2.1 Strong & Weak Acids & Bases
You have undoubtedly heard of the pH scale before and know that it has
something to do with indicating how strong or weak an acid is. In this
part of the unit we will work towards defining acid and base strength
in terms of pH, but there are several important steps along the way. It will be important that you understand each step.
Section 1-3 examined the difference between strong and weak electrolytes:
- Strong electrolytes conduct electricity well because the compound produces many ions in solution
- Weak electrolytes conduct electricity poorly because they produce few ions in solution
These terms apply equally well to acids and bases which are, of course, electrolytic solutions:
Strong acids produce many H+ ions
(or H3O+ ions)
weak acids produce few H+ ions
The stronger the acid,
the more H+ ions are produced |
AND
Strong bases produce many OH- ions
weak bases produce few OH- ions
The stronger the base,
the more OH- ions are produced. |
Strong acids and bases are essentially one-way reactions - the acid or base breaks down completely to produce ions. At equilibrium there are very few reactants left (very low concentration); only products - the ions.
Strong Acids |
Strong Bases |
|
|
HCl(aq) → H+(aq) + Cl-(aq) |
NaOH(aq) → Na+(aq) + OH-(aq) |
H2SO4 (aq) → H+(aq) + HSO41-(aq) |
Mg(OH)2 (aq) → Mg2+(aq) + 2 OH-(aq) |
Weak acids and bases, however, do not ionize completely. For weak electrolytes, equilibrium lies to the left side of the equation (the reactant side) and there will be few ions present. The double arrow is commonly used to indicate the partial ionization of the solution. Some examples:
Weak Acids |
Weak Bases |
|
|
HC2H3O2 (aq) ↔ H+(aq) + C2H3O2-(aq) |
NH3 (aq) + H2O(l) ↔ NH4+(aq) + OH-(aq) |
HCHO2 (aq) ↔ H+(aq) + CHO2-(aq) |
NH2CH3(aq) + H2O(l) ↔ NH3CH3+(aq) + OH-(aq) |
It is important that you don't confuse acid and base strength will dilution.
Dilute and concentrated, you should remember, refer to the relative amounts of solute and solvent in a solution. By contrast, acid and base strength specially refer to the concentration of ions in the solution.
A strong acid such as HCl is still a strong acid (completely ionizes to produce many ions) even when it is dilute (lots of water and relatively small amounts of HCl). Acetic acid is still a weak acid even when it is concentrated.
