1. Energy Changes & Phase Changes

2. It takes energy to heat stuff up! For pure substance in single phase, can calculate how much using Q = mC  T For pure substance in single phase, can calculate how much using Q = mC  T Q = energy in Joules Q = energy in Joules m = mass in grams m = mass in grams C = specific heat capacity C = specific heat capacity  T = change in temperature = T f - T i  T = change in temperature = T f - T i On the other hand, when something cools down, energy is released! On the other hand, when something cools down, energy is released!

3. Q = mC  T C = specific heat capacity = amount heat required to raise temperature of 1 gram of pure substance by 1  C C = specific heat capacity = amount heat required to raise temperature of 1 gram of pure substance by 1  C C is physical constant C is physical constant unique for every pure substance unique for every pure substance C H2O = 4.18 J/g  C H2O = 4.18 J/g  C

4. Heat Flow: hot to cold System 1 phase Environment

5. But what about phase changes? Sometimes more than one phase of substance is present Sometimes more than one phase of substance is present Example: melting ice Example: melting ice both liquid water and solid ice are present both liquid water and solid ice are present temperature is constant, so  T = 0, even though beaker ice water is absorbing heat temperature is constant, so  T = 0, even though beaker ice water is absorbing heat

6. All chemical & physical changes are accompanied by energy changes. Phase changes are physical changes Phase changes are physical changes sometimes energy is absorbed sometimes energy is absorbed sometimes energy is released sometimes energy is released energy change for given phase change can be measured or calculated energy change for given phase change can be measured or calculated

7. What are 6 possible phase changes and their names?

8. Potential Energy Energy of relative position Energy of relative position Molecules always attracted to one another Molecules always attracted to one another need to put energy into system to pull molecules apart from one another need to put energy into system to pull molecules apart from one another farther apart molecules are, higher their PE is farther apart molecules are, higher their PE is

9. Melting Ice As beaker of ice water sits on hot plate, ice is melting As beaker of ice water sits on hot plate, ice is melting system absorbing heat from hot plate and using it for phase change system absorbing heat from hot plate and using it for phase change temperature is constant: temperature is constant: heat energy from hot plate going into phase change or PE of system heat energy from hot plate going into phase change or PE of system heat energy not going to KE heat energy not going to KE

10. Identify a phase change as endothermic or exothermic. PE Solid Liquid Gas EndothermicExothermic Fusion Vaporization Sublimation Condensation Freezing Deposition

11. Heating & Cooling Curves way to investigate energy changes way to investigate energy changes Measure temperature as function of time at constant heating or cooling rate Measure temperature as function of time at constant heating or cooling rate

12. Time Temperature I II IIIIVV Solid Solid & Liquid Liquid Liquid & Gas Gas K.E.  K.E.↔ P.E.↔ P.E.  Melt pt. Boil pt.

13. Melting & Boiling Points Plateaus = Phase changes = PE changes Plateaus = Phase changes = PE changes as long as 2 phases are present, temperature is constant as long as 2 phases are present, temperature is constant Melting point, Boiling point Melting point, Boiling point specific temperature value – not range specific temperature value – not range

14. What happens to the temperature as heat is added at the boiling point? Nothing, until only 1 phase present! Nothing, until only 1 phase present!

15. To analyze a heating/cooling curve: Does curve go uphill or downhill? Does curve go uphill or downhill? Label phase(s) present in each region Label phase(s) present in each region Describe what happens to KE in each region Describe what happens to KE in each region Describe what happens to PE in each region Describe what happens to PE in each region Locate melting point and boiling point Locate melting point and boiling point

16. Heating curve of H 2 O.

17. The heating rate is 150 J/min If substance takes 4 minutes to melt, how much heat energy used to melt it?

18. What is melting point of substance? The boiling point?

19. Heat of Fusion Amount energy required to change 1 gram of pure substance from solid to liquid at its melting point Amount energy required to change 1 gram of pure substance from solid to liquid at its melting point Heat of Fusion = H f = physical constant Heat of Fusion = H f = physical constant H f for water = 333.6 J/g (Table B) H f for water = 333.6 J/g (Table B)

20. How much heat is absorbed when 10 grams of ice melts at 0 o C? Heat absorbed = Heat absorbed = mass substance x heat of fusion of substance Q = mH f = (10 g)(334 J/g) = 3340 J Q = mH f = (10 g)(334 J/g) = 3340 J Where does energy go? Where does energy go? Particles use it to overcome forces of attraction to move farther apart Particles use it to overcome forces of attraction to move farther apart

22. Heat of Vaporization Amount energy required to convert 1 gram of pure substance from liquid to gas at its boiling point Amount energy required to convert 1 gram of pure substance from liquid to gas at its boiling point Heat of vaporization = H v = physical constant Heat of vaporization = H v = physical constant H v for water = 2260 J/g H v for water = 2260 J/g

23. How much energy does it take to vaporize 10 g of water? Q = mH v Q = mH v Q = (10 g)(2260 J/g) = 22600 J Q = (10 g)(2260 J/g) = 22600 J It takes lot more energy to go from liquid to gas than from solid to liquid. Why? It takes lot more energy to go from liquid to gas than from solid to liquid. Why?

24. The particles are spreading out a lot more!

25. Heats of fusion & vaporization Determined in calorimetry experiments Determined in calorimetry experiments

26. Temperature Time Q = mH f Q = mH v Q = mC l  T Q = mC s  T Q = mC g  T

27. 3 equations for Q Q = mC  T Q = mC  T Q = mH f Q = mH f Q = mH v Q = mH v figure out which one to use for given problem figure out which one to use for given problem depends on section of heating curve depends on section of heating curve look for hints in problem look for hints in problem

28. Q = mC  T Temperature changed Temperature changed Temperature increased Temperature increased Temperature decreased Temperature decreased Initial temperature Initial temperature Start temperature Start temperature Final temperature Final temperature Ending temperature Ending temperature From ____ to ____ From ____ to ____ Water Water

29. Q = mH f Ice Ice Freezing Freezing Melting Melting At 0  C (for H 2 O) At 0  C (for H 2 O) At constant temperature At constant temperature

30. Q = mH v Steam Steam Boiling Boiling Condensation Condensation At 100  C (for H 2 O) At 100  C (for H 2 O) At constant temperature At constant temperature