1 Water molecules are polar.√ The electronegativity difference of each O-H bond is 3,5 – 2,1 = 1,4. √√
2 √√ The molecule has an angular shape √ causing the one end of the molecule to be δ- and the other end δ+. √ (5)
H2O(g), CO2(g) and other polar molecules trap heat, keeping the earth warm. Some ultraviolet
rays are reflected.
Atmosphere
3 Intermolecular forces in H2O are hydrogen bonds. √√ Intermolecular forces in H2S are dipole-dipole interactions. √√ Hydrogen forces are stronger than normal dipole- dipole forces. √ (5)
[10] PRE-KNOWLEDGE
A basic understanding of the following:
• The different physical properties of water related to strong hydrogen bonds. • Heat capacity
• The greenhouse effect.
2.2. Main Body (Lesson presentation) [30 min]
• Heat energy transferred by water and its effect on weather.
As has already being explained, hydrogen bonds are stronger than dipole-dipole forces. Although H2O(ℓ) does not have the regular structure of ice, hydrogen bonding still occurs. The extent of hydrogen bonding decreases as the temperature increases.
Disrupting hydrogen bonds requires a significant amount of energy and results in the high heat capacity of water.
As a result of the high heat capacity of water, it can absorb a great amount of energy before the temperature of the water rises. The high heat capacity of water is, in large part, why oceans and lakes have such an enormous effect on weather.
In autumn, when the temperature of the air is lower than the temperature of the ocean or lake, the water transfers energy as heat to the atmosphere, moderating the drop in air temperature.
For each degree drop in temperature a great amount of energy is available to be transferred and the temperature of the water decreases very slowly.
The temperature of oceans or lakes is normally higher than the average air temperature until late in autumn.
• The greenhouse effect of water.
Earth’s natural greenhouse effect makes life as we know it possible. Earth's gravity allows it to hold an atmosphere.
Water vapour (H2O(g)) and carbon dioxide (CO2(g)) in the atmosphere provide a temperature buffer (greenhouse effect) which helps maintain a relatively steady surface temperature.
The polarity of the covalent bonds in H2O-molecules and the strong hydrogen bonds between the molecules that vibrates at the same frequency as infrared light are responsible for the fact that water absorbs sunlight.
Water is the main absorber of the sunlight in the atmosphere and is responsible for about 70% of all atmospheric absorption of radiation, mainly in the infrared region where water shows strong absorption. It contributes significantly to the greenhouse effect ensuring a warm habitable planet.
The greenhouse gases, of which water accounts for most of the observed effect, allow Earth to warm by an average of 350C. These gases are vital and life on Earth would be impossible without them.
• The boiling point of water
We are used to the fact that water freezes at 0 0C and boils at 100 0C. Intermolecular forces increases as the molecular size increases.
Water is the smallest molecule of the hydrides in group VI, and the melting and boiling point of water is much higher than H2S, H2Se and H2Te. In fact, if we extrapolate the graph for boiling points, water would have a boiling point approximately -1000C. That is about 200 0C lower than the observed boiling point.
The hydrogen bonds between highly polar water molecules are stronger than normal dipole-dipole forces and more energy is needed to break these bonds. Energy needed for molecules to evaporate, is called heat of vaporization. 40,7 kJ∙mol-1 energy is needed for water molecules to evaporate – hence the higher than expected boiling point.
• The relationship between changing density of water aquatic life and temperature
When ice melts at 0 0C, the regular structure imposed on the solid state by hydrogen bonding breaks down, and a relative large increase in density occurs. Surprisingly, as the temperature of liquid water is raised from 0 0C to 4 0C, the density of the water increases. For almost all other substances known, density decreases as the temperature is raised.
The hydrogen bonding model also explains this observation.
At a temperature just above the melting point, some of the water molecules continue to cluster in ice-like arrangements and requires more space.
Only about 15% of the bonds are broken when the ice melts. The volume contracts as the ice structures disappear and the density increases more.
The density of water is a maximum at 4 0C. From this point the density declines with increasing temperature in the normal fashion.
Because of the way the density of water changes as the temperature approaches the freezing point, lakes do not freeze solidly from the bottom up in the winter.
As the lake water cools the density increases and the cooler water sinks while the warmer water rises.
Oxygen-rich water moves to the bottom of the lake to restore the oxygen used during the summer and nutrients are brought to the top layers of the lake. As the temperature decreases more, the colder water stays on the top of the lake, because water cooler than 4 0C is less dense than water at 4 0C. Ice can
begin to form on the surface, floating there and protecting the water below and aquatic life from further heat loss. Remember that ice is a poor conductor of heat.
Water freezes from the top down and protects the
LEARNER ACTIVITY[15 MINUTES]