Understanding:
• Hydrogen bonding and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water
Water has the capacity to absorb significant amounts of heat before changing state
- This is due to the extensive hydrogen bonding between water molecules – the H-bonds need to be broken before a change in state can occur and this requires the absorption of energy (heat)
Consequently, water is an excellent medium for living organisms as it is relatively slow to change temperature and thus supports the maintenance of constant conditions (internal and external)
Understanding:
• Comparison of the thermal properties of water with those of methane
Methane (CH4) provides a good basis for comparison with water due to the many similarities between their structures:
- Comparable size and weight (H2O = 18 dalton ; CH4 = 16 dalton)
- Comparable valence structures (both have tetrahedral orbital formations, but water is bent due to unbonded electron pairs)
Comparison of Water and Methane Molecules
Differences between Water and Methane
The differences in thermal properties between water and methane arise from differences in polarity between the molecules:
- Water is polar and can form intermolecular hydrogen bonds (due to high electronegativity of oxygen atom)
- Methane is non-polar and can only form weak dispersion forces between its molecules (carbon has a lower electronegativity)
This means water absorbs more heat before changing state (each H-bond has an average energy of 20 kJ/mol)
- Water has a significantly higher melting and boiling point
- Water has a higher specific heat capacity (energy required to raise the temperature of 1 g of substance by 1ºC)
- Water has a higher heat of vaporisation (energy absorbed per gram as it changes from a liquid to a gas / vapour)
- Water as a higher heat of fusion (energy required to be lost to change 1 g of liquid to 1 g of solid at 0ºC)
Table of Key Thermal Properties (Water versus Methane)
Application:
• Use of water as a coolant in sweat
The evaporation of water as sweat is a fundamental mechanism employed by humans as a means of cooling down
- The change of water from liquid to vapour (evaporation) requires an input of energy
- This energy comes from the surface of the skin when it is hot, therefore when the sweat evaporates the skin is cooled
- Because water has a high specific heat capacity, it absorbs a lot of thermal energy before it evaporates
- Thus water functions as a highly effective coolant, making it the principal component of sweat