Unit 8 Energy

• energy is the ability to do work or to produce heat

  • Potential Energy: the energy of position

  • Kinetic Energy: the energy of motion

    • depends on the object's mass

    • Equation:

      • E_k = ¹/₂mv²

The Nature 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 (converting at all times from one form to another).

Law of Conservation of Energy

Surface Types

How does the surface affect the energy transfer?

• Ball A always loses same amount of energy.

  • Energy change is independent of the pathway, whereas work and heat both are dependent on the pathway.

    • The way the energy transfer is divided between work and heat depends on the pathway.

• A state function is a property of the system that changes independently of its pathway.

  • In the previous ball 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 energy is distributed as heat or work depends on the nature of the hill’s surface (heat and work are not state functions).

State Function

Is it a state function?

• Distance traveled depends on the route taken (pathway), it is not a state function.

• Change in elevation depends only on the difference between the two cities’ elevations.  Change in elevation is a state function.

• the flow of energy due to the movement of particles

  • more movement = more heat energy

​​HEAT

• a measurement of the flow of energy

​​TEMPERATURE

• Temperature is a measure of the random motions of the components of a substance.

  • Therefore, the answer lies within the motion of the water molecules.

    • in the 40^oC water the particles are moving faster than the molecules in the 4^oC water.

      • so the hotter water has more heat energy

What's the difference between hot water and cold water?

• 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.

Consider this...

• 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 energy).

    • Heat can be defined as a flow of energy due to a temperature difference.

• 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?

What will eventually happen?

• Since energy is conserved, they must be equal. (the amount of energy released = the amont of energy absorbed)

Consider striking and burning a match.

• Energy is released through heat as the match burns.

Exothermic and Endothermic Processes

• 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 (focused on).

  • In this reaction, the system is the reactants and products.

    • the match burning

System vs Surroundings

• The surroundings are everything else in the universe.

  • The surroundings consist of everything else other than the reactants and products.

    • the air around the match

Exothermic vs Endothermic

• Exothermic – when a process releases energy in the form of heat.

  • Energy flows out of the system as heat. (EXITS)

  • The burning of the match is exothermic.

• Endothermic – when a process absorbs energy from the surroundings.

  • Energy flows into the system as heat. (ENTERS)

  • Boiling water to form steam is endothermic.

Exothermic vs Endothermic

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 and an increase in kinetic energy.

Where does the energy come from in an exothermic reaction?

• ​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.