What are acids?
Arrhenius acids • produce H+ ions in water. H2O
HCl(g) H+(aq) + Cl- (aq)
• are electrolytes.• have a sour taste. • turn litmus red.• neutralize bases.HCl Hydrochloric acidHNO3 Nitric acidH2SO4 Sulfuric acid
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What are bases?
Arrhenius bases • produce OH− ions in water.• taste bitter or chalky.• are electrolytes.• feel soapy and slippery.• neutralize acids.
NaOH sodium hydroxide
KOH potassium hydroxide
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What are Brønsted-Lowry acids and bases?
According to the BrØnsted-Lowry theory,• acids donate a proton (H+). • bases accept a proton (H+).
In the reaction of ammonia and water,• NH3 base that accept H+.
• H2O acid that donates H+.
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What is ionization of water!
In water,• H+ is transferred from one H2O molecule to
another. • one water molecule acts as an acid, while
another acts as a base.
H2O + H2O H3O+ + OH− .. .. .. .. :O: H + H:O: H:O:H + + :O:H−
.. .. .. .. H H H water water hydronium
hydroxide ion (+) ion (-) 4
However, pure water is neutralIn pure water, • the ionization of water molecules produces small, but
equal quantities of H3O+ and OH− ions.
• molar concentrations are indicated in brackets as [H3O+] and [OH−].
[H3O+] = 1.0 x 10−7 M
[OH−] = 1.0 x 10−7 M
Adding an acid to pure water • increases the [H3O+]. • cause the [H3O+] to exceed 1.0 x 10-7 M.• decreases the [OH−].
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Adding a base to pure water• increases the [OH−]. • causes the [OH−] to exceed 1.0 x
10− 7M.• decreases the [H3O+].
Ion Product of Water, Kw
The ion product constant, Kw, for water
• is the product of the concentrations of the hydronium and hydroxide ions.
Kw = [ H3O+] [ OH− ]
• can be obtained from the concentrations in pure water.
Kw = [ H3O+] [ OH− ]
Kw = [1.0 x 10− 7 M] x [ 1.0 x 10− 7 M]
= 1.0 x 10− 14
In neutral, acidic, or basic solutions, the Kw is always 1.0 x 10−14 6
Calculating [H3O+]
What is the [H3O+] of a solution if [OH−] is 5.0 x 10-8
M?
STEP 1: Write the Kw for water.
Kw = [H3O+ ][OH− ] = 1.0 x 10−14
STEP 2: Rearrange the Kw expression.
[H3O+] = 1.0 x 10-14
[OH−]
STEP 3: Substitute [OH−]. [H3O+] = 1.0 x 10-14 = 2.0 x 10-7
M 5.0 x 10- 8
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An example!
If lemon juice has [H3O+] of 2 x 10−3 M, what is the [OH−] of the solution?
Kw = [H3O+ ][OH− ] = 1.0 x
10−14
[OH− ] = 1.0 x 10 -14 = 5 x 10−12 M 2 x 10 - 3
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pH Scale
The pH of a solution• indicate the acidity of a solution. • values that usually range from 0 to 14.• acidic when the values are less than 7.• neutral with a pH of 7.• basic when the values are greater than 7
determined using • a) pH meter• b) pH paper• c) indicators that have specific colors at different
pH values.9
Calculating pH
pH is the negative log of the hydronium ion concentration.
pH = - log [H3O+]
Example: For a solution with [H3O+] = 1 x 10−4
pH = −log [1 x 10−4 ]pH = - [-4.0]pH = 4.0
Note: The number of decimal places in the pH equals
the significant figures in the coefficient of [H3O+]. 4.0
1 SF in 1 x 10-4
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Calculating pH
Find the pH of a solution with a [H3O+] of 1.0 x 10−3:
STEP 1 Enter [H3O+]
Enter 1 x 10-3 by pressing 1 (EE) 3 The EE key gives an exponent of 10 and change sign (+/- key or – key)
STEP 2 Press log key and change sign - log (1 x 10−3) = -[-3]
STEP 3 Adjust figures after decimal point to equal the significant figures in the coefficient.
3 3.00 Two significant figures in 1.0 x 10−3
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Neutralization ReactionsIn a neutralization reaction : an acid and a base
produce a salt and water. • an acid such as HCl reacts with a base such as
NaOH.HCl + H2O H3O+ + Cl−
NaOH Na+ + OH−
• the H3O+ from the acid and the OH− from the base form water.
H3O+ + OH− 2 H2O
HCl + NaOH NaCl + H2O
acid base salt water
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Balancing Neutralization Reactions
Write the balanced equation for the neutralization of
magnesium hydroxide and nitric acid.STEP 1 Write the acid and base.
Mg(OH)2 + HNO3
STEP 2 Balance H+ in acid with OH- in base. Mg(OH)2+ 2HNO3
STEP 3 Balance with H2O.
Mg(OH)2 + 2HNO3 salt + 2H2O
STEP 4 Write the salt from remaining ions. Mg(OH)2 + 2HNO3 Mg(NO3)2
+ 2H2O
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Buffers
When an acid or base is added
• to water, pH changes drastically.
• to a buffer solution, pH is maintained; pH does not change.
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