NSES Content Standard B  Physical Science: Conservation of energy and the increase in disorder Grades 9-12, page 180 Everything tends to become less organized and less orderly over time. Thus, in all energy transfers, the overall effect is that energy is spread out uniformly. Examples are the transfer of energy from hotter to cooler objects by conduction, radiation, or convection and the warming of our surroundings when we burn fuels.

 
Benchmark 4E The Physical Setting: Energy Transformations
Grades 9-12, page 86
Heat energy in a material consists of the disordered motions of its atoms or molecules. In any interactions of atoms or molecules, the statistical odds are that they will end up with less order than they began--that is, with the heat energy spread out more evenly. With huge numbers of atoms and molecules, the greater disorder is almost certain.

Benchmark 4E The Physical Setting: Energy Transformations
Grades 9-12, page 86
Transformations of energy usually produce some energy in the form of heat, which spreads around by radiation or conduction into cooler places. Although just as much total energy remains, its being spread out more evenly means less can be done with it.