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

Constructing an Energy Bar Chart Consider this example A cup of hot coffee cools as it sits on the table. 1.Determine what is in the system Everything else makes up the surroundings COFFEE CUP

Decide whether E ch is involved In this case, you start with coffee and end with coffee; particles are not rearranged to form new substances So, ignore E ch for now.

Assign values to E i 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

Assign values to E i Use two E i bars before and after (it is a liquid the whole time) cup coffee

Choose bars for E th depending on temperature To be consistent, let’s use the following system for these bars: 4 bars for 100 o C 2 bars for room temperature (20-25 o C) 1 bar for 0 o C 1/2 bar for < 0 o C

Choose bars for E th depending on temperature Use 3 bars for hot coffee and 2 bars for room temp coffee Other values might also work; try to be consistent in your representations cup coffee

Now show energy transfer Final situation has 1 less bar of E than initial; 1 bars had to leave the system 5 bars-1 bars =4 bars cup coffee

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 i increases (solid  liquid) The E th increases (temp rises) Now represent these changes in bar graph.

Initial & Final States E th Initial: E th is less than 1 bar since it starts at -8 o C. Final: E th is 2 bars since it is at room temp E i Initial: E i is 1 bar since it is a solid Final: E i is 2 bars since it is a liquid Ice tray

Account for Energy Energy must flow into system via heating 4 bars bars = 1.5 bars Ice tray