Chapter 21 Flashcards
What is a buffer solution?
A buffer solution is a solution which resists changes in pH when small amounts of acids or alkalis are added. They are used to keep the pH almost constant.
What do buffer solutions consist of?
A buffer can consists of weak acid - conjugate base or weak base - conjugate acid
What does the weak acid do in a buffer solution?
Removes added alkali
What does the conjugate base do in a buffer solution?
Removes added acid
How does the ethanoic acid and sodium ethanoate buffer solution work?
Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions
Sodium ethanoate is a salt which fully ionises in solution
What happens when H+ ions are added to a buffer solution..? (acid)
Conjugate base removes the added acid.
The increased H+ reacts with the A- (conjugate base) and forms more HA until equilibrium is re-established. (equilibrium shifts to the left)
What happens when OH- ions are added to a buffer solution..? (alkali)
Weak acid removes added alkali
OH- reacts with H+ and forms water. More HA dissociates to restore some H+. So equilibrium shifts to the right.
How do you calculate the pH of a buffer solution?
H+ = Ka x [HA]/[A-]
The values of H+ and A- are no longer thought to be equal. (As A- is added as a component in the buffer solution)
What is the buffer system used to control blood pH?
The carbonic acid- hydrogen carbonate buffer system.
H2CO3 –> H+ + HCO3
Body cells produce CO2 during aerobic respiration
This CO2 will combine with water in blood to form a solution containing H+ ions
CO2 (g) + H2O (l) ⇌ H+ (aq) + HCO3- (aq)
This equilibrium between CO2 and HCO3- is extremely important
If the concentration of H+ ions is not regulated, the blood pH would drop and cause ‘acidosis’
Acidosis refers to a condition in which there is too much acid in the body fluids such as blood
This could cause body malfunctioning and eventually lead to coma
How does the carbonic acid- hydrogen carbonate buffer system react when OH- or H+ is added?
If H+ is added it reacts with the conjugate base HCO3- to form more HA (H2CO3) Equilibrium shifts to the left.
If OH- is added it reacts with H+ to form water. Then more H2CO3 dissociates to restore H+ and shifts equilibrium to the right.
What should the pH of blood be?
between 7.35 and 7.45
What shape is a pH curve?
A s shape.
What is the equivalence point of a titration?
The volume of one solution that exactly reacts with the volume of the other solution. It is the centre of the vertical section of the pH curve.
What is the end point of a titration?
The indicator contains equal amounts of the weak acid and conjugate base so the colour is in-between the two extremes.
eg methyl orange- HA is red, A- is yellow. End point is orange
How do you choose which indicator to use?
The indicator must have a colour change which coincides with the vertical section of the pH titration curve.
