Acids and Bases Flashcards
What is activity?
a modified concentration which accounts for energy loss when an ion divests itself of the oppositely charged ions around it (loss of reactivity)
Activity equation?
(activity coefficient x concentration)/ standard molality
Why do you divide by molality?
to make activity unitless
what is activity in standard state?
1
What happens at infinite dilution?
coefficient tends to 1 and activity = concentration
What happens when an acid/base dissociates?
form conjugate base/conjugate acid
What does a stronger base cause?
weaker conjugate acid
why can you ignore effects of 2nd dissociation?
They will be that small that they wont really effect anything (cant ignore with H2SO4)
equation for Kw and pkw (Learn how to derive them)
Kw = Ka x Kb pKw = pKa + pKb = 14 Kw = [H30+][OH-], take logs pKw = pH + pOH
What is a buffer solution?
a solution that can resist a change in pH when small amounts of strong acid/base are added, contains weak acid a d salt of a weak acid (conjugate base)
What does the acid and conjugate base in the buffer do?
acid mops up -OH ions and conjugate base mops ip protons (as theyre both in large excess)
How do you derive the equation for pH of a buffer?
write the Ka using the chemical equation and take -logs of it
pH = pKa + log([A-]/[HA])
What does the pH of a buffer depend on?
The ratio of base to acid and the Ka
What is buffer capacity?
amount of acid/base a buffer can neutralis without a significant change in pH
When is buffer capacity max?
When [A-] = [HA]
What is the buffer range?
The pH over which a buffer operates effectively
What should you consider when choosing a buffer?
- choose a weak acid with a pKa close to the required pH
- choose the appropriate salt for the acid/base
- use the H-H equation to determine the ratio of [A-]/[HA]
equivalence point for strong base/strong acid?
7
equivalence point for weak acid/strong base?
8/9
equivalence point for weak base/strong acid?
5/6
What does the pH equal at the half equivalence point?
pH = pKa
