Ka, Kb

1
Ka, Kb
2
Comparing the pH of two acids

1. Predict the pH of HCl and HF (below)
2. Calibrate a pH meter
3. Measure the pH of HCl(aq) and HF(aq)
4. Complete the chart below

HCl (aq) HF (aq)
in mol/L (on label)
Net ionic equation
Predicted H
Predicted pH
pH measured
Actual H
Conductivity (demo) Higher / stronger Lower / weaker
0.05
0.05
HCl ? H Cl
HF ? H F
0.05
0.05
-log(0.05)1.3
-log(0.05)1.3
1.3
2.3 ?
10pH 0.05
10pH 0.005
3
Questions

• Read15.3. (pg. 607)
• Based on your results, which acid ionizes (forms
  ions) to a greater degree?
• Which two measurements taken in the lab support
  your answer to 1?
• What is another name for Ka?
• Solve PE 5, 6
• Write the Ka equation for HCl (aq) and HF (aq)
  from todays lab
• Solve for PE 8, 9 (use this equilibrium for
  butyric acid HBu ? H Bu)
• For HF(aq) set up a RICE chart, then solve for
  Ka. How does your value for Ka compare to the
  accepted value (pg. 608)?
• Try PE 10 (follow example 15.7 on pg. 610)

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Answers

• HCl ionizes more than HF
• HCl has a lower pH (indicating more H), a
  higher conductivity (indicating more ions)
• Ka acid ionization constant
• HNO2 ? H NO2, KaHNO2/HNO2
• HPO42 ? H PO43,KaHPO43/HPO42
• HCl ? H Cl, KaHCl/HCl
• HF ? H F, KaHF/HF

5
PE 8 - pg. 610 HBu ? H Bu
HBu
H
Bu
1
1
1
0.0100
0
0
-0.0004
0.0004
0.0004
0.0096
0.0004
0.0004
H 10 pH 10 3.40 3.98 x 10 4
1.67 x 10 5
6
PE 9 - pg. 610 HBu ? H Bu
HBu
H
Bu
1
1
1
0.0100
0
0
-0.001
0.001
0.001
0.009
0.001
0.001
H 10 pH 10 2.98 1.05 x 10 3
1.1 x 10 4
7
Question 7 HF ? H F
HF
H
F
1
1
1
0.05
0
0
-0.005
0.005
0.005
0.045
0.005
0.005
H 10 pH 10 2.3 0.005
5.6 x 10 4
Accepted value of Ka for HF is 6.4 x 10 4
8
10 HC2H4NO2 ? H C2H4NO2
HC2H4NO2
H
C2H4NO2
1
1
1
0.010
0
0
-x
x
x
0.010 - x
x
x
1.4 x 10 5
Since x is small 0.010 x 0.010
x 3.74 x 105 M, pH 3.43
1.4 x 10 5
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Ka summary

• Ka follows the pattern of other K equations
• I.e. for HA(aq) H2O(l) ? H3O(aq) A(aq)
• Ka H3OA / HA
• Notice that H2O is ignored because it is liquid
• HA cannot be ignored because it is aqueous
• This is different than with Ksp. In Ksp, solids
  could only be in solution as ions
• Acids can be in solution whether ionized or not
• The solubility of acids makes sense if you think
  back to the partial charges in HCl for ex.

10
Ka summary

• Generally Ka tells you about acid strength
• Strong acids have high Ka values
• A strong acid is an acid that completely
  ionizes. E.g. HCl H2O ? H3O Cl
• A weak acid is an acid that doesnt ionize
  completely. E.g. HF H2O ? H3O F
• Note dont get confused between strength and
  concentration. 1 M HCN has a smaller H, thus a
  higher pH, than 0.001 M HCl
• In general Ka lt 10 3 Weak acid
• 10 3 lt Ka lt 1 Moderate acid
• Ka gt 1 Strong acid

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Dissociation vs. Ionization

• Ionization and dissociation indicate ions form
• Dissociation ions form when a chemical comes
  apart. E.g. NaCl melts to form Na, Cl
• Ionization ions form when two chemicals react.
  E.g. HCl(aq) H2O ? H3O(aq) Cl(aq)
• Even though we write HCl ? H Cl , this is
  just an abbreviation. In reality HCl reacts with
  H2O, thus it is an ionization not a dissociation
• Note that NaCl can also dissociate in water.
  This is not an ionization, since water is only
  required to stabilize ions (it is not needed as a
  reactant involved in forming ions)

12
Kb the last K (I promise)

• Kb is similar to Ka except b stands for base
• The general reaction involving a base can be
  written as B(aq) H2O ? BH(aq) OH(aq)
• Thus Kb BH OH / B
• Recall shorthand for Ka is HA ? H A
• Kb has no shorthand form
• Read pg. 614 - 617
• Try PE 12 (a-c), 13, 14 (for 13, you do not need
  to know the chemical formula of morphine.
  Symbolize it with M)

13
PE 12

• CN(aq) H2O ? HCN(aq) OH(aq)
• Kb HCNOH / CN
• C2H3O2(aq) H2O ? HC2H3O2(aq) OH(aq)
• Kb HC2H3O2OH / C2H3O2
• C6H5NH2(aq) H2O ? C6H5NH3(aq) OH(aq)
• Kb C6H5NH3OH / C6H5NH2

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PE 13 - pg. 617 M H2O ? MH OH
M
MH
OH
1
1
1
0.010
0
0
-0.00013
0.00013
0.00013
0.00013
0.00013
0.00987
pOH 14 - pH 14 - 10.10 3.90 OH- 10-pOH
10-3.90 1.26 x 10-4
0.00013 0.00013
1.7 x 10-6
0.00987
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PE 14 - pg. 617 M H2O ? MH OH
NH3
NH4
OH
1
1
1
0.020
0
0
-x
x
x
x
x
0.020 - x
pOH -logOH- 3.22 pH 14 - pOH 10.78
x2
1.8 x 10-5 x 6.0 x 10-4
0.020
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Strength of conjugates

• Consider HCl(l) H2O ? Cl(aq) H3O(aq)
• The Ka for HCl is Cl(aq) H3O(aq) /
  HCl(aq)
• Also, Cl(aq) H2O(aq) ? HCl(l) OH
• The Kb for Cl is HCl(aq) / Cl(aq)
  H3O(aq)

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Relative values of Ka

• Recall for HX ? H X, Ka HX / HX
• Q - what does a large Ka indicate?
• A - equilibrium is far to the right (all
  dissociates)
• Thus a large Ka strong acid
• Look at Table 15.4 on page 608
• The text uses this definition
• Ka lt 103 is a weak acid
• 103 lt Ka lt 1 is a moderate acid
• 1 lt Ka is a strong acid
• These definitions are somewhat arbitrary, we will
  not focus on this. Just remember a high Ka means
  the acid is strong.

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