Salts Flashcards
What is the same and what is different in equimolar solutions of weak and strong acids or bases?
same-stoichiometry of reactions
different-pH values, conductivity, reaction rates
what is the acid dissociation constant represented by?
Ka or by pKa
Ka = [H3O+][A-]/[HA]
where A- = conjugate base of acid
H3O+ = conc. of hydronium ions
HA= conc. of the acid
pKa = -logKa
what formula is used for the approximate pH of a weak acid?
pH = 1/2pKa -1/2longc
Difference between Ka, pKa and pH?
Ka=strength of an acid in solution (high value=strong acid) (a measure of the degree of dissociation)
pKa=indicates whether an acid is strong or weak (high pKa=weaker acid)
pH= concentration of hydronium ions
What does a soluble salt of strong acid and strong base dissolve in water to produce?
neutral solution
What does a soluble salt of weak acid and strong base dissolve in water to produce?
alkaline solution
What does a soluble salt of strong acid and weak base dissolve in water to produce?
acidic solution
how can the changes in concentration of H3O+ and OH- ion of salt solutions be explained?
using the appropriate equilibria
What is a buffer solution?
one in which the pH remains approximately constant when small amounts of acid, base or water (diluted) are added
What does an acid buffer consist of?
a solution of weak acid and one of its salts made from a strong base
(salt must relate to acid)
how does an acid buffer solution work?
In an acid buffer solution, the weak acid provides hydrogen ions when w£3’ these are removed by the addition of a small amount of base.. The salt of the weak acid provides the conjugate base, which can absorb excess hydrogen ions produced by the addition of a small amount of acid.
What does a basic buffer consist of?
a solution of a weak base and one of its (related) salts
How does a basic buffer solution work?
In a basic buffer solution the weak base removes excess hydrogen ions, and the conjugate acid provided by the salt supplies hydrogen ions when these are removed
What are indicators?
weak acids for which dissociation can be represented as:
HIn(aq) + H2O(l) <—> H3O+(aq) + In-(aq)
In aqueous solution, what is the colour of an acid indicator distinctly different from?
That of its conjugate base
How is the colour of an indicator determined?
ratio of [HIn] to [In-]
HIn = weak acid indicator
In- = conjugate base
What is the theoretical point at which colour change occurs with an indicator?
when [H3O+] = KIn
When is the indicator colour change assumed to be distinguishable?
when [HIn] and [In-] differ by a factor of 10
What does the expression, pH = pKIn +- 1, estimate?
The pH range over which a colour change occurs
salt from strong acid v strong alkali
pH=7 neutral
There are no weak ions from strong acids and strong alkalis
eg sodium chloride, potassium phosphate
salt from weak acid v strong alkali
pH>7 alkaline
eg sodium ethanoate, potassium carbonate
salt from strong acid v weak alkali
pH<7 acidic
eg ammonium chloride, ammonium nitrate
How do you work out the pH of a salt
-Show the reaction
-take the products and split them into ions + their OH- and H+ from first reactants
-work out what side equilibria lies towards for each equation (ions or molecule)
What is an example of a salt made from a weak acid and strong base
soaps (alkaline salt)
What is the equation when dealing with ammonia/amines and why is it different?
NH3 + H2P <— -> NH4 + OH-
Ammonia is a weak base. It doesn’t contain hydroxide ions but reacts with water to produce them
What is the calculation to calculate the pH of a weak acid from its pam’s value and its concentration?
(Ka=[H3O+]x[A-]/[HA])
but [H3O+]=[A-]
so
Ka=[H3O+]^2/[HA]
How do indicators change colour at different pHs?
Indicator is a weak acid
If added to acid, H+ concentration increases, so equilibrium will shift to the left to use up H+
If added to alkali, OH- will react with H+, so equilibrium will shift to the right to produce more H+
indicator for strong acid/strong base
indicator must change colour about pH 7
indicator for strong acid/weak base
indicator must change colour about pH 5
methyl orange
indicator for weak acid/strong base
indicator must change colour about pH 9
phenolphthalein
indicator for a weak acid/weak base
titration hardly ever use
the equivalence point changes from case to case
How can buffers be used to calibrate pH meters?
To calibrate a pH meter, normally we use 3 types of buffer solutions: pH4, pH7 and pH10. The electrode will be placed inside each buffer (ascending pH order) and then the calibration button is pressed each time. When you use a pH meter you want to be sure that the pH meter is working ok before hand and displays the right pH values before you measure the pH of your required.
Why does adding water to a buffer not change the pH?
The concentration of the acid and salt will change by the same amount
