CHAPTER 21 - BUFFERS AND NEUTRALISATION Flashcards
What is a buffer solution
A system that minimises pH changes when small amounts of an acid or a base are added
What is a buffer comprised of
Weak acid (HA) and conjugate base (A-)
What are the two methods to prepare a buffer solution
Preparation from a weak acid and its salts
Preparation by partial neutralisation of the weak acid
How is a buffer solution prepared from a weak acid and its salts
Mixing a solution of (eg.) ethanoic acid - CH3COOH - with one of its salts, (eg) sodium ethanoate CH3COONa
Ethanoic acid is the source of the weak ass component of the buffer solution
Salts of weak acids are ionic compounds, and provide a convenient source of the conjugate base. When added to water, the salt completely dissolves, dissociating into ions, so that the source of the conjugate base component of the buffer solution.
CH3COOH <–> H+ + CH3COO- (all aq)
CH3COONa(s) + Aq –> CH3COO-(aq) + Na+ (aq)
How is preparation of a buffer by partial neutralisation of the weak acid prepared
Adding aqueous solution of alkali (eg. NaOH) to an excess of weak acid.
Weak acid is partially neutralised by alkali, forming conjugate base, but some of acid remains untreated, leaving a mixture of salt of weak acid and unreacted weak acid
How does an ethanoic acid equilibrium act as two reservoirs to remove added alkali and acid
In ethanolic acid equilibrium, the equilibrium position lies well towards ethanolic acid.
When CH3COO- ions are added to CH3COOH, the equilibrium position shift, even further to the left, reducing the already small concentration of H+ (aq) ions, and leaving a solution containing, mainly the two components, CH3COOH and CH3COO-
CH3COOH and CH3COO- act as to reservoirs that are able to act independently to remove added acid and alkali. This is achieved by shifting the buffers equilibrium system either to the right or left.
What is the equilibrium equation in a buffer solution of an conjugate acid-base pair
HA (aq) <—> H+ (aq) + A- (aq)
How does a conjugate base remove added acid in a buffer solution?
[H+] increases
H+ ions react with conjugate base - A-
The equilibrium position shits to the left, remain most of the H+ ions
HA (aq) <—> H+(aq) + A- (aq)
How does a weak acid remove added alkali in a buffer solution?
[OH-] increases
The small concentration of H+ ions reacts with the OH- ions
H+(aq) + OH-(aq) —> H2O (l)
HA dissociates, shifting equilibrium position to the right to restore most of the H+ ions
When is a buffer most effective at removing either added acid or alkali
when there are equal concentrations of the weak acid and its conjugate base
[HA(aq)] = [A-(aq)]
pH of 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
How do you calculate Ka
Ka = [H+][A-] / [HA] (all aq apart form Ka obvs)
(Pg 334)
What pH range does blood plasma need to be maintained between
7.35-7.45
What is one example of a buffer solution in the body
Carbonic acid-hydrogen carbonate (H2CO3/HCO3-) system in blood plasma
What happens if Blood pH falls below or rises above the desired pH range
Falls below:
acidosis, causes fatigue, shortness of breath, shock or death
Rises above:
Alkalosis, , causes muscle spasms, light-headedness and nausea
CALCULATE Ka EXAMPLE QUESTIONS
DO SOME
How is a build up of H2CO3 prevented
Converting it into CO2 which is then exhaled by lungs
What is the Henderson-Hasselbalch equation
An alternative way of calculating the pH of a buffer solution that allows you to really see how pKa and the base/acid ratio control the pH
pH=pKa+ log [A-] / [HA]
Describe an experiment using a pH meter/probe to see the effect of neutralisation
Add measured volume of acid to a conical flask using a pipette
Place the electrode of the pH meter in the flask
Add the aqueous base to the burette, and add to the acid in the conical flask, 1 cm³ at a time
After each addition, spell the contents. Record the pH and the total volume of the aqueous base added.
Repeat steps three and four, until the pH starts to change more rapidly. Then add the aqueous base drop wise for each reading until the pH changes less rapidly.
Now, add the aqueous base 1 cm³ at a time until an excess has been added, and the pH has been basic, with little change, for several additions
Or reverse - acid added to base
What is the equivalence point
The volume of solution that EXACTLY reacts with the volume of another solution
What is he endpoint of a titration
When the indicator contains equal concentrations of HA and A- and the colour will be between the two extreme colours
Read about indicator suitability
fairly straight forward, along with pH graphs
