THC_Energy Lesson
Energy
Have you noticed that there is something very important and fundamental that we have yet to talk about? Energy!
Energy may seem like an easy concept. We talk about it all the time -the energy crisis, sources of energy, the energy we get from food... But what is energy? It isn't something you can hold or touch. The very idea of energy took more than 100 years for scientists to agree upon. The agreed upon definition is "an object's ability to do work and supply heat." Hopefully you are now asking yourself, "What is work, and what is heat?" Those are great questions. We will get to them! First, let's define the two different ways an object can possess energy:
- Kinetic Energy (KE) - the energy of motion
- Potential Energy (PE) - stored energy
All forms of energy are measured in the SI unit, Joules (J). Because a joule is such a small measure of kinetic energy, it is often times more convenient to express energy in kilojoules. In the United States, energy is often measured in calories. A calorie, which is different from the food Calorie, is defined as the amount of energy needed to raise the temperature of 1 g of water by 1°C. Like a Joule, a calorie is a small unit that is often more useful to express in terms of kilocalories. When you see Calorie (notice the capitol C), in reference to food, that unit is actually kilocalorie. The food industry chose to abbreviate this as Cal. So, be very careful to notice calorie versus Calorie (which is really kcal). And since we use metric units in science, you will need to be able to convert between calorie and Joule. This relationship is 1 cal = 4.184 J.
The equations listed above for kinetic and potential energy are certainly useful when you are studying a roller coaster, for example. You can use them to determine the potential energy of the roller coaster at a certain height or the kinetic energy at a certain point during the ride. But, they aren't very useful from the perspective of chemistry.
On the scale of the roller coaster (the macro level), we might look at the roller coaster sitting still at the end of its ride and say that its kinetic energy is zero. While this is true on this macro level, it is not true on a molecular level. On a molecular level, the atoms, molecules, or ions in any object are always moving (as long as the temperature is above absolute zero). This means that all objects have some KE, even though they appear to be still on a macro level. So, on a molecular level, instead of looking at the kinetic and potential energy, we look at thermal and chemical energy.
Systems and Surroundings
Before we discuss thermal and chemical energy, we must first define some terms that are necessary to give us the proper perspective to discuss energy. We will use the terms system, surroundings, and sometimes boundary. The system refers to the specific thing we are studying. A system can be as large as an entire ecosystem or as small as the contents of a beaker. Everything outside of this system is called the surroundings. The division between the system and surroundings is the boundary. The boundary can be physical, like the walls of the beaker or it can be invisible, like the division between a warm and cold weather front.
A system can be open, closed, or isolated. Both open and closed systems allow energy exchanges to take place between the system and the surroundings. The difference between an open and closed system is that open systems allow not only energy to cross the boundary, but also allow matter to move across the boundary. An example of this is cooking a pot of soup. Here you can add spices and other ingredients. A closed system allows energy to cross, but does not allow matter to cross the boundary. An example of this is a pot of soup with the lid on. You can continue to supply energy to the pot from the stove, but matter cannot cross the boundary. In an isolated system, neither matter nor energy can cross the boundary. A thermos is a good example of an isolated system.
Fill in the chart below to see if you understood the different types of systems.
Molecular Kinetic Energy
Molecular KE is thermal energy. Thermal energy is the sum total of all of the microscopic randomized kinetic energy. While thermal energy cannot directly be measured, it can be observed by measuring temperature. The temperature of a body is direct measure of the quantity of thermal energy it contains. Specifically, the Kelvin temperature of a sample of matter is proportional to the average kinetic energy of the sample. But, know that temperature and kinetic energy are not the same thing. Although they are often used interchangeably, in a scientific setting, they are quite different. The particles in chemical systems are continually undergoing random motion. The temperature of a system is a direct measure of the average kinetic energy associated with this random motion.
The diagram below shows this process of energy transfer from a warmer to a cooler object. Let's imagine this in context of a situation that may be familiar to you. You are packing your lunch for school. You want to keep your sandwich cold, so you put it in a container with a reusable ice pack from the freezer.
a) Here, we see the ice pack and the sandwich before they are put together in the container. The sandwich is warmer than the ice pack. This is shown by the length of the arrow and the red color. The longer arrows on the sandwich side denote higher KE and higher temperature. The shorter arrows on the ice pack side denote lower KE and lower temperature.
b) Now the ice pack and sandwich are placed together inside the container. Remember that thermal energy always moves from high to low. So, some of the energy from the sandwich move to the ice pack side because the molecules are slowed down by collisions with molecules in the ice pack. Notice that the arrows representing the KE of the sandwich are smaller next tto the ice pack, and the arrows representing the KE of the ice pack are larger in the center.
c) The change in temperature will continue to spread through the sandwich and through the ice pack. After some time the kinetic energy of the sandwich and ice pack become equal. This shows that thermal equilibrium has been reached. What do you notice about the arrows on both sides and what does this mean?
Answer: The arrows are the same size indicating that they have the same kinetic energy and same temperature.
Molecular Potential Energy
The potential energy stored in molecules and compounds is called chemical energy. Potential energy is sometimes referred to as the energy of position. The easiest way to understand this is to first look at some "macro" examples of potential energy.
Think about pulling back a rubber band. The stretched rubber band has more potential energy than in its natural position. Or, when the North poles of two magnets are pushed together, their PE increases.
In both of these situations, the potential energy was determined by changes in position. Work was done on the rubber band to stretch it. Work was done on the repelling magnets to push them together. The important relationship to remember here is that, work done ON an object increases its PE. This can be tricky, so pay close attention to work done on versus work done by and object.
Let's stop for a minute and really focus on the word work. Just like the words heat and temperature, work is used commonly in our everyday lives. But in science, work refers to something very specific. Work is force applied multiplied by the distance the object moves.
W=FΔd
In the image below (from UC Davis Chem Wiki), the black arrow is doing work on the red box. This work is equal to the force applied by the black arrow multiplied by the distance the red box is moved (shown by the gray arrow).
In chemistry, we focus on work that is associated with changes in volume of a gas.
Look at the diagram to the right. This shows a piston, which is a cylinder with an adjustable top (like a syringe used in medicine). In the first picture, there is just one weight applying force on the piston.
The second picture shows the same cylinder, but with more weights added, therefore more force applied. This force causes the piston to move a certain distance, compressing the gas to a smaller volume.
This shows that work was done on the piston. Always focus on the system, not the surroundings, when you make your statements about work done on or by.
In a compression, like shown above, the volume of the gas decreases because work is done on the gas. Recall from above that when work is done on a system, the PE of the system increases . In an expansion, the volume of the gas increases. In this case, work would be done by the gas causing the PE of the system to decrease .
The work equation, W = F ∆ d , is not very useful when working with gases. You will recall that the quantities we use to describe gases include volume, pressure, temperature, and number of moles. Let's rewrite the work equation using some of these quantities that apply to gases.
Remember to work on the module practice problems as you complete each section of content.
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