ENERGY

substance-like quantity present in every system of particles always involved when a system undergoes change

ENERGY in a system: Thermal -- energy of motion (“E th account”)  depends on mass and velocity of particles  temperature of a system is a measure of how fast particles are moving Phase -- energy due to arrangement/orientation of particles which have an attractive force between them (“E ph account”)  attractions lower energy of a system  solids have lowest E ph, liquids have greater E ph, and gases have the greatest E ph

ENERGY in a system: Energy can be transferred between “accounts” within the system of particles OR Energy can be transferred into or out of a system of particles A change in temperature or phase is observable evidence of transfer of energy

TRANSFER OF ENERGY between the system and the surroundings Heating – transfer of energy through collisions of particles Working – transfer of energy when macroscopic objects exert forces on each other Radiating – transfer of energy by the emission or absorption of light

LAW OF CONSERVATION OF ENERGY Energy can not be created or destroyed Total amount of energy remains the same during all energy transfers

HEAT AND TEMPERATURE Heat is a measure of energy transferred into or out of a system through collisions of particles Temperature is a measure of how fast the particles are moving

MEASURE OF TEMPERATURE Celsius Scale freezing point of water is set at 0 o C boiling point of water is set at 100 o C interval between them is divided into 100 parts Kelvin Scale absolute zero – theoretically the lowest possible temperature  particles stop moving absolute zero = 0 K = o C the size of each degree (interval) is the same as Celsius

Energy Bar Charts – refer to Energy Bar Charts notes How to represent the role of energy in physical change © Modeling Chemistry 2007

Tracking Energy… We were able to discuss how energy was transferred to the system (heating) and how the system responded (changes in particle motion or arrangement. It would be helpful if we could keep track of how energy is transferred and stored.  Representation Tool = Energy Bar Charts.

Constructing an Energy Bar Chart Consider the Lab A beaker of liquid water is heated over a Bunsen burner. 1.Represent the process with a temperature – time graph. Then make an empty Energy Bar Chart

Constructing an Energy Bar Chart 2.Determine what is in the system  Write it in the center circle, which represents the system.  Everything else makes up the surroundings WATERa ter

Constructing an Energy Bar Chart 3. Decide whether E ch is involved  In this case, you start with water and end with water; particles are not rearranged to form new substances  So, ignore E ch for now.

Constructing an Energy Bar Chart 4. Assign values to E ph  Due to interactions between particles, the energy stored due to the arrangement of particles is ranked: solids < liquids < gases  We choose to represent these phases by using: Solids = 1 bar Liquids = 2 bars Gases = 4 bars

Constructing an Energy Bar Chart Assign values to E ph  Use two E ph bars before and after WATER

Constructing an Energy Bar Chart 5. Choose bars for E th depending on temperature  Use 2 bars for colder water and 4 bars for warmer water  Other values might also work; try to be consistent in your representations

Constructing an Energy Bar Chart WATER

Constructing an Energy Bar Chart 6. Now show energy transfer using arrows showing energy entering or leaving the system.  Final situation has 2 more bars of E than initial; 2 bars had to be added to the system  Label the arrow using the symbol of the transfer mechanism.

Constructing an Energy Bar Chart 4 BARS+ 2 BARS6 BARS WATER Q

Now, consider phase change A tray of ice cubes (-8 ˚C) is placed on the counter and becomes water at room temperature What do we know about the situation?  The system is the tray of ice cubes.  The solid water turns to liquid water - no change in E ch  The E ph increases (solid  liquid)  The E th increases (temp rises) Now represent these changes using a temp/time graph and bar chart.

Initial & Final States Choice of bars for E th arbitrary, but keep consistent.  We used 2 bars for room temp and usually use 1 bar for “cold” substances.  Temp < 0˚C should be < 1 bar.

Account for Energy transfer Energy must flow into system via heating