Vaporization Heat and the Heat Capacity of H20 comparison

In summary: The amount of energy needed to heat the vapor can be calculated using the heat capacity of water vapor. In summary, according to the Vaporization Heat table, 3.7KJ is the energy needed to heat 1 mol of H2O from 25°C to 100°C, but this is not enough to reach 100°C starting from 25°C. To fully convert water vapor at 25 °C to water vapor at 100 °C, you also need to compress the vapor.
  • #1
StarChem
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According to the Vaporization Heat table, the heat needed for 1 mol of H2O to evaporate at 100°C is 40.7KJ and 44.0KJ/mol is needed to evaporate H2O at 25°C. Thus 44.0-40.7=3.7KJ is the energy needed to heat H2O to 100°C from 25°C. However, according to the heat capacity of H2O, 3.7KJ will only warm the water by ~+48.6°C, which is not enough to reach 100°C starting from 25°C!

HEAT CAPACITY TABLE.jpg
HEAT OF VAPORIZATION.jpg
 
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  • #2
StarChem said:
Thus 44.0-40.7=3.7KJ is the energy needed to heat H2O to 100°C from 25°C

Nope, doesn't follow. To convert liquid water at 25°C to vapor at 100°C you have to either heat water and evaporate it, or evaporate water and heat the vapor - plus you will need to do work compressing the vapor, as saturated vapor pressure at 25°C is much lower than 1 atm that you will get at 100°C.
 
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  • #3
Thanks Borek for your reply.
I'm still confused though; how water can be evaporated at 25°C then heated, Is it by lowering the pressure? And what the compression work is needed for?
 
  • #4
StarChem said:
how water can be evaporated at 25°C then heated
Water evaporates at all temperatures, till the vapor gets saturated. Then you heat up the vapor - not much different from heating up air.

To convert water vapor saturated at 25 °C (about 3.17 kPa) to water vapor saturated at 100 °C (101 kPa) you need to both heat the vapor up and compress it.
 
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Related to Vaporization Heat and the Heat Capacity of H20 comparison

1. What is vaporization heat and how is it related to the heat capacity of water?

Vaporization heat is the amount of heat required to convert a liquid into a gas at a constant temperature. It is directly related to the heat capacity of water, which is the amount of heat required to raise the temperature of water by one degree Celsius. This is because both vaporization and temperature change require the input or release of heat energy.

2. How does the heat capacity of water compare to other substances?

The heat capacity of water is relatively high compared to other substances. This is due to its molecular structure, which allows it to absorb and retain a large amount of heat energy. In fact, water has one of the highest heat capacities of all commonly found substances, making it an important component in regulating the Earth's temperature.

3. Why is the heat capacity of water important?

The heat capacity of water is important for several reasons. Firstly, it helps to regulate the Earth's temperature by absorbing and releasing heat energy, which is crucial for maintaining a stable climate. Additionally, the high heat capacity of water makes it an effective coolant, allowing it to absorb and dissipate heat from other substances or systems.

4. How does vaporization heat affect the Earth's water cycle?

Vaporization heat plays a crucial role in the Earth's water cycle. When water is heated and vaporized, it rises into the atmosphere and eventually condenses into clouds. This process releases heat energy, which helps to drive weather patterns and ocean currents. When the water vapor condenses and falls as precipitation, it releases even more heat energy, which helps to regulate the Earth's temperature.

5. Can the heat capacity of water change?

The heat capacity of water is a physical property that remains relatively constant under normal conditions. However, it can be affected by certain factors such as pressure, temperature, and impurities. For example, as water freezes, its heat capacity decreases due to the formation of ice crystals. Additionally, the addition of impurities such as salt can also alter the heat capacity of water.

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