Changes in Energy

For Chemical and Physical Changes

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Heat vs Thermal Energy

These two are not the same thing! Thermal energy is, well, energy, and matter contains thermal energy but not heat. Heat is thermal energy in transit. This transfer of thermal energy only happens in one direction: from a higher temperature substance to a lower energy substance and NEVER in the opposite direction. This is the 2nd law of thermodynamics.

Temperature Scales and Conversions

There are three temperature scales that are currently in use. They all differ by the number of degrees between water freezing and boiling. The one that is most familiar to you is the Fahrenheit scale. This scale has water freezing at 32°F and boiling at 212 °F. The Celsius (same as Centigrade) is a "metric based scale" with water freezing and boiling at 0°C and 100°C respectively. The temperature scale used by many scientists is the Kelvin scale which is a more accurate measure of particle motion which is what temperature really is. Of special interest is the temperature at which all particle motion (kinetic energy) is supposed to stop. The temperature at which KE is the lowest it can be is absolute zero or 0 K. To convert between °C and °F are found below:

°C = 5/9(°F-32) or °F = 9/5°C +32

Thermal Energy and Temperature

The difference here is one of types of motion. Even though the picture here says heat (scientists can be sloppy about using the terms heat and thermal energy interchangeably). Thermal energy (internal energy) represents all types of motion (vibrational, rotational, and translation) articles can have as well as attractions and repulsions between the particles (the latter two represent potential energy) whereas temperature is just translational energy or kinetic energy. Something at a lower temperature can, in fact, have more thermal energy than something at a higher temperature...remember our demo!
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Thermochemistry-the balance between KE and PE

There are two basic forms of energy: kinetic energy (energy of motion) and potential energy (energy of position). Whenever energy is transformed (e.g., electrical from chemical (fuel)); you are turning potential into kinetic or vice versa. Potential energy is locked up in chemical bonds and can be released when those bonds are broken. When a molecule forms, potential energy drops (think ball on an incline). This is really the only reason molecules do form-the system rests at a lower energy. When chemical reactions occur, energy is put into breaking the bonds (ball has to be pushed back up the hill). Once the bonds break, the ball rolls down the hill again. There is a balance between this constant switching from potential to kinetic and that is what thermochemistry is all about.

Enthalpies: ∆H

Heats of reactions or enthalpies can also be calculated from other kinds of data (besides calorimetry). When reactants react to form products, bonds are broken and then reformed into new arrangements. It takes energy to break bonds (endothermic) and energy is released when bonds form (exothermic). This endothermic and exothermic business occurs in every chemical reaction. The balance between the two dictates whether a chemical reaction is overall exothermic (bond forming step is bigger) or endothermic (bond breaking step is bigger). If a reaction is endothermic, then enthalpy can be thought of as entering the reaction on the reactant side and if a reaction is exothermic, heat can be thought of as exiting the reaction on the product side.

C (s) + O2 (g) → CO2 (g) + 393.5 kJ

H2O (l) + 285.8 kJ → H2 (g) + 1/2 O2 (g)

Which is endothermic above? Exothermic?

Take the enthalpy out of the equations above and it could be written separately from the reaction. If it is, the sign of the enthalpy value tells you whether the reaction is endothermic or exothermic. If ∆Hrxn is negative, the reaction is exothermic. If ∆Hrxn is positive, the reaction is endothermic.

Calculating the amount of Heat Transfer-Calorimetry

As you will see in our activities, the amount of thermal energy (Q) transferred, aka heat, depends on three things: amount of substance (mass), the substance itself (specific heat), and the temperature of the substance before and after the transfer of heat (∆T). This equation takes all three into account: Q = mc∆T
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Chemistry 10.5 Heat of Reaction

Heats of formation

There are special chemical reactions that essentially define the energy change when making compounds from elements. These equations are formation equations and they can be used to calculate the energy for any chemical reaction using the equation below. For these equations, the energy change is essentially the energy "locked up" in the formation of a compound.
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Entropy-a loss of energy!

Thermal energy is the least useful type of energy in terms of being able to do work. Every time energy is transferred, the amount of energy available to do work decreases. This dispersal of energy into "non useful" energy is entropy. It can also be defined as amount of "randomness". Entropy (S) can be expressed mathematically as ∆S = ∆Q/T. It takes energy to decrease entropy. Think about how easy it is for your room to become messy but how hard it is to reverse it! Entropy encompasses two out of the three laws of thermodynamics.

Energy Changes for Physical Changes like Phase Changes

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Water is a different substance from what is predicted by the Kinetic Theory!

Water expands when it freezes-most substances do not!

The differences between solids, liquids, and gases is that the volume each phase increases in the order of solid<liquid<gas. Water, however, expands just before it freezes. Between 0°C and 4°C, water takes on a slightly different molecular structure that pushes the molecules farther apart. Kind of a good thing too because what would have happened during colder climate conditions if ice (solid water) sank to the bottom of the pond?

What can you do to help your understanding?

Chemistry is a subject that must be practiced everyday if possible. Work through the lecture examples stopping the video clips and then restarting to check yourself. Take advantage of the practice in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Sakai everyday. Keep a printed copy of the most recent course calendar (copies are kept on your content page in Blackboard) next to your work area. Email me with questions!!