Acids, Bases and Salts

ACIDS

Acid is a compound which when dissolved in water dissociates to give hydrogen ions as the only positive ions.

All acids contain hydrogen but not all substances that contain hydrogen are acids.

NOTE
The presence of hydrogen ions (H+) is what makes a solution acidic. Hydrogen ions are formed when a substance dissolved in water.

Natural sources of acids

Natural source Type of acid present
Lemon and orange Citric acid
Grapes Tartaric acid
Tea Tannic acid
Sour milk Lactic acid
Vinegar Acetic acid(ethanoic acid)
Bee sting, ant sting and hair of nettle plants Formic acid
Wall of stomach Hydrochloric acid

Classification of acids

Based on the origin

Organic acids -most of them are extracted from living organisms or organic matter. They contain carbon atoms in their compounds.

Mineral acids are inorganic acids that are derived from minerals or inorganic compounds. Most of them are manufactured in industries. Examples are sulphuric acid, hydrochloric acid and nitric acids.

Based on the constituent elements

Hydracids are acids that consist of hydrogen and a non-metallic element. They are also known as binary acids.

The name of these hydracids usually starts with “hydro” followed by the names of non-metallic element with the suffix “ic”. HCl hydrochloric acid, HBr hydrobromic acid. HI,HF.


Oxy-acids are the acids containing hydrogen, oxygen and a third element usually non-metal. Nitric acid, HNO3, sulphuric acid (H2SO4)

Based on the strength of the acid

Strong acids are acids that completely dissociate into ions when dissolved in water, resulting in a high concentration of hydrogen ions (H+).

Examples of strong acids include hydrochloric acid, sulphuric acid and nitric acid.

Weak acids are acids that partially dissociate into ions when dissolved in water and hence produce lower concentration of hydrogen ions.

Differences between

Strong acids Weak acids
They undergo complete dissociation in their aqueous solution. They undergo partial dissociation in their aqueous solutions.
Gives high concentration of hydrogen ions Gives low concentration of hydrogen ions
Their solution contains contain ions only. Their solutions contain ions and molecules.
They are good conductor of electricity. They are bad conductors of electricity.

Basicity of Acids

Basicity of an acid is the number of hydrogen ions which can be produced by one molecule of acid. Types of Basicity there are three types of basicity which are
  1. Monobasic acid is an acid which dissociates to produce only one hydrogen ion when dissolved in water. This acid is said to have a basicity of one
    2. example
  2. Dibasic acid is an acid which dissociates to produce two hydrogen ions when dissolved in water. This acid is said to have a basicity of two.
  3. Tribasic acid is an acid which dissociates to produce three hydrogen ions when dissolved in water. This acid is said to have a basicity of three
NOTE

In chemistry, the term proton is often used to refer to the hydrogen ion (H+) which is a hydrogen atom that has lost its one electron and has one positively charged proton at its nucleus. So the terms "-protic"as summarized below shows the number of protons or hydrogen ions produced in its aqueous state.

ACIDIons in aqueous solution The name of basicity
HCl H+, Cl- Monobasic Monoprotic
H2SO4 2H+, SO2-4 Dibasic Diprotic
H3PO4 3H+, PO43- Tribasic Triprotic
CH3COOH CH3COO- , H+ Monobasic Monoprotic
H3PO4 3H+, PO43- Tribasic Triprotic
(COOH)2 2H+, 2 (COO-) Dibasic Diprotic

PROPERTIES OF ACIDS

(a) Physical properties

  1. Acids have sour taste.
  2. Acids turn blue litmus paper red.
  3. Some acids are corrosive.
  4. Acids conduct electricity when dissolved in water.

(b) Chemical properties

  1. Acids react with metals which are above hydrogen in the reactivity series to form salt and hydrogen gas.

    2. Reactive metal + acid → salt + hydrogen gas.

    Ca + H2SO4 →CaSO4 + H2
    Mg + HNO3 →Mg (NO3)2 + H2
  2. Acids react with metal/ammonium hydroxides to form salt and water.

    3. Metal hydroxide + Acid → Salt + Water

    Ca(OH)2 + H2SO4 →CaSO4 + H2O
    Mg(OH)2 + HCl →MgCl2 + H2O
  3. Acids react with metal oxides to form salt and water

    4. Metal oxide + Acid → Salt + Water

    MgO + H2SO4 → MgSO4 + H2O
    CaO + HCl →CaCl2 + H2O
    The two reactions (2 and 3) above are known as neutralization reaction.
    Neutralization reaction is the reaction between acid and base to form salt and water only.
  4. Acids react with metal carbonates to form salt, water and carbon dioxide.

    5. Acid + metal carbonate →Salt + carbon dioxide + water

    H2SO4 + Na2CO3 →Na2SO4 + H2O + CO2
  5. Acids react with metal hydrogencarbonate to form salt, water and carbon dioxide.

    6. Acid + metal hydrogencarbonate → salt + water + carbon dioxide

    H2SO4 + NaHCO3 →Na2SO4 + H2O + CO2

BASES

Base is an oxide or hydroxide of metal that reacts with acids to form salts and water only. Exception to this definition is ammonium hydroxide which is also a base.

Alkali is a base that is soluble in water. All alkalis are bases and not bases are alkali.

Alkali is a compound which when dissolved in water forms hydroxide ions, (OH-) as the only negatively charged ions.

Examples of alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2 and ammonium hydroxide (NH4OH).

Strength of Alkalis/Bases

Strong bases are bases which ionize completely and hence produce more concentration of hydroxide ions in their aqueous solutions. In their aqueous solution they will be present in the form of ions.
Weak bases are bases which ionize partially and produce less concentration of hydroxide ions.

