High School Chemistry/Energy

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Just like matter, energy is something that we are all familiar with and use on a daily basis. Before you go on a long hike, you eat an energy bar; every month, the energy bill is paid; on TV, politicians argue about the energy crisis. But have you ever wondered what energy really is? When you plug a lamp into an electric socket, you see energy in the form of light, but when you plug a heating pad into that same socket, you only feel warmth. When you eat a bowl of spaghetti, the energy it provides helps you to function throughout the day, but when you eat five bowls of spaghetti, some of that energy is turned into body fat.

If you stop to think about it, energy is very complicated. Still, we use energy for every single thing that we do, from the moment we wake up to the moment we go to sleep and even during the night while we sleep our body is using energy to do things such as growing bones. Without energy, we couldn't turn on lights, we couldn't brush our teeth, we couldn't make our lunch, and we couldn't travel to school. Although we all use energy, very few of us understand how we use it.

Lesson Objectives[edit]

  • Define heat and work.
  • Distinguish between kinetic energy and potential energy.
  • State the law of conservation of matter and energy.

Ability to Do Work or Produce Heat[edit]

When we speak of using energy, we are really referring to transferring energy from one place to another. When you use energy to throw a ball, you transfer energy from your body to the ball, and this causes the ball to fly through the air. When you use energy to warm your house, you transfer energy from the furnace to the air in your home, and this causes the temperature in your house to rise. Although energy is used in many kinds of different situations, all of these uses rely on energy being transferred in one of two ways. Energy can be transferred as heat or as work. Unfortunately, both "heat" and "work" are common words, so you might think that you already know their meanings. In science, the words "heat" and "work" have very specific definitions that are different from what you might expect. Do not confuse the everyday meanings of the words "heat" and "work" with the scientific meanings.

When scientists speak of heat, they are referring to energy that is transferred from an object with a higher temperature to an object with a lower temperature as a result of the temperature difference. Heat will "flow" from the hot object to the cold object until both end up at the same temperature. When you cook with a metal pot, you witness energy being transferred in the form of heat. Initially, only the stove element is hot – the pot and the food inside the pot are cold. As a result, heat moves from the hot stove element to the cold pot. After a while, enough heat is transferred from the stove to the pot, raising the temperature of the pot and all of its contents (Figure 1.23).

Figure 1.23: Energy is transferred as heat from the hot stove element to the cooler pot until the pot and its contents become just as hot as the element. The energy that is transferred into the pot as heat is then used to cook the food.
Molecules transferring energy upon contact.

We've all observed heat moving from a hot object to a cold object, but you might wonder how the energy actually travels. Whenever an object is hot, the molecules within the object are shaking and vibrating vigorously. The hotter an object is, the more the molecules jiggle around. As you'll learn in the next section, anything that is moving has energy, and the more it's moving, the more energy it has. Hot objects have a lot of energy, and it's this energy that is transferred to the colder objects when the two come in contact. The easiest way to visualize heat transfer is to imagine a domino effect.

Heat is being transferred from a hot object to a colder object. In detail: a. As the red molecules in the hot object jiggle and vibrate, they hit some of the blue molecules in the colder object. This transfers energy from the hot molecules to the colder molecules, causing these molecules to vibrate faster. b. - d. Just like dominoes, heat passes along the chain until the energy is spread equally between all of the molecules.

Take a close look at the figure to the right. When the vibrating molecules of the hot object bump into the molecules of the colder object, they transfer some of their energy, causing the molecules in the colder object to start vibrating vigorously as well. As these molecules vibrate, they bump into their neighbors and transfer some of their energy on down the chain. In this way, energy passes through the whole system until all of the molecules have about the same amount, and the initial objects are at the same temperature.

Heat is only one way in which energy can be transferred. Energy can also be transferred as work. The scientific definition of work is force (any push or pull) applied over a distance. Whenever you push an object and cause it to move, you've done work, and you've transferred some of your energy to the object. At this point, it's important to warn you of a common misconception. Sometimes we think that the amount of work done can be measured by the amount of effort put in. This may be true in everyday life, but it isn't true in science. By definition, scientific work requires that force be applied over a distance. It doesn't matter how hard you push or how hard you pull. If you haven't moved the object, you haven't done any work.

