Acids and Bases

Acid and Bases

1. General Definitions:

  Acid: a substance which when added to water produces hydrogen ions [H⁺].

  Base: a substance which when added to water produces hydroxide ions [OH^-].

2. Properties:

Acids:

·         react with metals to  form hydrogen (H_2(g))

·         react with compounds containing CO₃^2- and form carbon dioxide and water

·         turn litmus red

·         taste sour (lemons contain citric acid, for example) DO NOT TASTE ACIDS IN THE LABORATORY!!

Bases:

·         feel soapy or slippery

·         turn litmus blue

·         they react with most cations to precipitate hydroxides

·         taste bitter DO NOT TASTE BASES IN THE LABORATORY!!

3. pH

Definition of acidic, basic, and neutral solutions based on pH 

acidic: if pH is less than 7 (Hydrochloric Acid)
basic: if pH is greater than 7 (Sodium Hydroxide)
neutral: if pH is equal to 7 (Water)

5. Strength of Acids and Bases:

Acids

1. Strong Acids:

·         completely dissociate in water, forming H⁺ and an anion.

example: HN0₃ dissociates completely in water to form H⁺ and N0₃^1-.

The reaction is

HNO_3(aq)  →   H⁺_(aq) + N0₃^1-_(aq)

A 0.01 M solution of nitric acid contains 0.01 M of H⁺ and 0.01 M N0₃^- ions and almost no HN0₃ molecules. The pH of the solution would be 2.0.

·         There are 6 strong acids. The remainder of the acids therefore are considered weak acids.

    HCl                              Hydrochloric Acid 

    H₂SO_{4                        Sulphuric Acid}

    HNO_{3                          Nitric Acid}

    HClO_{4                         Perchloric Acid}

        HBr                             Hydrobromic Acid

    HI                               Hydroiodic Acid

2. Weak acids:

·         a weak acid only partially dissociates in water to give H⁺ and the anion

Bases

1. Strong Bases:

•   They dissociate 100% into the cation and OH^- (hydroxide ion).
• example:  NaOH_(aq)  →   Na⁺_(aq) + OH^-_(aq)
• Which are the strong bases?

The hydroxides of Groups I and II.

2. Weak Bases:

What compounds are considered to be weak bases?

1. Most weak bases are anions of weak acids.

Weak bases do not furnish OH^- ions by dissociation.

Acid-Base Properties of Salt Solutions:

definition of a salt:

• an ionic compound made of a cation and an anion, other than hydroxide.
• the product besides water of a neutralization reaction

For Example : weak base_(aq) + H₂O_(aq)  →   weak acid_(aq) + OH^-_(aq)

6. Indicators

Litmus Paper:             - Acid turns blue litmus paper red.

                                 - Alkali turns red litmus paper blue.

Methyl Orange:          - Used to identify strong acids.

                                 - Changes color at pH 4.
                                 - Strong acids turn it from orange to red.
                                 - Alkalis turn it from orange to yellow.
Phenolphthalein:      - Used to identify strong alkalis.
                                 - Changes color at pH 9.
                                 - Strong alkalis turn it from colorless to pink.
                                 - Remains colorless in acids.
Universal indicator:  - Remains green in neutral solutions.
                                 - Strong acids turn it from green to red.
                                 - Strong alkalis turn it from green to violet.

7. Chemical Equations

Acids + metals → salt + hydrogen.
Hydrochloric acid + sodium → sodium chloride + hydrogen gas
Na + HCl → NaCl + H2

Acids + carbonates → salt + carbon dioxide + water.
Calcium carbonate + nitric acid → calcium nitrate + carbon dioxide + water
CaCO3 + 2 (HNO3) → Ca (NO3) 2 + CO2 + H2O

Acids + bases → salt + water
Sulfuric acid + potassium hydroxide → Potassium sulfate + water
H2SO4 + KOH → K2SO4 + 2 (H2O)

Alkalis + ammonium salts → salt + water + ammonia gas
Sodium hydroxide + ammonium nitrate → sodium nitrate + water + ammonia gas
NaOH + NH4NO3 → NaNO3 + H2O + NH3