Lecture 17b- Water Curve Calcs
- 1. BELLWORK‐ heat flow in pool When you enter a swimming pool, the water may feel quite cold. ABer a while, though, your body “gets used to it,” and the water no longer feels so cold. Use the concept of heat to explain what is going on.
- 2. The specific heat capacity (c) of any substance is the amount of heat required to raise the temperature of 1gram of that substance by 1°C. • Every substance has its own specific heat.
- 3. Water has a high specific heat. c= 4.184 J/(g·°C) Water can absorb or release a lot of heat before changing temperature
- 4. The heat absorbed during a change in temperature is calculated using the equation Specific Mass of Change Heat heat sample in temp (J) capacity (g) (°C) J/(g·°C) ΔT= Tfinal-Tinitial
- 5. PracIce How much energy is required to raise 50g of water from 25°C to 40°C? q = c m ΔT q = unknown c = 4.18J/(g°C) for water m = 50g ΔT = 15°C q = 4.18 x 50 x 15 = 3135 J of energy
- 6. ∆Hfus = the quanIty of heat absorbed during melIng = the quanIty of heat released during freezing. For water H2O(s) H2O(l) ∆Hfus = 6.01kJ/mol H2O(l) H2O(s) ∆Hfus = -6.01kJ/mol
- 7. How many grams of ice at 0°C will melt if 2.25kJ of heat are added? Known • iniIal and final temps are 0 °C • ΔHfus = 6.01kJ/mol • ΔH = 2.25kJ 2.25 kJ x 1mol = 0.374 moles would 6.01kJ be melted
- 8. How much energy is needed to melt 50g of ice? Known The heat of • ΔHfus = 6.01kJ/mol fusion is PER • 50g MOLE so convert grams to moles 50g H2O x 1mole 2.8 moles x 6.01 kJ = 18g 1mole =16.8 kJ
- 9. The quanIty of heat absorbed by a evaporaIng liquid is exactly the same as the quanIty of heat released when the vapor condenses = ∆Hvap For water H2O(l) H2O(g) ∆Hvap = 40.7kJ/mol H2O(g) H2O(l) ∆Hvap= -40.7kJ/mol
- 10. Phase change occurs at a specific temperature (aka the melIng point and boiling point) Temperature is constant during a phase change
- 11. Temperature is constant during a phase change During a phase change heat is used to overcome intermolecular a`racIons.
- 12. The ΔHvap is always larger than ΔHfus ∆Hfus is the energy required to overcome some intermolecular a`racIons. ∆Hvap is the energy required to overcome all intermolecular a`racIons.
- 13. IN YOUR NOTES-- Include a labeled drawing of the water curve and all of the notes in black and red on the slides to follow.
- 14. To calculate energy changes (ΔH) for a state change moles x ΔHfus OR moles x ΔHvap
- 15. To calculate energy required to increase or decrease temperature within a single state q= c m ΔT Use the specific Use the heat of specific heat Energy steam of liquid water Use the specific heat of ice
- 16. To calculate the energy change over more than one secIon of the curve… Calculate the energy for each step and add them together.
- 17. Calculate the ΔH associated with changing 100g of steam at 125°C to ice at ‐50 °C. 125°C 1.Determine if a phase change occurs within the temperature range. • water will condense at 100°C • water will freeze at 0 °C ‐50°C
- 18. Calculate the ΔH associated with changing 100g of steam at 125°C to ice at ‐50 °C. 125°C 2. Split problem into steps. 125 °C steam 100 °C steam steam water 100 °C water 0 °C water ‐50°C water ice 0 °C ice ‐50 °C ice
- 19. 3. Calculate the enthalpy change for each step. 125 °C steam 100 °C steam q = 1.89 J/(g°C) x 100g x ‐25°C = ‐4725J steam water 100g x 1 mol/18g = 5.55mol x 40.7kJ/mol = ‐226kJ 100 °C water 0 °C water q = 4.18 J/(g°C) x 100g x ‐100°C = ‐41,800J water ice 5.55mol x 6.01 kJ/mol = ‐33.4kJ 0 °C ice ‐50 °C ice q = 2.10 J/(g°C) x 100g x ‐50°C = ‐10,500J
- 20. 4. Add the values for each step to get the total energy change ‐4725J = ‐4.725 kJ + ‐226kJ + ‐41,800J = ‐41.8 kJ + ‐33.4kJ + ‐10,500J = ‐10.5 kJ Whoops! The units aren’t the same!!
- 21. 4. Add the values for each step to get the total energy change ‐4.725 kJ + ‐226 kJ The total enthalpy + ‐41.8 kJ change and the + ‐33.4 kJ changes in each step + ‐10.5 kJ are all negaIve ΔHtotal = ‐316.4 kJ values because cooling water is an exothermic process
- 22. EXOTHERMIC STATE CHANGES Heat comes out of substance(system) ∆H is negative Happens in a freezer Freezing(solidification) & condensation ENDOTHERMIC STATE CHANGES Heat goes into substance ∆H is positive Happens on a stove Melting(fusion), vaporization, sublimation
- 23. The insulated device used to measure heat changes in chemical or physical processes is called a calorimeter. qwater = - qrxn The heat change for the water in the calorimeter is equal to the heat change of the reaction but opposite in sign.