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The stronger the acid, the weaker the conjugate base. ... weaker base. Therefore sulfuric acid is the stronger acid. ... H2SO4 is a strong acid while HSO4.
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Key topics: pH scale; acid-base properties of water Ka = acid ionization constant; Kb = base ionization constant Polyprotic acids BrØnsted Acids and Bases Acid : proton donor Base : proton acceptor acid/base reaction: proton transfer Each side of the reaction has an acid and a base! Conjugate base: what remains after the acid donates a proton Conjugate acid: formed when the base accepts a proton
an acid and its conjugate base : conjugate acid-base pair a base and its conjugate acid : conjugate acid-base pair The two species in a conjugate pair differ only by H
There are two conjugate pairs in any acid-base reaction e.g. , HSO 3
- is the conjugate acid of what species? HSO 3 - is the conjugate base of what species? Solution: SO 3 2 – , H 2 SO 3 e.g. , (a) Write an equation in which HSO 4 - reacts (with water) to form its conjugate base. (b) Write an equation in which HSO 4 - reacts (with water) to form its conjugate acid. Solution: HSO 4 -
HSO 4 -
Oxoacids (O—H bond): contain H, O, and a central non-metal Acidity determined by the polarity of the O—H bond:
e.g. , CH 3 COOH > CH 3 CH 2 OH
attract H
less, making it a weaker base. Therefore sulfuric acid is the stronger acid.
The Acid-Base Properties of Water Water is amphoteric : it can act as an acid or a base. Water also autoionizes to a very small degree The equilibrium constant Kw does not include the liquid, so
7
14 For any temperature, pure water is neutral ([OH
] the solution is neutral If [H
] the solution is acidic If [H
] the solution is basic e.g. , at 25ºC, [H
] = 4.6 x 10
]. Is the solution acidic or basic or neutral?
(from chemwiki.ucdavis.edu) Solution: [H
][OH ] = 1. 0 ⇥ 10 14 ) [OH ] =
] and pOH = log[OH ] and pX = log X
) pKw = pH + pOH (= 14 at 25 C) For pure water at 25°C, pH = pOH = 7 (neutral). When pH < 7, pOH > 7 and the solution is acidic. When pH > 7, pOH < 7 and the solution is basic.
List of strong bases. Note: the Group 2A hydroxides are completely dissociated in solutions of 0.01 M or less. These are insoluble bases which ionize 100% (the tiny amount that is soluble dissociates completely). e.g., An aqueous solution of a strong base has pH 12.24 at 25 °C. Calculate the concentration of base in the solution (a) if the base is NaOH and (b) if the base is Ba(OH) 2. Answer: pH = 12.24 means that pOH = 14 – 12 .24 = 1. Therefore [OH
- ] = 10 - 1. = 0. With NaOH, we must have [NaOH] = 0.017 M With Ba(OH) 2 , we have [Ba(OH) 2 ] = (0.017 / 2) = 8.7 x 10 - 3 M
Weak Acids and Acid Ionization Constants Weak acids do not fully dissociate in water. The extent to which a weak acid ionizes in water depends on
(aq) + A (aq)
Ka is called the acid ionization constant. pKa = - log Ka The larger the value of Ka, the stronger the acid. We can calculate pH from Ka, or Ka from pH. The math is simple if: [H
]eq < (0.05) [HA]initial or [HA]initial > 400 Ka Otherwise, we will need the quadratic formula or the method of successive approximations.
At equilibrium we have
2
The “simple” math is to put (0. 50 x) ⇡ 0. 50 Then we have Ka = x 2
2
4 whereas if we use x = 0.019 we get Ka = 7.5 x 10
A weak acid ionizes more the weaker its concentration. (data for acetic acid) Next, we use this value of x in the (0.5 0 – x) part of Ka, giving Ka = x 2
x 2 = (0.0015)Ka ) x = 1. 64 ⇥ 10 4 M Then we put this value of x in the denominator:
2
2
which gives x = 1.55 x 10
x 2
] ) [H
] = 10 5. 03 = 9. 33 ⇥ 10 6 M HA H
Initial 0.015 0 0 Change - 9.33 x 10
6
2
9
All these Bronsted bases have a lone pair of electrons on a nitrogen atom which can accept a proton. Weak Bases and Base Ionization Constants To define the base ionization constant Kb we write: B( aq ) + H2O( l ) HB
( aq ) + OH
Conjugate Acid-Base Pairs Consider dissolving NaCl in water. Since Cl
is the conjugate base of a weak acid (HF), it is called a strong conjugate base and it will generate HF:
Let us examine HF in more detail: F
Ka = [F ][H 3 O
] [HF] Add these two together: 2H 2 O ⌦ OH
K = Ka ⇥ Kb = [OH ][H 3 O
] = Kw Therefore pKa + pKb = pKw = 14 (at 25 C) This is the conjugate acid-base pair relationship. e.g. , Determine (a) Kb of the weak base B whose conjugate acid HB
has Ka = 8.9 x 10
(from www.chem.wisc.edu) Diprotic and Polyprotic Acids Some acids can undergo two or three ionizations: Note that the conjugate base in the first ionization serves as the acid in the second ionization. The second ionization constant is very small compared to the first one (usually at least 1000x smaller). This is because it is harder to remove a proton (H
) from a negatively charged species. This lets us make some simple approximations. e.g. , Calculate the concentrations of H 2 SO 4 , HSO 4
ions in a 0.14 M sulfuric acid solution at 25 ºC. Ka1 = very large ; Ka2 = 1.3 x 10