So far, we've talked about the two ways in which energy can be transferred from one place, or object, to another. Energy can be transferred as heat, and energy can be transferred as work. But the question still remains – what IS energy? We'll try to at least partially tackle that question in the next section.

Types of Energy: Kinetic and Potential[edit]

Machines use energy, our bodies use energy, energy comes from the sun, energy comes from volcanoes, energy causes forest fires, and energy helps us to grow food. With all these seemingly different types of energy, it's hard to believe that there are really only two different forms of energy – kinetic energy and potential energy. Kinetic energy is energy associated with motion. When an object is moving, it has kinetic energy. When the object stops moving, it has no kinetic energy. While all moving objects have kinetic energy, not all moving objects have the same amount of kinetic energy. The amount of kinetic energy possessed by an object is determined by its mass, and its speed. The heavier an object is and the faster it is moving, the more kinetic energy it has.

Kinetic energy is very common, and it's easy to spot examples of it in the world around you. Sometimes we even try to capture kinetic energy and use it to power things like our home appliances. If you're from California, you might have driven through the Tehachapi Pass near Mojave or the Montezuma Hills in Solano County and seen the windmills lining the slopes of the mountains (Figure 1.24). These are two of the larger wind farms in North America. As wind rushes along the hills, the kinetic energy of the moving air particles turns the windmills, trapping the wind's kinetic energy so that people can use it in their houses and offices.

Figure 1.24: A wind farm in the Tehachapi Mountains of Southern California. Kinetic energy from the rushing air particles turns the windmills, allowing us to capture the wind's kinetic energy and use it.

Capturing kinetic energy can be very effective, but if you think carefully, you'll realize that there's a small problem. Kinetic energy is only available when something is moving. When the wind is blowing, we can use its kinetic energy, but when the wind stops blowing, there's no kinetic energy available. Imagine what it would be like trying to power your television set using the wind's kinetic energy. You could turn on the TV and watch your favorite program on a windy day, but every time the wind stopped blowing, your TV screen would flicker off because it would run out of energy. You’d probably only be able to watch about half of the episodes, and you’d never know what was going on!

Of course, when you turn on the TV, or flip on the lights, you can usually count on them having a constant supply of energy. This is largely because we don't rely on kinetic energy alone for power. Instead, we use energy in its other form – we use potential energy. Potential energy is stored energy. It's energy that remains available until we choose to use it. Think of a battery in a flashlight. If you leave a flashlight on, the battery will run out of energy within a couple of hours, and your flashlight will die. If, however, you only use the flashlight when you need it, and you turn it off when you don’t, the battery will last for days or even months. The battery contains a certain amount of energy, and it will power the flashlight for a certain amount of time, but because the battery stores potential energy, you can choose to use the energy all at once, or you can save it and only use a small amount at a time.

Any stored energy is potential energy. Unfortunately, there are a lot of different ways in which energy can be stored, and that can make potential energy very difficult to recognize. In general, an object has potential energy because of its position relative to another object. For example when you hold a rock above the earth, it has potential energy because of its position relative to the ground. You can tell that this is potential energy because the energy is stored for as long as you hold the rock in the air. Once you drop the rock, though, the stored energy is released.

There are other common examples of potential energy. A ball at the top of a hill stores potential energy until it is allowed to roll to the bottom. When you hold two magnets next to each other, they store potential energy too. For some examples of potential energy, though, it's harder to see how "position" is involved. In chemistry, we are often interested in what is called chemical potential energy. Chemical potential energy is energy stored in the atoms, molecules, and chemical bonds that make up matter. How does this depend on position?

Figure 1.25: Scientists use their knowledge of what the atoms and molecules look like and how they interact to determine the potential energy that can be stored in any particular chemical substance. This is a molecule of water.

