How Much Heat To Turn Water To Steam?

[mike2007]

How much heat must be added to 120g of water at an initial temperature of 60C to
heat it to the boiling point?
completely convert the 100C water to steam?

Specific heat capacity of water = 1.0cal/g-C
It would take 120*1 to raise 120g of water up to 1C
difference in temperature = 100C - 60C = 40C
It would then take 120*40 to raise it to the boiling point
120*40 = 4800cal

specific heat of steam is 0.48 cal/g-C but what it the temperature of steam?

 

[Astronuc]

Temperature of steam at 1 atm (0.101325 MPa, or 14.7 psia) is 100°C (212°F). Steam represents a phase change.

Steam however can be superheated, but that would be in a vessel with higher steam pressure, otherwise the steam expands and cools.


Remember, the problem increasing the water temperature, then transforming liquid water at 100°C to steam (water vapor) at 100°C.

See
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase2.html#c3
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/phase.html

 

[mike2007]

But what do i use for the calculation of water to steam. Doesnt the system have to have some amount of heat added to convert it to steam.

 

[Feynman diagram]

You need the latent heat of vapourization of water for the last part of the calculation, not the specific heat of steam ;)

 

[Astronuc]

Look at the first link I provided.

The energy required to change a gram of a liquid into the gaseous state at the boiling point is called the "heat of vaporization". This energy breaks down the intermolecular attractive forces, and also must provide the energy necessary to expand the gas.

Converting liquid water to steam at 100°C (0.101325 MPa) requires a heat addition of 539 cal/g (2260 J/gm).

 

[mike2007]

The latent heat for boiling is 540 cal/g. That means that 1g requires 540 cal of heat to steam

Thus, 120g requires 120 x 540 = 64800 cal to steam

 

[Astronuc]

Correct.

And then add the two heat requirements.