Topic 4 Flashcards
1
Q
Ionic equilibrium, pH, pKa
A
- the cell is an aqueous environment
- water is the main solvent for biological systems
- water has a slight tendency to dissociate into a hydrogen ion - a proton, H+, and a hydroxide ion (OH-)
H20 <-> H+ + OH-
- only a small fraction fo the H2O molecules become ionized
2
Q
Generalized reaction
A
A + B <-> C + D
3
Q
Equilibrium constant Keq
A
- The degree of dissociation
- Keq for any given chemical reaction is fixed at a specified temperature
- defines the composition at equilibrium regardless of the starting concentrations
4
Q
Kw
A
- the ion product constant of water at 25 degrees
- Kw = 10^-14 M^2
5
Q
Can [H+] and [OH-] change?
A
Yes
- if one increases the other will decrease to maintain Kw
6
Q
What is the basis of the pH scale?
A
Kw
7
Q
Regardless of pH
A
[H+][OH-] = 10^-14 M^2
pH +pOH = 14
- the lower the pH, the higher the [H+]
8
Q
pH Scale
A
- a convient way to express [H+] concentration, pH = -log[H+]
- the higher the [H+] of a solution (the more acidic), the lower the pH
- is logarithmic, so a differ ne in 1pH unti is a 10x difference in H+ concentration
9
Q
Measuring pH
A
- indicator dyer
- colourless in acidic to neutral solutions and pink in basic solutions
- glass electrode
10
Q
Strong acids and bases
A
- almost completely ionize (dissociate) in aqueous solutions
- strong acids (HA) will almost completely dissociate into H+ plus their conjugate base
11
Q
Weak acids and bases
A
- hold onto their protons tightly and thus dissociate only partially
- the equilibrium constant, Ka, of a weak acid is :
Keq = ( [H+][A-] ) / [HA] = Ka - the larger the Ka, the greater the tendency for HA to dissociate
- as with [H+], because the numbers are very small, for convenience the strength of acids is expressed in terms of pKa, pKa = -log Ka
- a strong acid will have a large Ka and hence a small pKa
12
Q
Henderson-Hasselbalch equation
A
- describes the relationship between pH and the ratio of acid to base in a solution
- tell you exactly how much pH changes as you add a base (A-) to an acidic solution or vise versa
13
Q
When [HA] = [A-], pH =pKa
A
The pKa is equal to the pH at which [HA] = [A-] (ie. the concentrations of the acid and the conjugate bases are equal)
- the lower the pKa, the stronger the acid
14
Q
Titration curves
A
- allow determination of pKa
- a plot of the dependence of pH of a solution on the amount of base added
- let you determine how much weak acid is present in a solution based on the amount of strong base required to neutralize that solution or to bring it to a specific pH
15
Q
Titration curve midpoint
A
- the point where [A-] = [HA] or [proton acceptor] = [proton donor]
- at this point the pH is equal to the pKa
- the lower the pKa, the stronger the acid
- the weak acid and the conjugate bases are present in ~ equal concentrations
- the HA can neutralize any base added by dissociating to provide more H+ and the A- can neutralize any acid added by associating with excess H+, resulting in minimal pH change within the buffering range
- weak acids have the highest buffering capacity when the pH is equal to their pKa
- the weak acid solution acts as a “buffer”
16
Q
Buffer
A
- a mixture of H+ donor and its H+ acceptor
- have the ability to maintain a relatively constant pH within a certain pH range
- the pH at which a buffer is most effective will depend on its pKa
- biological buffers maintins a relatively constant pH within a compartment of an organism which is extremely important for survival
17
Q
Weak acids in biology and medicine
A
- buffer systems defend cells and organisms against pH changes
- enzymes have optimal activities at different pHs - small changes can alter their activity
- some acid-based conjugate pairs have pKas close to physiological pH so can be in a pronated or deprotonated form at physiological pH - act as buffers
- different enzymes are active at different pHs
18
Q
pH of blood plasma can indicate disease
A
- normal pH is ~7.4
- people with sever uncontrolled diabetes have blood pH below 7.4 (acidosis)
- alkalosis is when the pH is above pH 7.4
- blood plasma pH is maintained by the carbonic acid/bicarbonate systems
19
Q
Triprotic acid
A
- phosphoric acid behaves as this
- 3 ionizable hydrogen atoms
- each acid has its own pKa
