Thermodynamics:
Heat and Enthalpy
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In this module:
Introduction
Heat and Temperature
Heat Capacity
Heat of Fusion
Heat of Vaporization
Enthalpy

What happens when a phase change (e.g.liquid to solid, solid to liquid, liquid to gas) is involved? For example, it obviously takes heat to melt an ice cube, but if you measure the temperature, you will see that it stays at 0 ºC through the whole process.

Scientists deal with this by defining a new constant called the heat of fusion :

heat of fusion = q / m

The heat of fusion has units of J/g. Different substances have different heats of fusion. The heat of fusion of water at 0 ºC is 333 J/g, for example.

How much heat is required to melt 50.0 g of ice cubes at 0 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 ice 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. The question asks for an amount of heat, so the answer should be an amount of energy and have units of Joules.

Step 4: Predict the approximate size of your answer. The heat of fusion of water is 333 J/g at 0 ºC. Therefore, the answer should be about 300 x 50 = 15,000 J.

What is the mass of the substance being heated? 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 50.0 g.

How much heat is required to melt the ice cubes? Substitute the values of mass and heat of fusion into the equation above. Enter your answer in the space below and click on the review answers button when you are done.

Correct! 16,700 J of heat are required to melt the ice cubes.
Remember to include the units with your answer.
While numerically correct, your answer has the wrong number of significant digits. Try again.
16,700 J of heat are required to melt the ice cubes.
That is incorrect. Please try again.

Heat of Fusion