The acidity constant
pH is a number that is a property of a solution (pH = -log[H3O+]) where [H3O+] is in .
pH is measured with a pH meter or pH paper
pH depends on which acids and bases are present and their concentration.
The equilibrium constant Ka is a number that is a property of an acid-base conjugate pair.
Examples of acid-base conjugate pairs are
Comparison of Ka for two acid-base conjugate pairs indicates the relative tendency of the
two acids to donate a proton and the relative tendency of the two bases to accept a proton.
The relationship between Ka and the concentrations of H3O+, acid and conjugate base in a solution at equilibrium is given at the right.
Solutions containing weak acids and/or their conjugate bases achieve equilibrium on mixing.
The form of the reaction quotient expression is defined by reaction of one mole of the acid with water. acid + H2O
H3O+ + conjugate base
Reaction quotient expressions have products in the numerator and reactants in the denominator.
Each concentration is raised to a power equal to its coefficient in the balanced equation for the reaction.
For stronger acids, Ka is higher as the tendency of the acid to donate H+ is higher.
The forward reaction is more significant (higher [conjugate base] than [acid] at equilibrium).
For stronger bases, Ka is lower as the tendency of the base to accept H+ is higher.
The reverse reaction is more significant (higher [acid] than [conjugate base] at equilibrium).
pH is measured with a pH meter or pH paper
pH depends on which acids and bases are present and their concentration.
The equilibrium constant Ka is a number that is a property of an acid-base conjugate pair.
Examples of acid-base conjugate pairs are
CH3CO2H (acid) and CH3CO2– (conjugate base)
NH4+ (acid) and NH3 (conjugate base)
NH4+ (acid) and NH3 (conjugate base)
Comparison of Ka for two acid-base conjugate pairs indicates the relative tendency of the
two acids to donate a proton and the relative tendency of the two bases to accept a proton.
The relationship between Ka and the concentrations of H3O+, acid and conjugate base in a solution at equilibrium is given at the right.
| Ka = | [conjugate base ][H3O+] |
| [acid] | |
| number | reaction quotient |
Solutions containing weak acids and/or their conjugate bases achieve equilibrium on mixing.
The form of the reaction quotient expression is defined by reaction of one mole of the acid with water. acid + H2O
H3O+ + conjugate baseReaction quotient expressions have products in the numerator and reactants in the denominator.
Each concentration is raised to a power equal to its coefficient in the balanced equation for the reaction.
For stronger acids, Ka is higher as the tendency of the acid to donate H+ is higher.
The forward reaction is more significant (higher [conjugate base] than [acid] at equilibrium).
The reverse reaction is more significant (higher [acid] than [conjugate base] at equilibrium).