Molecular Biology: 2.2 Water Flashcards
Why are water molecules known to have polar covalent bonds?
Water molecules are formed from the covalent bonding between an oxygen atom and two hydrogen atoms. Oxygen is more electronegative compared to hydrogen atoms and so its nucleus is more attractive to electrons than the nuclei of hydrogen atoms (due to its high proton number than hydrogen). Therefore, there is an unequal sharing of electrons in water molecules, making it a polar covalent bond.
What is the result of the polarity of water molecules?
Each oxygen–hydrogen bond is a polar covalent bond, and results in a slight negative charge at the oxygen end of the molecule and a slight positive charge at the end with the two hydrogens. Because of the triangular shape
of a water molecule, the two ends of each molecule have opposite charges and therefore form oppositely charged poles.
What holds water molecules together?
Hydrogen bonding is a form of intermolecular force wherein a hydrogen in one polar molecule is attracted to a slightly negative atom of another polar covalent molecule.
What is the result of the intermolecular forces in water molecules?
This intermolecular bonding between water molecules gives water distinct properties not seen in other substances:
Thermal properties – Water can absorb much heat before changing state (requires breaking of hydrogen bonds)
Cohesive / adhesive properties – Water will ‘stick’ to other water molecules (cohesion) and charged substances (adhesion)
Solvent properties – Water dissolves polar and ionic substances (forms competing polar associations to draw materials apart)
What makes water cohesive and adhesive?
Water has the capacity to form intermolecular associations with molecules that share common properties
Because water is polar it will be attracted to other molecules that are polar or have an ionic charge
Outline the cohesive property of water
Cohesion is the ability of like molecules to stick together
Water is strongly cohesive (it will form hydrogen bonds)
Explain the significance of the cohesive property of water
The cohesive properties of water explain its surface tension
The hydrogen bonding between water molecules allows the liquid to resist low levels of external force (surface tension)
The high surface tension of water makes it sufficiently dense for certain smaller organisms to move along its surface
This property is also useful in transportation of water through xylem vessels, as water molecules are able to be delivered as a continuous column under low pressure, withstanding the suction forces that could separate water molecules and break the column if they were not cohesive in nature.
Outline the adhesive property of water
Adhesion is the ability of dissimilar molecules to stick together
Water will form intermolecular associations with polar and charged molecules
Explain the significance of the adhesive property of water
The adhesive properties of water explain its capillary action
Attraction to charged or polar surfaces (e.g. glass) allows water to flow in opposition of gravitational forces (capillary action)
This capillary action is necessary to allow water to be transported up plant stems via a transpiration stream
Outline the thermal property 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)
Water therefore as a high specific heat capacity, high latent heat of vaporisation, and high boiling point
How does water make a good evaporative coolant?
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
List the differences between methane and water
Water has a significantly higher melting (0ºC) and boiling point (100ºC) whereas methane has a melting point of -182ºC and boiling point of -160ºC
Water has a higher specific heat capacity (4.2 J per g per ºC) whereas methane= 2.2 J per g per ºC
Water has a higher heat of vaporisation (2257 J/g) whereas methane= 760J/g
Water as a higher heat of fusion
Why are there differences between methane and water?
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)
Outline the solvent properties of water
Water is commonly referred to as the universal solvent due to its capacity to dissolve a large number of substances
Water can dissolve any substance that contains charged particles (ions) or electronegative atoms (polarity)
This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken intramolecular forces (such as ionic bonds) and result in the dissociation of the atoms
The slightly charged regions of the water molecule surround atoms of opposing charge, forming dispersive hydration shells
How to identify hydrophilic substances?
Hydrophilic molecules typically contain functional groups that result in the molecules being polar. Carbohydrates are a good example of polar molecules; their relative solubility in water is attributed to their multiple hydroxyl (alcohol) functional groups.