PROPERTIES OF BASES

  1. Physical properties

    1. Bases have bitter taste.
    2. Bases have a soapy and slippery texture when in solution.
    3. Bases turn red litmus paper blue.
    4. Bases can conduct electricity when dissolved in water (they are electrolyte).
    5. Some strong bases can be corrosive and cause skin and tissue damage.

  2. chemical properties

    1. bases react with acids to form salt and water. This reaction is called neutralization reaction.
      2. NaOH + HCl →NaCl + H2O
      MgO + HCl →MgCl2 + H2O
    2. Alkalis when heated with ammonium salts and release ammonia gas.
      3. NaOH + NH4Cl →NaCl + H2O + NH3

APPLICATION NEUTRALIZATION REACTION

  1. Management of soil PH
    2.  Most plants grow well when the PH of the soil is close to 7.

    Neutralization reactions can be used to adjust the PH of soil, making it more suitable for plant growth.

    Excess acidic soil can be treated by addition of basic compounds such as oxides, hydroxides and carbonates of calcium and magnesium.

    Also, high soil alkalinity can be lowered by addition of acidic substances such as sulphur and ammonium based fertilizers.

  2. Treating insect stings.
    3.  Bee and ant stings inject an acidic liquid to the skin or flesh.

    The sting can be neutralized by rubbing with baking soda (sodium hydrogen carbonate).

    Other sting which can be treated this way is nettle sting from plants which contain formic acid.

    Wasp stings are alkaline and can be neutralized with vinegar.
  3. Treating factory waste.
    4.  Liquid waste from most industries and factories often contains acid.

    Neutralization reactions are used to treat industrial waste before it is released into the environment or water bodies. For example most of these wastes can be treated by addition of slaked lime.

  4. Relieving indigestion
    5.  Indigestion is mostly caused by the presence of excess hydrochloric acid in the stomach.

    Neutralization reaction can help relieve indigestion.

    Antacids are a type of medication that contain alkaline substances such as magnesium hydroxide (Mg(OH)2 also known as milk of magnesia, which react with stomach acid to form salt and water.

  5. Neutralizing accidental spills.
    6.

    If acid spills on laboratory benches or working surfaces can be neutralized by basic compounds such as quicklime CaO and slaked lime, Ca(OH)2.

    Also if alkali spills on the surface can also be treated by acidic compounds.

ACID-BASE INDICATORS

Indicator is a substance that shows whether another substance is an acid, a base or neutral by changing its color.

It changes color depending on whether a solution is acidic, basic or neutral.


Common indicator in chemistry laboratory
Indicator In acid In neutral In alkali
Litmus paper RedPurple/blue Blue
Methyl orange Pink Orange Yellow
Phenolpthalein Colorless Colorless Pink

Universal indicator is the mixture of simple indicators which changes through a range of different colours. Universal indicator changes color depending on how strong the acid or base is. That is to say it depends on the PH of the solution.

PH SCALE

PH is the negative logarithm of the concentration of hydrogen ions of the solution.
PH = -log [H+] or PH =log 1/[H+]
PH scale is the scale used to measure the acidity, neutrality and alkalinity of the solution. It ranges from 0 to 14, whereby;

SALTS

Salt is a compound formed when the hydrogen of an acid is replaced by a metal ion or ammonium ion.

OR

A salt is a compound formed when all the replaceable or part of the replaceable hydrogen ions of an acid has been replaced by a metal or ammonium ion.

There so many salts in our world which we live. The name of salt is derived from the acid from which it is formed, as follows:
Acid Name of the salts Example
Carbonic acid (H2CO3) Carbonates Sodium carbonate (Na2CO3)
Ethanoic acid (CH3COOH) Ethanoates Sodium ethanoate (CH3COONa)
Hydrochloric acid (HCl) Chlorides Calcium chloride (CaCl2)
Nitric acid (HNO3) Nitrates Copper (II) nitrate (Cu(NO3)2)
Sulphuric acid (H2SO4) Sulphates Potassium sulphate (K2SO4)
Phosphoric acid(H3PO4) phosphates Sodium phosphate (Na3PO4)

Solubility of salts in water

Select the salts in the solubility rule and categorize those salts on whether they are soluble or insoluble in water.

METHODS OF PREPARING SALTS

  1. Methods of preparing soluble salts
    1. Action of an acid on a metal (MAZIT)
      2. Acid + more reactive metal →salt + hydrogen
      Zn + H2SO4 →ZnSO4 + H2
    2. Action of acid on insoluble carbonate
      3. Insoluble carbonate + acid →salt + water + carbon dioxide
      PbCO3 + HNO3 →Pb(NO3)2 + H2O + CO2
    3. Action of an acid on an insoluble oxide
      4. Insoluble oxide + acid →salt + water
      ZnO + H2SO4 →ZnSO4 + H2O
    4. Action of an acid on hydroxide by titration.
      5. NaOH + HCl →NaCl + H2O
      NH3(aq) + HCl →NH4Cl + H2O
  2. Methods of preparing insoluble salts
    Insoluble salts are usually prepared by precipitation reaction (double decomposition reaction).
    Pb(NO3)2 + HCl →PbCl2 + HNO3
    Other salts that can be prepared this way are calcium carbonate, copper carbonate, zinc carbonate, lead sulphate.
Show chemical reactions that produce these salts.

PROPERTIES SALTS

  1. Physical properties

    1. Most are crystalline solids
    2. they have high melting points and boiling points.
    3. Many salts are electrolytes (they conduct electricity in their solution or molten states.)
    4. Salts are soluble, sparingly (partially) soluble and insoluble in water.
    5. Some salts change when exposed to air
    Chemical properties of most salts check out here

    Types of salts

    USES OF SALTS

    Explore Form Three topics