Water and Energy. What can thermal energy do? Change a substances temperature Change a substances state of matter Any amount thermal energy absorbed by.

Water and Energy

What can thermal energy do?

• Change a substances temperature

• Change a substances state of matter

Any amount thermal energy absorbed by a substance can be used to do either or, but not both

Whenever the water is absorbing the thermal energy from the sun some of it is used to change the temperature of snow, ice, water and vapour

Some of it is used to evaporate or melt some of the water

Consider this

Water has a relatively large heat capacity

•It takes a lot of energy to raise the temperature of water

•This means that unlike other substances water can absorb lots of the sun’s energy and it will not change its temperature dramatically

•Waters high heat capacity also means that it looses energy relatively slow

•So as the sun goes down and night falls the water will still remain a relatively constant temperature

What if water had a low heat capacity?

If water had a low heat capacity

•Only a little bit of thermal energy would be needed to raise the temperature

•So all day long as the Sun’s energy was absorb by the water the temperature would raise dramatically

•This would also mean that during the night water would loose it’s heat very fast and the temperature would drop quickly!!!

What can we take from this

• Water’s high heat capacity allows it to moderate temperature despite the huge fluctuations of thermal energy that it receives during the day and night.

• Without it waters temperature would change drastically throughout the course of a day

The Specific Heat Capacity

• How much energy it takes to raise the temperature for 1.0 g of a substance by 1° C is called the specific heat capacity

• We can find out how much energy is needed to change an objects temperature with this equation:

Q = mcΔT

Effects of Mass on Heat

• Large object

• = mcΔT

•Q is bigger

• Smaller object

• = mcΔT

Q is smaller

Conclusion: bigger objects need more energy to raise their temperature

Effect of SHC on Heat

• Large Specific Heat

• = mcΔT

•Q is bigger

• Smaller Specific Heat

• = mcΔT

Q is smaller

Conclusion: bigger SHC need more energy to raise their temperature by the same amount

4.18 J/g·°C

0.32 J/g·°C

Effect of Temp Change on Heat

• Large Temp Change

• = mcΔT•Q is bigger

• Smaller Temp change

• = mcΔT

Q is smaller

Conclusion: bigger SHC need more energy to raise their temperature by the same amount

Energy that doesn’t raise temperature

• Not all of the energy that is absorbed by water is used to raise it’s temperature.

• Some of the energy is used to evaporate the water and is not used to increase the temperature

• The amount of energy it takes to convert 1.0 g of water to a gaseous state is called the heat of vaporization

• We can find how much energy it will take to evaporate liquid water with this equation

Q = mΔH°vap

Notice there is no symbol for temperature because none of the energy required (Q) is being used to change the temperature during a state change

Energy that doesn’t raise temperature con’t

• The same can be said for changing solid water into liquid water (fusion, aka: melting)

• We call the energy required to convert 1.0 g of solid water to the liquid state the heat of fusion

• We can find how much energy it will take to evaporate liquid water with this equation

Q = mΔH°fus

Notice there is no symbol for temperature because none of the energy required (Q) is being used to change the temperature during a state change

Compared to other substances

• Water’s heat of vaporization and heat of fusion are relatively larger than other substances.

• This means it will need to absorb lots of energy to melt or evaporate water – energy that will not be used to change water’s temperature

Heating Curve