Structure and properties of water

Polarity of water + reason why?

-Oxygen atom is electronegative=attracts electrons away from the H atoms it is bonded to creating a partial -ve charge on O and partial +ve charge on each H

-Imbalance of charge create a dipole + other molecules that also contain a dipole can align forming bonding interactions/hydrogen bonding.

-Hydrogen bonding occurs between water in bulk state (strong bonds but weaker than ionic + covalent and stronger than Van Der Waals)

Water: polar solvent

DIPOLE=POLARITY

Structure of water

Shape: Tetrahedral/ Bent

Bond angle: 104.5°

109.5°-2.5-2.5= 104.5° (Each lone pair of e- reduce bond angle by 2.5°)

Bonding between water molecules+ how it is formed?

-Presence of polarised OH bonds + oxygen lone pair= formation of hydrogen bonds between water molecules leading to extended H bonding network

-delta positive H interacts with delta negative O= oppositely charged dipoles attract each other.

Water molecules interaction with other molecules+is energy required?

-Disruption of hydrogen bonding between water molecules to solubilise another species requires energy.

Why?

-Energy is required to break the hydrogen bonds between water molecules.

-Energy lost in disruption of bonds between water= regained by forming new bonding interactions which can be H-bonding or dipole-dipole or dipole-ion interactions.

What molecules can make H bonds with water?

-Environments with free lone pairs or polarised X-H bonds (due to more electronegative atoms) can make hydrogen bonds with water.

-Polar environments= -OH groups, carbonyls, C-N, C-O,C-F and N-H (due to lone pair).

-Ions= interactions between water + ionised species are stronger hydrogen bonds.

Molecules that do not interact with water?

-Environments with low polarity or H-bonding capacity= do not interact with water e.g C-C, C-H regions + higher halogens.

-Molecules can contain both polar +non-polar regions.

Enthalpy diagram including delta H and delta H with explanation of dissolution of CaCl2

Arrow up (↑)= Endothermic reaction-energy is absorbed

Arrow down (↓)= Exothermic reaction- energy is released from system

Explanation of energy diagram of dissolution in terms of enthalpy and entropy

Left side (solute + water)

Arrow=↑

Enthalpy +Entropy:

Left side of energy diagram

1.Enthalpy of hydration (↑):

-Energy required to break H bonds between water molecules.

-Energy required to break lattice interaction (ionic bonds) in solid solute e.g CaCl2.

ΔH=+ve

2.Entropy:

-Energy required to reduce entropy of water molecules in hydration sphere.

Explanation of energy diagram of dissolution in terms of enthalpy and entropy

Right side (aqeuous solute species)

Arrow= ↓

3.Entropy:

-Energy released by increased entropy of ions e.g Ca2+ and Cl- ions becoming aqeuous=more disordered state.

ΔS=+ve

4.Enthalpy of dissolution(↓) :

-Energy released by formation of new bonds between water and solute.

ΔH=-ve

Gibbs free energy equation

ΔG= ΔH-TΔS

Explanation of energy diagram of the dissolultion of a solute in terms of enthalpy + favourability of reaction

-Exothermic enthalpy of dissolution=favourable in terms of water interaction-> overall more energy in dissolution of solute is released than absorbed to break H-bonds in water.

-Endothermic enthalpy of dissolution=unfavourable in terms of water interactions-> more energy absorbed to break H-bonds in water than released for dissolution of solute.

What do water molecules do to accomodate solute?

-H bonding between water in bulk allows water molecules to move freely in their environment.

-To accommodate the solute-> water forms hydration sphere of molecules =solvation shell.

-Molecules move less freely than in bulk state=reduction in entropy as shell makes them less disordered (rigid structure with limited movement)