Acids
Sour taste
• Change the color of acid-base indicators
•
(ex. turn litmus to red)
Some react with metals and release hydrogen gas
• React with bases to produce salts and water
•
(salt = cation from a base and anion from an acid)
Contain H+ when dissolved in aqueous solutions
• pH less than 7
• Binary acids, oxyacids and organic acids
•
Bases
Taste bitter
• Change the color of acid-base indicators
•
(ex. turn litmus to blue)
Dilute solutions feel slippery
• React with acids to produce salts and water
• pH more than 7
• Many soluble bases (alkali) contain hydroxide ions
(OH-)
•
Definitions of Acids and Bases
1. Arrhenius
◦ Acid : compound that creates H+ in a solution
◦ Base : compound that creates OH- in a solution
 The H+ ion attracts other molecules so strongly that it normally
exists as the hydronium ion (H3O+) when in water
H2O
HCl 
 H3O (aq ) + Cl– (aq )
Definitions of Acids and Bases (con’t)
2. Brønsted-Lowry
• Most commonly used definition for acids and bases
• Created by two scientists around the same time (1923)
• A Brønsted-Lowry acid is a molecule or ion that is a
proton donor (hydrogen ion, H+).
• Ex. Hydrogen chloride acts as a Brønsted-Lowry acid
when it reacts with ammonia.
HCl + NH3 → NH4+ + Cl-
• Water can act as a Brønsted-Lowry acid
NH3(aq) + H2O(l) → NH4+ + OH-
• A Brønsted-Lowry base is a molecule or ion
that is a proton acceptor (gains a H+).
• Ammonia accepts a proton from hydrochloric acid.
NH3 + HCl
base
→
NH4+ + Cl-
acid
• In a Brønsted-Lowry acid-base reaction, protons
are transferred from one reactant to another (the
acid to the base).
Conjugate acids and bases
•
The substance that remains after a Brønsted-Lowry acid
has given up a proton is the conjugate base.
•
The substance that is formed when a base accepts a
proton is the conjugate acid.
HF(aq)
acid
+
H2O(l) → F-(aq) +
base
conjugate
base
H3O+(aq)
conjugate
acid
•
Monoprotic acids
• Only have one acidic proton (one hydrogen ion)
• ex. HClO4, HCl, HNO3
•
Polyprotic acids (diprotic, triprotic)
• Can donate more than one proton per
molecule
• ex. H2SO4, H3PO4
Interesting fact…
Water can
act as an
acid or a
base!!!
Amphoteric compound: Any compound that can react as
either an acid or a base
Measuring acidity or basicity
pH is a measure of how acidic or basic a
solution is.
(“p” stands for potential and “H” stands for hydrogen; therefore, the
potential of a substance to attract hydrogen ions)
pH ranges from 0 to 14.
• Acidic solutions have pH values < 7
• A pH of 7 is neutral.
• Basic solutions have pH values >7
pH scale
pH
Concentration of pure water at 25°C:
[H+] = 1.0 ×10−7 M
[OH−] = 1.0 × 10−7 M
Brackets, [ ], symbolize the molarity concentration
Calculating pH

The pH of a solution is the negative logarithm of the hydrogen
ion concentration, [H+].
pH = −log [H+]
◦ example: for a neutral solution [H+] = 1×10−7
pH = −log [H+] = −log(1 × 10−7) = −(−7.0) = 7.0

The pOH of a solution is defined as the negative logarithm of
the hydroxide ion concentration, [OH−].
pOH = −log [OH–]

pH + pOH = 14.0
Practice problems
1. What is the pH of coffee if the [H+] is 1 x 10−5 M?
STEP 1 Enter the [H+] value:
Enter 1 x 105 (press 1 EE 5, then change sign )
STEP 2 Press log key and change the sign:
log (1 x 10−5) = [5]
STEP 3 Make the number of digits after the decimal point equal
to the number of significant figures in the coefficient:
[H+]= 1 x 10−5
Answer:
pH is 5.0
13
Stomach acid (HCl) has a concentration of
approximately 2 x 10−4 M. What is the pH
of the solution?
STEP 1 Enter the [H+] value: 2 x 104
(press 2 EE 4, then change sign)
STEP 2 Press log key and change the sign:
log (2 x 10−4) = [3.7]
STEP 3 Make the number of digits after the decimal
point equal to the number of significant figures in
the coefficient [H+] = 2 x 10−4
Answer: pH is = 3.7
Calculating [H+] from pH
What is the concentration of nitric acid
(HNO3) that has a pH of 3.80?
STEP 1 Enter the pH value, change sign: –3.80
STEP 2 Convert pH to concentration:
Use 2nd function key and then10x key
or inverse key and then log key
answer = 1.584893 −06
STEP 3 Adjust the significant figures in the coefficient (2
digit following decimal point = 2 digit in the coefficient):
Answer: [HNO3] = 1.6 x 10−6 M