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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