MCAT Chemistry > Acids & Bases
There are many different definitions on what exactly bases and acids do, however the MCAT will test all of them so understanding how these important molecules react is essential in being successful come test day. Not only in chemistry, but acids and bases play key roles in the human body systems and understanding these concepts is essential in becoming a successful doctor.
- Acids dissociate in water into H+ ions
- Bases dissociate in water into OH- ions
Bronsted-Lowry System contains the concept of conjugate base pairs
- Acids donate H+ ions
- Bases accept H+ ions (OH-, NH3, F-)
- Acids are species that are electron pair acceptors
- Bases are species that are electron pair donators
The nomenclature is related to parent anion name.
Acids end with the suffix –ide or by adding the prefix hydro- and ending with the suffix –ic
- F- = Fluoride
- HF = hydrofluoric acid
- Br- = Bromide
- HBr = hydrobromic acid
Oxyoacids are acids formed from oxyanions.
If the anion ends in –ite (less O), acid will be –ous acid, if the anion ends in –ate (more O), than the acid will end with –ic acid
Hypo = inadequate, Hyper = excessive
- ClO- = hypoclorate = HClO = hypochlorous acid
- ClO2- = chlorite = HClO2 = chlorous acids
- ClO3- = chlorate = HClO3 = chloric acid
- ClO4- = perchlorate = HClO4 = perchloric acid
- NO2- = nitrite = HNO2 = nitrous acid
- NO3- = nitrate = HNO3 = nitric acid
Water Auto-Ionization is a property that allows water act as an amphoteric species, which is a substance that in the presence of a base reacts like an acid, and in the presence of an acid, acts like a base.
See pH and pOH
Strong Acids and Bases
Strong Acids and Bases completely dissociate into constituent ions in aqueous solutions
NaOH + H2O → Na+ + OH-
See Strong Acids
See Strong Bases
Weak Acids and Bases
Weak Acids and Bases are weak to the degree that which they partially dissociate.
The smaller the value of Ka, the weaker the acid and the less dissociation
HA + H2O ←→ H3O+ + A-
Ka = [H30+]([A-]/[HA])
- Ka < 1.0 = weak acid/base.
The pH of a solution containing a weak acid/weak base depends on the strength of the reactants. When a weak acid reacts with a strong base, the pH of the solution at equilibrium is basic because salt hydrolyzes to reform acid and with concurrent formation of the OH- ions from H2O, the reaction will shift away from autoionization, [H+/H3O+] decreases, [OH-] greater than [H+].
CH3COOH + NaOH ←→ Na+ + CH3COO- + H2O
CH3COO- + H2O ←→ CH3COOH + OH-
- Acidic = Ka > Kb
- Basic = Ka > Kb
See Weak Acids
See Weak Bases
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