OH-]2 / [B]
% ionization = 100 x Mdissociated / Minitial
Weak acids will dissociate more in a dilute solution
Find Ka for a 1.00 M HF solution with 2.7% ionization.
Do Example 3
f. The pH scale : f. The pH scale pH ranges from 0-14, where pH < 7 is acidic, pH = 7 neutral, and pH > 7 is basic
pH = -log [H+]
pOH = log [OH-] (the opposite of pH)
pH + pOH = 14
Kw (the ionization constant of water) is 1.0 x 10-14
Kw = [H+] [OH-]
Kw = (Ka) (Kb)
pKa = -log Ka
pKb = -log Kb
Do Examples 1 & 2
Not the whole story! : Not the whole story! Cannot use pH = -log [M of acid] for weak acids, because not all of it has dissociated into actual H+ ions.
1.5 M HCl has 1.5 M H+ because of total dissociation, but 1.5 M CH3COOH doesn’t have 1.5 M H+ because it’s a weak acid
Need to use Ka expression to calculate actual [H+]
For polyprotic acids (e.g. H2SO4), there are multiple Ka values for the successive ionizations
1st ionization: H2SO4 ? H+ + HSO4- Ka1
2nd ionization: HSO4- ? H+ + SO42- Ka2
Ka1 >> Ka2
Calculate the pH of a 1.13 M oxalic acid solution if Ka1 = 6.5 x 10-2 and Ka2 = 6.1 x 10-5. Find the oxalate ion and hydroxide ion concentrations.
Do Example 4
Slide 10 :