1. Chapter 16 Acid-Base Equilibria
CHEMISTRY The Central Science 9th Edition
Chapter 16 Acid-Base Equilibria
Chapter 16

2. 16.1: Acids and Bases: A Brief Review
Acids: taste sour
cause dyes to change color
Bases: taste bitter
feel soapy
Arrhenius: acids increase [H+], bases increase [OH-] in AQUEOUS solution
Arrhenius: acid + base  salt + water
Problem: the definition confines us to aqueous solutions
Chapter 16

3. 16.2: Brønstead-Lowry Acids and Bases
The H+ Ion in Water
The H+(aq) ion is a proton with no electrons
In water, the H+(aq) form clusters
The simplest cluster is H3O+(aq)
A hydrated proton
Larger clusters are H5O2+ and H9O4+
Generally H+(aq) and H3O+(aq) are used interchangeably
Chapter 16

4. Proton Transfer Reactions Brønsted-Lowry theory:
Acid donates H+ and base accepts H+
Brønsted-Lowry base does not need to contain OH-
HCl(aq) + H2O(l)  H3O+(aq) + Cl-(aq)
HCl donates a proton to water (HCl is an acid)
H2O accepts a proton from HCl (H2O is a base)
Water can behave as either an acid or a base
Amphoteric substances can behave as acids and bases
With a stronger base, it is an acid (and vice versa)
Chapter 16

5. Conjugate Acid-Base Pairs
Whatever is left of the acid after the proton is donated is called its conjugate base
Whatever remains of the base after it accepts a proton is called a conjugate acid
After HA (acid) loses its proton it is converted into A- (base)
HA and A- are conjugate acid-base pairs
After H2O (base) gains a proton it is converted into H3O+ (acid)
H2O and H3O+ are conjugate acid-base pairs
Conjugate acid-base pairs differ by only one proton
Chapter 16

6. Relative Strengths of Acids and Bases
The stronger the acid, the weaker the conjugate base
Chapter 16
Text, P. 618

7. Generalizations about Acid and Base Behavior
Strong acids completely transfer their protons to water
The conjugate base of a strong acid has negligible acid-base properties
The conjugate acid of a strong base has negligible acid-base properties
Weak acids partly dissociate in aqueous solution
The conjugate bases of weak acids are weak bases
Substances with negligible acidity (CH4) contain H but don’t behave as acids in water
Their conjugate bases are strong bases (form (OH)- in water)
Chapter 16

8. Proton Transfer Reactions
Strong acids and bases react with water to liberate H+(aq) and (OH)-(aq) ions
The leveling effect of water: one can’t distinguish among the strengths of the strong acids or strong bases
Place HCl, HBr or HI in dilute acetic acid and there is a noticeable difference in their strengths
H+ is the strongest acid that can exist in equilibrium in aqueous solution
OH- is the strongest base that can exist in equilibrium in aqueous solution
Chapter 16

9. Use chart ( P. 618) to determine which base is stronger (CO3-2)
Proton Transfer Reactions
In acid-base reactions, the equilibrium favors transfer of the proton from the stronger acid to the stronger base
Strong acid in water: Equilibrium favors the right side
HSO4-(aq) + CO3-2(aq) SO4-2(aq) + HCO3-(aq)
Compare bases: CO3-2 is from HCO3-
SO4-2 is from HSO4-
Use chart ( P. 618) to determine which base is stronger (CO3-2)
The stronger base and acid are consumed
Proton is transferred to the stronger base
Production of the weaker electrolytes is favored
Chapter 16

10. The reaction favors the consumption of the stronger acid and stronger base and the formation of the weaker acid and weaker base (the weaker electrolytes)
Chapter 16

11. Use chart ( P. 618) to determine which base is stronger (C2H3O2-)
Proton Transfer Reactions
Weak acid in water: Equilibrium favors the left side
The anion of the acid is a stronger base, so it accepts a proton from the H3O+
HC2H3O2(aq) + H2O(l) H3O+(aq) + C2H3O2-(aq)
Use chart ( P. 618) to determine which base is stronger (C2H3O2-)
The stronger base and acid are consumed
Proton is transferred to the stronger base
Chapter 16

12. Complete Sample Problems: # 5, 7, 9, 11, 15
The reaction favors the consumption of the stronger acid and stronger base and the formation of the weaker acid and weaker base (the weaker electrolytes)
Complete Sample Problems:
# 5, 7, 9, 11, 15
Chapter 16

13. 16.3: The Autoionization of Water
In pure water the following equilibrium is established:
at 25 C
The above represents the autoionization of water
The ion-product constant of water
Chapter 16

14. 16.4: The pH Scale In most solutions [H+(aq)] is quite small We define
In neutral water at 25 C, pH = pOH = 7.00
In acidic solutions, [H+] > 1.0  10-7, so pH < 7.00
In basic solutions, [H+] < 1.0  10-7, so pH > 7.00
The higher the pH, the lower the pOH, the more basic the solution (range for pH is 0-14)
Chapter 16

15. Text, P. 623

16. In general for a number X,
Other “p” Scales
In general for a number X,
For example, pKw = -log Kw
pH, logs and Sig Figs:
The number of places after the decimal of a pH, pOH, etc. is the same as the number of sig figs in the concentration you are taking the log of
Chapter 16

17. Most accurate method to measure pH is to use a pH meter
Measuring pH
Most accurate method to measure pH is to use a pH meter
Indicators change color as pH changes
Indicators are less precise than pH meters
Many indicators do not have a sharp color change as a function of pH
Chapter 16

18. Text, p. 625