Heat of Fusion & Heat of Vaporization: Definitions & Equations

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  • 0:03 Heat of Fusion and Equation
  • 4:20 Heat of Vaporization…
  • 6:41 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

When a substance changes states, like from a solid to a liquid, a lot is happening. This lesson will examine the heat of fusion and the heat of vaporization. It will also explain both formulas.

Heat of Fusion and Equation

It's Saturday night and you are looking forward to a fun and exciting evening with your friends. What wild and crazy events do you have planned? Maybe pizza? A movie? Some root beer floats? Nope, nope and nope.

Well then, what's this 'fun and exciting' evening? You and your friends are about to have a chemistry party. So, grab your test tubes, goggles and lab coats and prepare to have a wild and crazy chemistry-filled night.

You'll start the evening by gathering some ice cubes and placing them on the kitchen table. Sure, watching ice melt doesn't sound particularly exciting, but there's actually a lot going on with the molecules inside that ice cube.

Before we talk about the molecules, let's go over some vocabulary.

  • Melting or fusion is when a solid turns into a liquid.
  • Freezing or solidification is when a liquid turns into a solid.

I'm sure you've heard the words 'melting' and 'freezing' before, but fusion and solidification may be new to you.

Okay, back to your ice cube. Heat energy from the table causes the molecules in the ice cube to start moving faster. They also get farther and farther apart.

In order to change a solid to a liquid, there is a certain amount of energy required. This is called the heat of fusion. Remember, fusion means melting, so you can see where it gets its name. Anyway, the heat of fusion is the amount of energy required to change a substance from a solid to a liquid at its melting point.

It's worth noting, it takes energy for something to melt, but energy is given off when something freezes. Also worth noting, there is a heat of solidification, which is the energy released when a liquid turns into a solid at its freezing point. Remember, solidification means turning a liquid into a solid, so you can see how heat of solidification gets its name. But, for now, the focus of your chemistry party is the heat of fusion and that melting ice cube, so back to that!

The heat of fusion can be measured in joules per gram, or J/g, or calories per gram, or cal/g. To keep things simple, we'll just use cal/g in this lesson.

Both joules and calories are measurements of energy. You may be familiar with Calories (with a capital C), which is also known as a kilocalorie or the calorie you see on a food label. Just so you can relate to what a calorie is, there are 1,000 calories in 1 food calorie (or kilocalorie).

So, your ice cube melted and now you have a puddle of water on your kitchen table. You might be thinking your chemistry party is coming to an end, but don't worry! Things are about to get even more exciting. There is a formula you can use that involves the heat of fusion!

And here it is:

the heat of fusion = heat energy / mass

Where:

  • heat of fusion is H subscript f
  • mass is m
  • and heat energy is q

Since it is a chemistry party, I think we should try out this formula! Let's try to figure out how much heat energy was required to turn that ice cube into a liquid puddle at its melting point! In other words, we will use the formula to solve for q.

  1. We need to determine the mass of the ice cube in grams. Don't worry if you don't have a scale available. I'll help you out. Your ice cube's mass is 4.0 grams, so m = 4.0 grams.
  2. Figure out the heat of fusion for water. There are tables available on the Internet and in some chemistry textbooks. Each substance has its own heat of fusion, and water's heat of fusion is 79.7 cal/g.
  3. Plug these into the equation and solve for q.

Multiply by 4.0 g on both sides so you can get q by itself.

Your answer is 318.8 calories. So, it takes 318.8 calories to turn a 4.0 g ice cube into a liquid, which is less than 1 food calorie! Remember, there are 1,000 calories with a little 'c' in 1 of the Calories with a big 'C' that we're familiar with from food labels.

Heat of Vaporization and Equation

Just when you thought your ice melting party couldn't get any more exciting, let's boil some water! But let's get a couple of new vocabulary words out of the way first.

  • Boiling or vaporization is when a liquid is changed into a gas.
  • Condensation is when a gas is changed into a liquid.

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