Calculating energy changes with phase changes
GA standards SC6 Students will understand the effects motion of atoms and molecules in chemical and physical processes. a. Compare and contrast atomic/molecular motion in solids, liquids, gases, and plasmas. b. Collect data and calculate the amount of heat given off or taken in by chemical or physical processes. c. Analyzing (both conceptually and quantitatively) flow of energy during change of state (phase).
Hmmmmmm? Why didn’t the temperature of the water in your beaker go above the boiling point (100 C) even though you kept heating it? All the energy was being used to evaporate the water Going from liquid to gas takes energy. What’s happening at the molecular level? Water molecules are attracted to each other. It takes energy to separate them (liquid to gas)
Solid – liquid – gas at molecular level lowest energy. molecules are in fixed place Liquid Middle energy Molecules rolling around each other Gas Highest energy Molecules move independently in straight paths, randomly bumping into each other
Liquid to gas (cont’d) Once all the water is gone, the temperature of the steam can be increased if you keep heating it. gas liquid to gas liquid Temperature Energy
Liquid to gas (cont’d) The energy it takes to change 1 g of a substance from liquid to gas is called the heat of vaporization Hv Hv water = 539.4 cal/g How much energy will it take to evaporate 15g of water? Q = Hvm Q = 539.4 cal/g x 15g Q = 8091 cal
Solid to liquid What about melting an ice cube? Same holds true. It takes energy to separate the water molecules and get them moving around each other. While the ice is melting, the temperature holds steady at 0 C. Once the ice is all melted, the liquid can be heated.
Once all the water is gone, the temperature of the steam can be increased if you keep heating it. liquid solid to liquid solid (ice) Temperature Energy
Calculating melting energy Hf is the energy it takes to melt 1 g of a substance If the substance is freezing, Hf is negative Hf water = 333.0 J/g How much energy does it take to melt 15 g of ice? Q = Hf m Q = 330.0 J/g x 15g Q = 4950 = 5.0 x 103 J
Heating curve of water
Cooling Curve of Water
How many joules of heat are given off when 5 How many joules of heat are given off when 5.0 g of water cools from 75oC to 25oc? (Specific heat of water = 4.184 J/goC)
How many calories are given off when 85 g of steam condense to liquid water? (Hv= 539.4 cal/g)
How many joules does it take to melt 35g of ice at 0oC? (Hf = 333 J/g)
Given: Specific heat of How many joules are required to convert 10.0g of ice at -10.0oC to steam at 150. oC? Given: Specific heat of Specific heat of water = 4.184 J/g C° Specific heat of Steam = 2.03 J/g C° Specific heat of ice = 2.06 J/g C° Hv water = 539.4 cal/g Hf water = 333.0 J/g 5 part calculation
Your assignment Page 497 Practice Problem 14.2 Page 497 Section 14.1 Review 4-7 Page 503 Section 14.2 Review 1