Thermodynamics:
Heat and Enthalpy
Home
Table of Contents
Thermodynamics Gateway Page
In this module:
Introduction
Heat and Temperature
Heat Capacity
Heat of Fusion
Heat of Vaporization
Enthalpy

A similar process occurs when a liquid changes to a gas. In this case, the constant that is used is the heat of vaporization :

heat of vaporization = q / m

which also has units of J/g.

If a process involves changing temperature and changing phases, the heats required for each process must be added together to get the total heat required.

What will be the final temperature if 2.70 kJ of heat are added to 1.00 g of liquid water that is initially at 25 ºC?

Step 1: Define the system and surroundings. Write your answer in the space below, then click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

Because the water is changing the most, it is the best choice for the system.

Step 2: Identify and assign signs to all the kinds of energy and work that enter or leave the system. Write your answer in the space below, then click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

Energy is entering the system in the form of heat. It has a positive sign. There is no other energy or work entering or leaving the system.

Step 3: Predict the units your answer should have. Write your answer in the space below, then click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

The answer is a final temperature, so the answer should have units of degrees Celsius.

Step 4: Predict the approximate size of your answer. Write your answer in the space below, then click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

Since the amount of water is so small and the amount of energy so large, the water will probably wind up in the gaseous state. Thus the final temperature will probably be larger than 100 ºC.

What is the mass of the system? Enter the mass in the space below and click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

The mass is given as 1.00 g.

How much energy is required to raise the temperature of the system to 100 ºC? Write your answer in the space below and click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

313.8 J of heat are required to heat the water to 100 ºC.

How much energy is required to completely vaporize the water? The heat of vaporization of water at 100 ºC is 2260 J/g. Write your answer in the space below and click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

The energy required is 2260 J.

How much energy is left over? Write your answer in the space below and click on the check button.

Now compare your answer with the one below.
Please enter your answer in the space at left.

126.2 J of energy are left over.

To what final temperature will this much energy raise the water vapor? The heat capacity of water vapor is 1.87 J/gºC. Write your answer in the space below and click on the review answers button.


Correct! The final temperature will be 167 ºC.
While numerically correct, your answer has the wrong number of significant digits. Try again.
The final temperature will be 167 ºC.
That is incorrect. Please try again.

Heat of Vaporization