D17.2 Energy, Temperature, and Heat

Thermal energy is kinetic energy associated with the random motion of atoms and molecules. When thermal energy is transferred into an object, its atoms and molecules move faster on average (higher KEaverage), the object’s temperature increases, and we say that the object is “hotter”. When thermal energy is transferred out of an object, its atoms and molecules move more slowly on average (lower KEaverage), the object’s temperature decreases, and we say that the object is “colder”.

Heating (or heat), represented by q, is the transfer of thermal energy between two bodies at different temperatures. Heat transfer of energy continues until both objects have reached the same temperature; that is, until thermal equilibrium has been reached.

Three drawings are shown and labeled a, b, and c, respectively. The first drawing labeled a depicts two boxes, with a space in between and the pair is captioned “Different temperatures.” The left hand box is labeled H and holds fourteen well-spaced red spheres with lines drawn around them to indicate rapid motion. The right hand box is labeled L and depicts fourteen blue spheres that are closer together than the red spheres and have smaller lines around them showing less particle motion. The second drawing labeled b depicts two boxes that are touching one another. The left box is labeled H and contains fourteen maroon spheres that are spaced evenly apart. There are tiny lines around each sphere depicting particle movement. The right box is labeled L and holds fourteen purple spheres that are slightly closer together than the maroon spheres. There are also tiny lines around each sphere depicting particle movement. A black arrow points from the left box to the right box and the pair of diagrams is captioned “Contact.” The third drawing labeled c, is labeled “Thermal equilibrium.” There are two boxes shown in contact with one another. Both boxes contain fourteen purple spheres with small lines around them depicting moderate movement. The left box is labeled H and the right box is labeled L.
Figure: Heat transfer of energy. (a) Two samples of matter are initially at different temperatures, higher (H) and lower (L). (b) When the two samples come into thermal contact, there is transfer of kinetic (thermal) energy from the hotter to the cooler matter. (c) The two samples reach thermal equilibrium when both are at the same temperature, and their molecules have the same average kinetic energy. The hotter sample has heated the cooler sample by transfer of energy.

Consider what happens during the heat-transfer process: energy transfer occurs as a result of collisions between molecules. (The boxes drawn around the groups of molecules in the figure are to delineate which molecules are in which group, but the  molecules contact each other.) On average the molecules in the hotter sample move faster than the molecules in the cooler sample, so on average a molecule from the hotter sample can transfer more energy than a molecule in the cooler sample. Thus, as long as the temperature is different, energy transfers from hotter to cooler.

After thermal equilibrium has been reached, energy transfer by molecular collisions continues to occur, but now the probability of energy transfer from sample H to sample L equals the probability of energy transfer from sample L to sample H. On the macroscopic scale there is no change in temperature of either sample, but on the atomic scale energy transfer continues at equal rates H→L and L→H. The atoms do not stop moving and transferring energy but the transfers are equal and opposite. This is a general characteristic of equilibrium.

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.