ENERGY
Jan 21, 2016
ENERGY
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 (“Eth 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 (“Eph account”) attractions lower energy of a system solids have lowest Eph, liquids have greater Eph, and gases have the greatest Eph
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 Scalefreezing point of water is set at 0oCboiling point of water is set at 100oCinterval between them is divided into 100 parts
Kelvin Scaleabsolute zero – theoretically the lowest possible temperature
particles stop movingabsolute zero = 0 K = - 273oCthe 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
WATERater
Constructing an Energy Bar Chart
3. Decide whether Ech is involved In this case, you start with water and end
with water; particles are not rearranged to form new substances
So, ignore Ech for now.
Constructing an Energy Bar Chart
4. Assign values to Eph 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 Eph
Use two Eph bars before and after
WATER
Constructing an Energy Bar Chart
5. Choose bars for Eth 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 BARS 6 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 Ech
The Eph increases (solidliquid)
The Eth increases (temp rises)
Now represent these changes using a temp/time graph and bar chart.
Initial & Final States
Choice of bars for Eth 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