Phase change diagram and computing how much energy it takes
to change liquid water at its boiling point to a vapor.
The diagram shows the uptake of heat by 1 kg of water, as it passes from
ice at -50 ºC to steam at temperatures above 100 ºC, affects the
temperature of the sample.
E: Steam absorbs heat and thus increases its temperature.
D: Water boils and absorbs latent heat of vaporization.
C: Rise in temperature as liquid water absorbs heat.
B: Absorption of latent heat of fusion.
A: Rise in temperature as ice absorbs heat.
from- http://www.physchem.co.za/Heat/Latent.htm
Heat of Vaporization-the amount of heat required to convert a given mass
of a liquid into the vapor without a change in temperature.
For water at its boiling point of 100 ºC, the heat of vaporization is
2260 J g-1.
Note that the units J g-1 means joules PER gram since the negative
exponent means to put the “g” in the denominator (clever eh!). So as to
confuse you more I provide the formula without the negative exponent.
Your textbook uses calories in this computation but calories are NOT SI
units, so to be more “scientific” I use joules with the symbol J…upper
case J. There are 4.18 calories per joule. The heat of vaporization in
calories is 540 calories…that is what your book uses in chapter 23, page
347. To convert calories to joules you multiply the calories you have by
4.18 and you have the heat of vaporization in joules…
540 x 4.18 = 2260 J
Heat of Vaporization of Water Hv = 2260 J /g
The heat of vaporization is how much energy in joules it takes to make
water at its boiling point change to a vapor by breaking many of the
hydrogen bonds that keep the water in its liquid phase.
q= m Hv
q is Heat
m is the mass of the water in grams
Hv is the heat of vaporization for water 2260 J
This means that to convert 1 g of water at 100 ºC to 1 g of steam at 100
ºC, 2260 J of heat must be absorbed by the water. Conversely, when 1 g of
steam at 100 ºC condenses to give 1 g of water at 100 ºC, 2260 J of heat
will be released to the surroundings.
1. How much energy is required to vaporize 10.g of water at its boiling
point?