Topic 2 -Role of Water

Question 1

Remember to read over assumed knowledge rev. notes!

Answer 1

Do it BEFORE studying this topic

Question 2

What is ampipathic?

Answer 2

parts of molecule have an affinity for water, others do not.

Question 3

Water can act as both an …. and a ….

Answer 3

acid and a base

Question 4

What properties does Hydrogen bonding give to water?

Answer 4

High MP, BP & heat of vaporisation

Question 5

Any molecule that has a H atom directly attached to a …. or a …. is capable of H bonding

Answer 5

oxygen or a nitrogen

Question 6

What property of H acceptor atoms allows them to form a bond w/ H?

Answer 6

They have a lone pair electron

Question 7

H atoms covalently bonded to C atoms do not participate in H bonding. Why?

Answer 7

C is not electronegative enough to cause unequal sharing w/ H

Question 8

What molecules can dissolve in water?

Answer 8

Charged biomolecules (Na+, Ca-)
Uncharged but polar molecules (sugars e.g. hydroxyl groups)
Alcohols, aldehydes, ketones and compounds containing N-H covalent bonds

Question 9

Polar molecules (hydrophilic) are water soluble. T or F?

Answer 9

true

Question 10

Non-polar molecules (hydrophobic) are water insoluble. T or F?

Answer 10

true. includes lipids & fats

Question 11

Ampipathic molecules are water soluble & insoluble. T or F?

Answer 11

true

Question 12

To achieve thermodynamic stability when placed in water, what do ampipathic compounds do?

Answer 12

Minimise amount of hydrophobic area exposed to water (polar solvent) by grouping together. This increases entropy.

Think micelles, lecture pages 8-10

Question 13

The strength of hydrophobic interactions results from…?

Answer 13

increasing entropy in water molecules by exclusion from hydrophobic regions which provides the greatest thermodynamic stability.
=> Important for membrane formation & protein folding

Question 14

Name the four types of non-covalent interactions which occur b/w biomolecules in aqueous solution.

Answer 14

Hydrogen bonds
Ionic interactions
Van der Waals interactions
Hydrophobic interactions

Question 15

What is Keq? State the equation

Answer 15

equilibrium constant -gives the position of a reaction is fixed for any given chemical reaction at a specific temp.

      [product] Keq = [reactant] 

        [C] [D] Keq =   [A] [B]

Question 16

Write the Keq (equilibrium constant) for the reversible ionisation of water.
What is the concentration of pure water at 25˚C?

Answer 16

[H+] [OH-]
Keq = [H2O]

[H2O] at 25˚C= 55.5M

Question 17

State the equation for the ion product of water (Kw) at 25˚C

Answer 17

[H+] [OH-] = 1 x 10^-14 M^2

Question 18

As the ion product of water is constant, whenever [H+] is greater than 1 x 10^-7, [OH-] must be…?

Answer 18

less than 1 x 10^-7

Question 19

A proton donor (acid) and its corresponding proton acceptor (base) make up a …?

Answer 19

conjugate acid base pair

Question 20

State the pH equation

Answer 20

-log [H+]

Question 21

State the ionisation (dissociation) constant equation (Ka)

Answer 21

[H+] [A-]
Ka = [HA]

HA = acid
A- = base

Question 22

What is pKa?

Answer 22

The pH at which there is 50% ionisation so [HA]=[A] (acid=base)

Question 23

The stronger the tendency to dissociate a proton, the stronger the acid and the lower its…?

Answer 23

pKa

Question 24

State the pKa equation

Answer 24

-log Ka

Question 25

Why do we use titrations?

Answer 25

to determine the amount of acid in a solution

Question 26

When [HA]=[A-] the pH at this point is the same as the…?

Answer 26

pKa

Question 27

What is a buffer and what is a buffer system made of?

Answer 27

A buffer are substances that resist changes in pH

A buffer system is made of a weak acid (proton donor) and its conjugate base (proton acceptor).

Question 28

State the Henderson-Hasselbach equation and what it tekks us

Answer 28

[A-]
pH = pKa + log [HA]

Describes the relationship b/w the pKa & pH of a weak acid