7.2 STRENGTH OF ACID AND ALKALI : SLISS 2012

1. • NURUL ASHIKIN BT. ABD RAHMAN

2. Learning Outcome
• State the use of a ph scale correctly
• Relate pH value with acidic or alkaline properties of a
substance.
• Relate concentration of hydrogen ions with ph value and
concentration of hydroxide ions with ph value.
• Relate strong or weak acid with degree of dissociation
• Relate strong or weak alkali with degree of dissociation
• Conceptualise qualitatively strong and weak acids
• Conceptualise qualitatively strong and weak alkalis.

4. Indicator Colour
Acid Neutral Alkali
Litmus Solution Red Purple Blue
Phenolphthalein Colourless Colourless Pink
Methyl orange Red Orange Yellow
Universal Red Green Purple
indicator

5. To indicate the degree of acidity or alkalinity of asolution
[H+] increases [OH-] increases
Increasing acidity neutral Increasing alkalinity
pH scale 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Weak Alkali Strong Alkali
Strong Acid Weak Acid
Example: NH3 Example:
Example: HCl, Example:
NaOH, KOH
HNO3, H2SO4 CH3COOH,
H2SO3, H3PO4
pH [H+] [OH-]

6. STRONG ACID WEAK ACID
Has a high degree of ionization Has a low degree of ionisation
(ionised completely). (partially ionized)
Has a higher concentration of Has a lower concentration of
hydrogen ions. hydrogen ions.
Has a lower pH value. Has a higher pH value.
H2O
H2O
Ex: HCl (l) H+ (aq) + Cl- Ex: CH3COOH (aq) H+ (aq) + CH3COO- (aq)
Degree of ionization a measurement of the percentage
of total number of acids molecules that will ionize
when it dissolve in water.

7. STRONG ALKALI WEAK ALKALI
Has a high degree of ionization Has a low degree of ionisation
(ionised completely). (partially ionized)
Has a higher [OH-]. Has a lower [OH-]
Has a higher pH value. Has a lower pH value.
H2O H2O
Ex: NaOH (l) Na+(aq) + OH- (aq) Ex: Mg(OH)2 (aq) Mg2+ (aq) + OH- (aq)

8. EXERCISE
1) HCl 6) KOH
2) CH3COOH 7) HNO3
3) H2SO4 8) Ba(OH)2
4) NaOH 9) Mg(OH)2
5) NH3 10)Ca(OH)2

9. HCl Cl- H+
H2O Strong
Acid
Before ionization After ionization
H2O
Weak
Acid
CH3COOH CH3COOH CH3COO- H+

10. NaOH Na+ OH-
H2O Strong
Alkali
Before ionization After ionization
H2O
Weak
Alkali
NH3 NH3 NH3+ OH-