Energy, Temperature, And Heat

The Nature of Energy Energy is the ability to do work or produce heat. Potential energy Energy of position Kinetic energy Energy of motion E = ½ mv2

Law of Conservation of Energy One of the most important characteristics of energy is that it can be conserved. The law of conservation of energy states that energy can be converted from one form to another but can be neither created nor destroyed. That is, the energy of the universe is constant.

Ball A – higher position – more potential energy Ball A – released – strikes ball B giving arrangement in second picture Ball A – potential energy decreased but since energy is conserved where is the energy lost by A? Ball B – potential energy has been increased which means work (force acting over a distance) has been done on B

Ball B – final position lower than initial position of A so where is the rest of the energy? Ball A loses some kinetic energy to hill as it rolls down – called frictional heating Ball A – energy stored (potential energy) distributed to B through work and the surface of hill by heat

Imagine experiment repeated using different surfaces Imagine experiment repeated using different surfaces. How does that effect the energy transfer? Ball A always loses same amount of energy. The way the energy transfer is divided between work and heat depends on the pathway. Energy change is independent of the pathway, whereas work and heat both are dependent on the pathway.

State Functions A state function is a property of the system that changes independently of its pathway. In the preceding example, energy is a state function but work and heat are not state functions. The change in energy is always the same (state function) but the way the resulting energy is distributed as heat or work depends on the nature of the hill’s surface (heat and work are not state functions).

Distance traveled? Or Change in Elevation? Distance traveled depends on the route taken (pathway), it is not a state function. Change in elevation depends only on the difference between’s the two cities elevation. Change in elevation is a state function.

Temperature and Heat How does hot water differ from cold water? The answer lies in the motion of the molecules. Temperature is a measure of the random motions of the components of a substance. Therefore, the water molecules in the 90.oC water are moving more rapidly than the molecules in the 10.oC.

Consider an experiment in which hot water is placed next to cold water in an insulated box. The hot water is separated from the cold by a thin metal wall. The hot water molecules are moving faster and will transfer energy through the metal wall to the cold water. This flow of energy is called heat. Heat can be defined as a flow of energy due to a temperature difference.

What will eventually happen? The two water samples will eventually reach the same temperature since their mass is equal. How does the energy lost by the hot water compare to the energy gained by the cold water? Since energy is conserved, they must be equal.

Exothermic and Endothermic Processes Consider striking and burning a match. Energy is released through heat as the match burns. To evaluate this reaction we must divide the universe into two parts: the system and the surroundings. The system is the part of the universe we are studying. The surroundings are everything else in the universe. In this reaction, the system is the reactants and products. The surroundings consist of everything else other than the reactants and products.

Exothermic – when a process results in the evolution of heat. Energy flows out of the system as heat. The burning of the match is exothermic. Endothermic – when a process absorbs energy from the surroundings. Energy flows into the system as heat. Boiling water to form steam is endothermic.

Where does the energy, released as heat, come from in an exothermic reaction? The potential energy of the reactants is higher than the potential energy of the products. The heat flow into the surroundings results from a lowering of the potential energy of the reaction system. In any exothermic reaction, some of the potential energy stored in the chemical bonds is converted to thermal energy via heat. For an endothermic reaction, the potential energy of the reactants would be lower than the potential energy of the products.