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An overview of the brønsted-lowry theory of acids and bases, including definitions, examples, and reactions. It covers the concepts of conjugate acids and bases, acid-base equilibria, and strong and weak acids and bases. Students will learn how to identify acids and bases, determine conjugate species, and calculate equilibrium constants.
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Worksheet 18 - Acids and Bases
The Brønsted-Lowry definition of an acid is a substance capable of donating a proton (H+^ ), and a base is a substance capable of accepting a proton. For example, the weak acid, HF , can be dissolved in water, giving the reaction:
HF (aq) + H 2 O (l) ' H 3 O +^ (aq) + F-^ (aq)
acid conjugate base base conjugate acid
In this reaction, HF is the species losing the proton (H+^ ), making it the acid. Water is the species accepting the proton, to form the hydronium ion , H 3 O +^ , making it the base. The F-^ (aq) is called the conjugate base of HF. It can gain a proton in the reverse reaction. H 3 O +^ is the conjugate acid of H 2 O, since it can lose a proton in the reverse reaction. The stronger an acid, the weaker its conjugate base will be and the stronger the base, the weaker its conjugate acid.
The equilibrium concentrations of these species will be determined by the relative strengths of the acids and bases. The strongest acid will dissociate to the greatest extent. H 3 O +^ (H+) is the strongest acid that can exist in an aqueous system. So, the equilibrium in this system will favor the reactants, HF and H 2 O, the weaker acid and base. The equilibrium state is described by an equilibrium constant, K a , in the case of acids, and K b in the case of bases. These are related by the expression K w = K a x K b = 1 x 10-^.
a) H 2 O b) OH-^ c) NH 3
d) NH 4 +^ e) NH 2 -^ f) CO 3 2-
-
-
2-
a) Write the equation for the reaction of HSO 3 -^ with H 2 O in which it acts like an acid and identify the acid-base pairs. Circle the strongest acid. Then write an expression for K a. The value of K a at 25o^ C is 1.23 x 10-7^. Use an arrow to indicate which side is favored by the equilibrium
7 3
3
2 (^3 1). 23 10 − −
− + = = × HSO
K (^) a
b) Write the equation for the reaction of HSO 3 -^ with water in which it acts like a base and identify the acid-base pairs. Circle the strongest base. Then write an expression for K b. What is the value of K b at 25o^ C, given that the K a for H 2 SO 3 = 1.58 x 10-2^? Use an arrow to indicate which side is favored by the equilibrium.
-
-
3
2 3 HSO
K (^) b
13 2
14
−
− = × ×
a
w b K
c) If HSO 3 -^ is placed in water, is the resulting solution acidic or basic?
-
+
-
d) Compute a value for the equilibrium constant for the reaction shown below:
6 7 3
2 3
fromparta
−
SO HO K a
Circle the strongest acid and indicate which side is favored by the equilibrium.
100 molH
100 molH 1 molHCl
1 molH 1 L
a) 0.001 M HCl b) 0.76 M KOH
pOH -log0.76 0. 119
pH
c) 2.8 x 10 -4^ M Ba(OH) 2 (Think carefully about this one!)
-
-
pOH -log 5. 6 10 3. 25
4
4 4
−
− − −
pH