As you learned earlier, the world, and all of the chemicals in it are made up of atoms and molecules. These store potential energy that is dependent on their positions relative to one another. Of course, you can't see atoms and molecules. Nevertheless, scientists do know a lot about the ways in which atoms and molecules interact, and this allows them to figure out how much potential energy is stored in a specific quantity (like a cup or a gallon) of a particular chemical (Figure 1.25). Different chemicals have different amounts of potential energy because they are made up of different atoms, and those atoms have different positions relative to one another.

Since different chemicals have different amounts of potential energy, scientists will sometimes say potential energy depends not only on position, but also on composition. Composition affects potential energy because it determines which molecules and atoms end up next to each other. For example, the total potential energy in a cup of pure water is different than the total potential energy in a cup of apple juice, because the cup of water and the cup of apple juice are composed of different amounts of different chemicals.

At this point, you might be wondering just how useful chemical potential energy is. If you want to release the potential energy stored in an object held above the ground, you just drop it. But how do you get potential energy out of chemicals? It's actually not that difficult. You use the fact that different chemicals have different amounts of potential energy. If you start with chemicals that have a lot of potential energy and allow them to react and form chemicals with less potential energy, all the extra energy that was in the chemicals at the beginning but not at the end is released.

Law of Conservation of Matter and Energy[edit]

So far we've talked about how energy exists as either kinetic energy or potential energy and how energy can be transferred as either heat or work. While it's important to understand the difference between kinetic energy and potential energy and the difference between heat and work, the truth is, energy is constantly changing. Kinetic energy is constantly being turned into potential energy, and potential energy is constantly being turned into kinetic energy. Likewise, energy that is transferred as work might later end up transferred as heat, while energy that is transferred as heat might later end up being used to do work.

Even though energy can change form, it must still follow one fundamental law – Energy cannot be created or destroyed, it can only be changed from one form to another. This law is known as the Law of Conservation of Energy. In a lot of ways, energy is like money. You can exchange quarters for dollar bills and dollar bills for quarters, but no matter how often you convert between the two, you won’t end up with any more or any less money than you started with. Similarly, you can transfer (or spend) money using cash, or transfer money using a credit card, but you still spend the same amount of money, and the store still makes the same amount of money.

As it turns out, the law of conservation of energy isn't exactly the whole truth. If you think back, you’ll remember that energy and matter are actually interchangeable. In other words, energy can be created (made out of matter) and destroyed (turned into matter). As a result, the law of conservation of energy has been changed into the Law of Conservation of Matter and Energy. This law states that the total amount of mass and energy in the universe is conserved (does not change).

This is one of the most important laws you will ever learn. Nevertheless, in chemistry we are rarely concerned with converting matter to energy or energy to matter. Instead, chemists deal primarily with converting one form of matter into another form of matter (through chemical reactions) and converting one form of energy into another form of energy.

Figure 1.26: A hot air balloon transfers energy in the form of heat from the flame to the air particles in the balloon. The design of the hot air balloon takes this energy and changes it from heat to work.

Let's take a look at several examples, where kinetic energy is switched to potential energy and vice versa. Remember Wile E. Coyote with his anvil poised at the top of the cliff? As long as Wile E. Coyote holds the anvil and waits for Road Runner, the anvil stores potential energy. However, when Wile E. Coyote drops the anvil, the original potential energy stored in the anvil is converted to kinetic energy. The further the anvil falls, the faster it falls, and more and more of the anvil’s potential energy is converted to kinetic energy.

The opposite, of course, happens when you throw a ball into the air. When the ball leaves your hand, it has a lot of kinetic energy, but as it moves higher and higher into the sky, the kinetic energy is converted to potential energy. Eventually, when all of the kinetic energy has been converted to potential energy, the ball stops moving entirely and hangs in the air for a moment. Then the ball starts to fall back down, and the potential energy is turned into kinetic energy again.

