Thursday, 22 January 2015

7.2 The strength of acids and alkalis

The pH scale
■ pH scale

A numerical scale that has a range between 0 to 14.

To measure the acidity and alkalinity of a solution.

A measurement of the H+ ion in a solution. (pH = -log[H+])

Definition : -log[H+], where [H+] represents the concentration of hydrogen ion.
■ Relationship between hydrogen ions, H+ and hydroxide ion, OH-.

An aqueous solution contains hydrogen ions, H+ and hydroxide ion, OH- formed from the dissociation of water.

pH scale can be used to compare the concentration of hydrogen ion, H+ in an acidic solution.

pH scale can be used to compare the concentration of hydroxide ion, OH- in an alkaline solution.
Solution pH Relationship between concentration of hydrogen ion and hydroxide ion
Acids Less than 7 Concentration of H+ ions is higher than OH- ion.
Alkalis More than 7 Concentration of H+ ions is less than OH- ion.
Neutral Equal to 7 Concentration of H+ ions is equal than OH- ion.

The animation below shows the relationship between hydrogen ions, H+ and hydroxide ion, OH-.
■ Measurement of the pH value of a solution.

Universal indicator
A mixture of indicators
Gives different colours corresponding to different pH values.

pH meter
An electric meter
Measure pH value of a solution accurately
■ This video contains information on the strength of acids and alkalis



Degree of dissociation of acids in water
■ Degree of dissociation/ionisation

Strength of an acid or an alkalinity.

Percentage or fraction of molecules that dissociates into ion when dissolved in water.
■ Acid can be divided into two categories:

Strong acid: an acid dissociates completely in water to form hydrogen ion, H+ of high concentration.

Weak acid: an acid dissociates partially in water to produce hydrogen ion, H+, of low concentration.
■ Degree of dissociation of strong acids and weak acids in water

Strong acids and weak acids have the same chemical properties but the rate of reaction and electrical conductivity of weak acid is lower.

Weak acid: an acid dissociates partially in water to produce hydrogen ion, H+, of low concentration.
Acid Strong acid Weak acid
Degree of dissociation in water Dissociates completely Dissociates partially
Concentration of hydrogen ion High concentration of hydrogen ion Low concentration of hydrogen ion
pH value Low High
Ionisation of acid in water Example: Hydrochloric acid solution
Example: Ethanoic acid solution
■ Examples of strong acid:

Acid Chemical equation
Hydrochloric acid HCl(g)H2 OH+(aq) + Cl-(aq)
Sulphuric acid H2SO4(aq) H2O + 2H+(aq) + SO42-(aq)
Nitric acid HNO3(aq) H2OH+(aq) + NO3-(aq)
■ Examples of weak acid:

Acid Chemical equation
Ethanoic acid CH3COOH(aq)H 2OH+(aq) + CH3COO-(aq)
Ethanadioic acid H2C2O4(aq) H2O2H+(aq) + C2O4 2-(aq)
■ This video shows on a molecular level the differences in ionization between strong and weak acids.



Degree of dissociation of alkalis in water
■ Alkalis can be divided into two categories:-

Strong alkalis: an alkali dissociates completely in water to form hydroxide ion, OH- of high concentration.

Weak alkalis: an alkali dissociates partially in water to produce hydroxide ion, OH- of low concentration.
■ Degree of dissociation of strong alkali and weak alkali in water

Alkali Strong alkali Weak alkali
Degree of dissociation in water Dissociates completely Dissociates partially
Concentration of hydroxide ion High concentration of hydroxide ion Low concentration of hydroxide ion
pH value High Low
Ionisation of alkali in water Example: Sodium hydroxide solution
Example: Ammonia solution
■ Examples of strong alkali:

Alkali Chemical equation
Sodium hydroxide solution NaOH(s)H2 ONa+(aq) + OH-(aq)
Potassium hydroxide solution KOH(s) H2O + K+(aq) + OH-(aq)
■ Examples of weak alkali:

Alkali Chemical equation
Ammonia aqueous NH3(g) + H2O(l)H2OHH4+(aq) + OH-(aq)
Calcium hydroxide Ca(OH)2(s) H2OCa2+(aq) + OH-(aq)


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