2.2 Water

Question 1

Water structure

Answer 1

H2O = 2 hydrogen atoms covalently bonded to an oxygen atom - sharing of electrons amongst atoms (uneven).

Question 2

Why is water described as polar?

Answer 2

due to slight charge difference across poles (dipole) of the molecule → forms weak associations with other polar molecules (slightly negative poles will attract slightly positive poles of molecules, vice versa).

Question 3

Hydrogen bonding - dipolarity of H2O

Answer 3

form polar associations with other charged molecules (polar/ionic). Water forms hydrogen bonds with other water molecules (between +H & -O of 2 molecules). Intermolecular bonding between water molecules = distinct properties.

Question 4

Hydrogen bonding - thermal properties

Answer 4

capacity to absorb significant amounts of heat before changing state due to the extensive H-bonding between H2O molecules.

H-bonds must be broken to change state (requires absorption of energy).

Thus, supports the maintenance of constant conditions in living organisms, as H2O creates a slow change in temperature.

Question 5

Hydrogen bonding - cohesive/adhesive properties

Answer 5

stick’ to other H2O molecules, ‘stick’ to charged substances (intermolecular associations with polar and ionic molecules.

C = surface tension, allows liquid to resist low levels of external force + sufficiently dense for certain smaller organisms to move along the surface.

A = Capillary action - attraction to ionic/polar surfaces allows H2O to flow in opposition of gravitational forces + necessary to transport water up plant stems (transpirational stream)

Question 6

Hydrogen bonding - solvent properties

Answer 6

dissolves polar and ionic substances, forms competing polar associations which draw substances apart.

Question 7

Methane vs Water - similarities vs differences

Answer 7

Comparable size, weight, valence structures (tetrahedral orbital formations). CH4 – nonpolar, weak dispersion forces between molecules. H2O – polar, forms intermolecular H-bonds (high electronegativity of O atom).

DIFFERENCES:

H2O has a higher specific heat capacity (energy required to raise the temperature of 1g by 1 degree), a higher heat of vaporisation (energy absorbed per gram in change from liquid to gas), and higher heat of fusion (energy required to change 1g of liquid to 1g of solid // LOST).

Question 8

hydrophilic

Answer 8

can freely associate/dissolve in water, e.g. polar molecules & ions

Question 9

hydrophobic

Answer 9

cannot freely associate/dissolve in water, e.g. large, non-polar molecules, e.g. fats and oils.

Question 10

water as a universal solvent

Answer 10

water has the capacity to dissolve a large number of substances.

Any ionic or polar substance can dissolve.
Polar attraction of large quantities of water sufficiently weakens intramolecular forces to result in the dissociation of atoms. Charged regions of water molecules surround atoms of opposing charge to form dispersive hydration shells.

Question 11

ability to travel in blood plasma: SOLUBLE

Answer 11

1. Sodium chloride (ionic compound).
2. Oxygen (soluble in low amounts, mainly transported in haemoglobin).
3. Glucose (hydroxyl groups > -OH).
4. Amino acids (ionised state > amine/carboxyl groups are charged).

Question 12

ability to travel in blood plasma: INSOLUBLE

Answer 12

Lipids

(non-polar, thus hydrophobic. Can travel if they form complexes with proteins in which the hydrophilic portions of proteins shield internal hydrophobic components of the lipids in order to travel through the bloodstream, e.g. lipoproteins).