Just as kinetic energy and potential energy are interchangeable, work and heat are interchangeable too. Think of a hot-air balloon (Figure 1.26). To operate a hot-air balloon, a flame at the base of the balloon is used to transfer energy in the form of heat from the flame to the air molecules inside the balloon. The whole point of this heat transfer, though, is to capture the heat and turn it into work that causes the balloon to rise into the sky. The clever design of the hot-air balloon makes the conversion of heat to work possible.

Lesson Summary[edit]

  • Any time we use energy, we transfer energy from one object to another. Energy can be transferred in one of two ways – as heat, or as work.
  • Heat is the term given to energy that is transferred from a hot object to a cooler object due to the difference in their temperatures.
  • Work is the term given to energy that is transferred as a result of a force applied over a distance.
  • Energy comes in two fundamentally different forms – kinetic energy and potential energy.
  • Kinetic energy is the energy of motion.
  • Potential energy is stored energy that depends on the position of an object relative to another object.
  • Chemical potential energy is a special type of potential energy that depends on the positions of different atoms and molecules relative to one another. Chemical potential energy can also be thought of as depending on chemical composition.
  • Energy can be converted from one form to another.
  • The total amount of mass and energy in the universe is conserved.

Review Questions[edit]

  1. Classify each of the following as energy primarily transferred as heat or energy primarily transferred as work:
    (a) The energy transferred from your body to a shopping cart as you push the shopping cart down the aisle.
    (b) The energy transferred from a wave to your board when you go surfing.
    (c) The energy transferred from the flames to your hotdog when you cook your hotdog over a campfire.
  2. Decide whether each of the following statements is true or false:
    (a) When heat is transferred to an object, the object cools down.
    (b) Any time you raise the temperature of an object, you have done work.
    (c) Any time you move an object by applying force, you have done work.
    (d) Any time you apply force to an object, you have done work.
  3. Rank the following scenarios in order of increasing work:
    (a) You apply 100 N of force to a boulder and successfully move it by 2 m.
    (b) You apply 100 N of force to a boulder and successfully move it by 1 m.
    (c) You apply 200 N of force to a boulder and successfully move it by 2 m.
    (d) You apply 200 N of force to a boulder but cannot move the boulder.
  4. In science, a vacuum is defined as space that contains absolutely no matter (no molecules, no atoms, etc.) Can energy be transferred as heat through a vacuum? Why or why not?
  5. Classify each of the following energies as kinetic energy or potential energy:
    (a) The energy in a chocolate bar.
    (b) The energy of rushing water used to turn a turbine or a water wheel.
    (c) The energy of a skater gliding on the ice.
    (d) The energy in a stretched rubber band.
  6. Decide which of the following objects has more kinetic energy:
    (a) A 200 lb. man running at 6 mph or a 200 lb. man running at 3 mph.
    (b) A 200 lb. man running at 7 mph or a 150 lb. man running at 7 mph.
    (c) A 400 lb. man running at 5 mph or a 150 lb. man running at 3 mph.
  7. A car and a truck are traveling along the highway at the same speed.
    (a) If the car weighs 1500 kg and the truck weighs 2500 kg, which has more kinetic energy, the car or the truck?
    (b) Both the car and the truck convert the potential energy stored in gasoline into the kinetic energy of motion. Which do you think uses more gas to travel the same distance, the car or the truck?
  8. You mix two chemicals in a beaker and notice that as the chemicals react, the beaker becomes noticeably colder. Which chemicals have more chemical potential energy, those present at the start of the reaction or those present at the end of the reaction?

Vocabulary[edit]

chemical potential energy
Potential energy stored in the atoms, molecules, and bonds of matter.
force
Any push or pull.
heat
Energy that is transferred from one object to another object due to a difference in temperature. Heat naturally flows from a hot object to a cooler object.
kinetic energy
Energy associated with motion.
Law of Conservation of Energy
Energy cannot be created or destroyed; it can only be changed from one form to another.
Law of Conservation of Mass and Energy
The total amount of mass and energy in the universe is conserved.
potential energy
Stored energy. Potential energy depends on an object's position or mixture's composition.
work
A force applied over a distance.


Matter · Chemistry - A Physical Science