Chemistry

1
Chemistry

• Calculating Heat

2
Transfer of Energy into a Substance

• Solid water (ice) is heated at a steady rate
  until some has boiled away

3
Transfer of Energy into a Substance

• At the plateaus, the particles rearrange as the
  substance changes phase. Energy is stored as Ei.

4
Transfer of Energy into a Substance

• On the inclines, particles are moving faster as
  the temperature of the substances increases.
  Energy is stored as Eth.

5
Transferring Energy Out of a Substance

• Boiling water is cooled at a steady rate until it
  freezes.

6
Transferring Energy Out of a Substance

• At the plateaus, the particles are rearranging as
  the Ei decreases and it changes phase

7
Transferring Energy Out of a Substance

• On the declines, the particles are slowing down
  as the Eth of the substance decreases.

8
Heat Capacity (c)

• The amount of energy required to raise the
  temperature of one gram of a substance one
  degree Celsius.
• 4.18J/gC or 1 calorie for liquid water
• Thus it takes 4.18Joules of energy to raise one
  gram of liquid water one degree Celsius.

9
Which Requires More Energy?

• Raise 10 grams of liquid water one degree
  Celsius?
• or
• Raise one gram of liquid water 100 degrees
  Celsius?

10
Calculating Heat required to change temperature
(Eth)

• The energy required to change the temperature of
  a substance is equal the
• mass of the substance X the heat capacity of the
  substance X the change in Temperature.

11
Determining Change in Temperature (?T)

• Change in temperature is the difference from
  beginning temperature to final temperature
• Change in temperature will be positive if energy
  is being added and negative if energy is being
  lost

12
Energy Transfer during a Phase Change

• We cant use the earlier equation because there
  is no temperature change during a phase change.
• This new equation is based on the amount of
  energy required to rearrange the particles during
  a phase change.

13
Heat of Fusion (?Hf)

• The amount of energy required to rearrange the
  particles of one gram of solid to one gram of
  liquid (or one gram of liquid to one gram of
  solid)
• Heat of fusion for water is 334J/g.

14
Heat of Fusion (?Hf)

• Heat of fusion can be positive or negative
  depending on the direction of the energy transfer

-Hf
Q
Substance
Substance
Q
Hf
15
Heat of Vaporization (?Hv)

• The amount of energy required to change one gram
  of liquid into one gram of gas (or one gram of
  gas into one gram of liquid)
• The heat of vaporization of water is 2260J/g

16
Heat of Vaporization (?Hv)

• Heat of vaporization can be positive or negative
  depending on the direction of energy transfer.

-Hv
Q
Substance
Substance
Q
Hv
17
Calculating Heat during a Phase Change

• The heat required for a phase change is equal to
  the mass of the substance X the heat of fusion /
  heat of vaporization of the substance.

18
Practice Problems

• A cup of coffee (140 g) cools from 75C down to
  comfortable room temperature 20C. How much
  energy does it release to the surroundings?

Eth
Ei
Eth
Ei
Cup of Coffee
19
Practice Problems

• Suppose during basketball practice, you lost 2.0
  lbs of water due to sweating. If all of this
  water evaporated, how much energy did the water
  absorb from your body? (1kg 2.2lbs)

Eth
Ei
Sweat
Eth
Ei