How can we measure the strength of acids and alkalis?

How can we measure the strength of acids and alkalis?

1. 1. An acid produces hydrogen ions in aqueous solution. The acidity of a solution is a measure of the concentration of the hydrogen ions in the solution.

1. A base produces hydroxide ions in aqueous solution. The alkalinity of a solution is a measure of the concentration of the hydroxide ions in the solution.
2. In 1909, Soren Sorensen proposed the pH scale for measuring acidity or alkalinity of an aqueous solution.

The pH scale

1. The pH scale normally has a range of values from 0 to 14 to indicate how acidic or alkaline an aqueous solution is.
2. The pH value measures the concentration of hydrogen ions or hydroxide ions.
   pH < 7 → acidi solution
   pH = 7 → neutral solution
   pH > 7 → alkaline solution
3. The lower the pH value, the higher the concentration of hydrogen ions.
   The higher the pH value, the higher the concentration of hydroxide ions.
4. The pH of values some common solutions used in daily life are shown below.
   
5. The pH value of an aqueous solution can be measured by using
   (a) Universal Indicator
   (b) pH meter
   (c) acid-base indicators
   

Table shows examples of acid-base indicators.

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Strong and weak acids

• The strength of an acid or alkali depends on the degree of dissociation of the acid or alkali in water.
• The degree of dissociation measures the percentage of acid molecules that ionise when dissolved in water.
• A strong acid is an acid which ionises or dissociates completely in water to produce a high concentration of hydrogen ions.
• Hydrochloric acid is a strong acid. It is 100% ionised in water. All the hydrogen chloride molecules that dissolve in the water ionise completely into hydrogen ions and chloride ions.
  
• A weak acid is an acid which ionises partially in water to produce a low concentration of hydrogen ions.
• Ethanoic acid is a weak acid. Dilute ethanoic acid is about 0.4% ionised, only 4 out of every 1000 ethanoic acid molecules ionise. As fast as the acid molecules ionise to produce ions, the ions combine back again to give the original acid molecules in a reversible reaction.
  
  
• Examples of strong and weak acids are shown in Table.
  

  Acid	Name	Particles
  Strong acid	Hydrochloric acid, HCl	H+, Cl–
  	Nitric acid, HNO3	H+, NO3–
  	Sulphuric acid, H2S04	H+, HSO4–, SO42-
  Weak acid	Carbonic acid, H2C03	H+, HCO3–, CO32-, H2CO3
  	Ethanoic acid, CH3COOH	H+, CH3COO–, CH3COOH
  	Sulphurous acid, H2SO3	H+, HSO3–, SO32-, H2SO3

• Strong acid and weak acid are defined as follows.
  A strong acid is completely ionised in water to produce a high concentration of hydrogen ions.
  A weak acid is partially ionised in water to produce a low concentration of hydrogen ions.
• The pH of an acid solution changes with concentration of the acid. The concentration of the acid affects the concentration of hydrogen ions produced. For example:
  10 mol dm^-3 hydrochloric acid: pH = 1
  01 mol dm^-3 hydrochloric acid: pH = 2
• pH values can be used to compare the acid strength of different acids. All the acids to be compared must be of the same concentration.
  For example:
  10 mol dm^-3hydrochloric acid: pH = 1
  10 mol dm^-3 ethanoic acid: pH = 3
• For two different acids of the same concentration, the acid with the lower pH value is the stronger acid, i.e. higher degree of ionisation in water.

Strong and weak alkalis

• Sodium hydroxide is a strong alkali. It ionises fully when dissolved in water.
  
• A strong alkali is an alkali which is fully ionised in water to produce a high concentration of hydroxide ions.
• A weak alkali is an alkali which ionises partially in water to produce a low concentration of hydroxide ions.
• Ammonia is an example of a weak alkali. It is only partly ionised in water, which means the ionisation of ammonia in water is incomplete. Only a small amount of ammonia molecules are ionised in water to produce ammonium ions and hydroxide ions.
  
• Examples of strong and weak alkalis are shown in Table.
  

  Alkali	Name	Particles
  Strong alkali	Sodium hydroxide, NaOH	Na+, OH–
  	Potassium hydroxide, KOH	K+, OH–
  	Barium hydroxide, Ba(OH)2	Ba2+, OH–
  Weak alkali	Ammonia, NH3	NH3, NH4+, OH–
  	Methylamine, CH3NH2	CH3NH2, CH3NH3+, OH–
  	Hydrazine, N2H4	N2H4, N2H5+, OH–

• Strong alkali and weak alkali are defined as follows.
  A strong alkali is completely ionised in water to produce a high concentration of hydroxide ions.
  A weak alkali is partially ionisesed in water to produce a low concentration of hydroxide ions.
• The pH of an alkali solution changes with concentration of the alkali. The concentration of the alkali affects the concentration of hydroxide ions produced.
  For example:
  10 mol dm^-3m^-3 sodium hydroxide solution pH = 13
  01 mol dm^-3 sodium hydroxide solution pH = 12
• pH values can be used to compare the strength of different alkalis. All the alkalis to be compared must be of the same concentration. For example:
  10 mol dm^-3 sodium hydroxide: pH = 13
  10 mol dm^-3 ammonia solution: pH = 11
• For two different alkalis of the same concentration, the alkali with the higher pH value is the stronger alkali, i.e. higher degree of ionisation in water.