Protons and their impact

Question 1

What is the pH equation?

Answer 1

pH = -log10[H+]
lower pH = higher concentration of H+

Question 2

What is the pH of water?

Answer 2

The pH of pure water is 7.
pH = 7 = -log10[H+] =[10^-7]
So [H+] = 10^-7M

Question 3

What is dynamic equilibrium?

Answer 3

The state of a chemical process in which the forward and reverse reactions occur at equal rates, so the concentrations of reactants and products do not change with time.

Question 4

What is Le Chatelier’s principle?

Answer 4

A change to equilibrium - temperature, pressure or concentration, will cause the system to shift its equilibrium position to counteract the effect.

Question 5

How does the equilibrium of an acid change when lowering pH?

Answer 5

Acids become ionised by losing H+.
Putting it in a solution of low pH - solution is rich in H+
Equilibrium shifts to the left, so there is more reactant and less ionisation.

Question 6

How does the equilibrium of an acid change when increasing pH?

Answer 6

In high pH, the solution is deficient in H+.
So equilibrium shifts to the right, there is more ionisation and more conjugate base.

Question 7

What is the relationship between pH and ionisation of acid?

Question 8

How do you know the %ionised, or where each species lies?

Answer 8

Use pKa - the compound is 50% ionised when pH = pKa

Question 9

What is the relationship between pH, pKa and ionisation?

Question 10

What is pKa?

Answer 10

A logarithmic constant
It is directly proportional to the free energy of the acid-base reaction (how acidic the compound is), and so gives an insight into how the equilibrium changes with pH.

Question 11

What is the Henderson-Hasselbach equation?

Answer 11

pH = pKa + log ([A-]/[HA])

Question 12

How do you calculate % ionisation?

Answer 12

%compound ionised = 100/1+10^charge(pH-pKa)
Where charge = -1 for acids (negatively ionised)
+ 1 for bases (positively ionised)

Question 13

What is the equilibrium for bases?

Answer 13

HB+ <–> H+ + B

Question 14

What is the equilibrium for acids?

Answer 14

HA <–> H+ + A-

Question 15

What is a conjugate acid base pair?

Answer 15

The deprotonated acid is the conjugate base - it accepts a proton in the reverse direction.
Strong acids give weak conjugate bases.

Question 16

How does lowering the pH affect equilibrium of a base?

Answer 16

In low pH, it is rich in protons, so equilibrium shifts left, and there is more ionisation.
Bases become ionised by gaining protons.

Question 17

How is equilibrium of bases affected when you increase pH?

Answer 17

In high pH, the solution is deficient in H+.
Equilibrium shifts right, and there is less ionisation

Question 18

What is Kb?

Answer 18

Kb is the same as Ka
Ka = [H+][B] / [HB+]

Question 19

What is physiological pH?

Answer 19

Cellular pH is generally 7.2-7.4.
In the body pH can also range from 1.5 (stomach) to 8.5 (duodenum).

Question 20

Why does ionisation matter?

Answer 20

Change in pH can have a big effect on ionisation and ionisation has a big effect on :
Potential conformation of protein
The behaviour of a molecule (if acid/base) in the body.

Question 21

How many ionisable groups are there?

Answer 21

pKa can describe ionisation properties of a single ionisable functional group.
But biological molecules have many ionisable groups, so has many pKa.
Changing the pH could affect multiple ionisable groups, and in different ways in terms of more or less ionisation.

Question 22

What is the isoelectric point?

Answer 22

When the net charge of a protein is 0, as there is an equal number of + and - charged groups.
The protein is at its minimum aqueous solubility.
It means proteins can be separated.

Question 23

How is phosphate’s acidity important for DNA structure?

Answer 23

Phosphate is highly acidic (pKa 0.1) therefore all phosphates in the backbone are ionised at pH >2.
So phosphate wants to be in the water, and the base pairs stick in.
DNA and RNA are always negatively charged - can separate out using gel electrophoresis.