acids, bases and PH year 2 Flashcards
(11 cards)
Bronsted- Lowry acids
-dissociate and release H+ ions in aqueous solution
bronsted-lowry bases
-dissociate and release OH- ions in aqueous solutions
buffer solution
A system that minimises pH changes when small amounts of acid or base are added
- contains two components to remove added acid or alkali- a weak acid and its conjugate base
- the weak acid removes added alkali
- the conjugate base removes added acid
- when acids and alkalis are added to a buffer, the two components in the buffer solution react and will eventually be used up. As soon as one component has all reacted, the solution loses its buffering ability towards added acid or alkalis
- as the buffer works the pH DOES change- but only by a small amount
preparation from a weak acid and its salt
A buffer solution can be made by mixing a solution of ethanoic acid with a solution of one of its salts, eg sodium ethanoate
- when ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small
- ethanoic acid is the source of the weak acid component of the buffer solution. Salts of weak acids provide a convenient source of the conjugate base. When added to water, salt completely dissolves and so the salt is the source of the conjugate base component of the buffer
- In ethanoic acid equilibrium, the equilibrium position lies well to the left. When the CH3COO- ions are added, the equilibrium position shifts even further to the left, reducing the already small concentration of H+ and leaving a solution containing reservoirs of mainly the two components, CH3COOH and CH3COO-. These reservoirs are able to act independently to remove added acid and alkali. This is achieved by shifting the buffers equilibrium system to the left or the right
preparation by partial neutralisation of the weak acid
A buffer solution can also be prepared by adding an aqueous solution of an alkali, eg NaOH, to an excess of the weak acid. The weak acid is partially neutralised by the alkali, forming the conjugate base. Some of the weak acid is left over unreacted, resulting in a solution containing a mixture of the salt of the weak acid and any unreacted acid
action of the buffer system
The conjugate base-pair HA/A- in an acid buffer solution controls the pH
- when acid/H+ added, H+ conc increases, they react with the conjugate base A-, and the equilibrium position then shifts left, removing most of the H+
- when OH-/ alkali is added. OH- conc increases, the small concentration of H+ reacts with OH-, forming water. HA dissociates, shifting the equilibrium position to the right to restore most of the H+ ions
choosing the components for a buffer solution
A buffer is most effective at removing either added acid or alkali when there are equal concentrations of the weak acid and its conjugate base
When [HA]= [A-]:
-The pH of the buffer solution is the same as the pKa value of HA
- the operating pH is typically over about two pH units, centred at the pH of the pKa value
The ratio of the concentrations of the weak acid and its conjugate base can then be adjusted to fine-tune the pH of the buffer solution
calculating the pH of a buffer solution
When calculating pH of a weak acid, you make the approximation that [H+] = [A-]
For a buffer solution this is no longer true as A- has been added as one of the components to the buffer
- to work out the pH of a buffer solution, rearrange the Ka expression, without the approximations, to find [H+]
pH titration curves
Acid-base titrations use indicators to monitor neutralisation reactions accurately. The results can then be analysed to find out some unknown information about the acid or base
- when the base is first added (if in the burette), the acid is in great excess and the pH increases very slightly. As the vertical section is approached, the pH starts to increase more quickly as the acid is used up more quickly
Eventually, the pH increases rapidly during addition of a very small volume of base, producing the vertical section. Only drops of solution will be needed for the whole vertical section
After the vertical section, the pH will rise very slightly as the base is now in great excess
- can also be plotted with the acid added from the burette to the base in the clonical flask. The graph is just flipped
the equivalence point
the volume of one solution that reacts exactly with the volume of the other solution. The solutions have then exactly reacted with one another and the amounts used matching the stoichiometry of the reaction. The equivalence point is the centre of the pH titration curve
acid-base indicators
An acid base indicator is a weak acid, HA, that has a distinctively different colour from its conjugate base, eg methyl orange:
HA is red and A- is yellow
At the end point of a titration, the indicator contains equal concentrations of HA and A- and the colour will be in between the two extreme colours. For methyl orange, the colour at its end point is orange
- the equilibrium position is shifted towards the weak acid in acidic conditions or towards the conjugate base in basic conditions, changing the colour as it does so
In a titration in which a strong base is added to a strong acid, methyl orange is initially red as the presence of H+ forces the equilibrium position well to the left
On addition of a basic solution containing OH-
- OH- react with H+ in the indicator. The weal acid HA dissociates, shifting the equilibrium to the right. The colour changes, first to orange at the end point and finally to yellow as the equilibrium is shifted to the right
If methyl orange is initially added to a basic solution and acid is added. The H+ ions react with the conjugate base, A-
The equilibrium position shifts to the left
- the colour changes, first to orange at the end point and finally to red when the equilibrium position has shifted left
- at the end point [A-] = [HA], so Ka = [H+] , and pKa= pH
In a titration you must use an indicator that has a colour change which coincides with the vertical section of the pH titration curve; Ideally the end point and equivalence point would coincide
