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Thermodynamics Chapters 16 and 13.

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Presentation on theme: "Thermodynamics Chapters 16 and 13."— Presentation transcript:

1 Thermodynamics Chapters 16 and 13

2 Energy Energy-ability to do work Law of Conservation of Energy Kinetic
Temperature-average kinetic energy of particles Potential Chemical Potential Energy-stored in bonds Measured in Joules Law of Conservation of Energy Can’t be created or destroyed Changes forms

3 Heat Transfer of energy flows from warmer to cooler object
Not the same as temperature! Measured in Joules or calories Heat required to raise 1 g of water 1°C is a calorie 1 calorie = 4.184J 1 food Calorie = 1000 calories Notice the capital C for the food Calorie!

4 A B Why does A feel hot and B feel cold?
Heat flows from A to your hand = hot. Heat flows from your hand to B = cold. 80ºC A 10ºC B

5 More About Energy: Heating and Cooling Curves
Shows Or energy Gas - KE  Boiling - PE  Liquid - KE  Melting - PE  Solid - KE 

6 Specific Heat Amount of heat (energy) required to raise temp of 1 g of a substance by 1°C Measured in J/g°C or J/gK (or calories/g°C or J/kg°C or calories/kg °C or cal/gK) Just like gas constant have to look at the units you are given in a problem!

7 Specific Heat Unique for different substances
Higher specific heat=more energy If comparing 2 substances… Substance with higher specific heat will take longer to heat (or cool)

8

9 Calculating Heat Can’t measure heat directly Formula: Q=mCDT Q = heat
m = mass in g or kg C = specific heat DT = change in temperature Tfinal – Tinitial

10 Calculating Heat If temp increases… If temp decreases
Tf is greater than Ti Tf-Ti will be positive Q will be positive Heat is gained If temp decreases Tf is less than Ti Tf-Ti will be negative Q will be negative Heat is lost

11 Practice Problem Q=mCDT C=Q/mDT C=114J/(10g)(25°C-50.4°C) C=0.449J/g°C
The temperature of a sample of iron with a mass of 10g changed from 50.4°C to 25°C with the release of 114 J of heat. What is the specific heat of iron? Q=mCDT m=10g Ti=50.4°C Tf=25C Q=114J C=? C=Q/mDT C=114J/(10g)(25°C-50.4°C) C=0.449J/g°C

12 Measuring Heat Calorimeter Insulated container to measure heat
Really you measure the temperature change and then calculate heat

13 Calorimeter Coffee cup Calorimeter
hill.com/sites/ /student_ view0/chapter48/bomb_calorimeter .html Coffee cup Calorimeter

14 Chemical Energy and the…
UNIVERSE!!! System—whatever your reaction is in Surroundings—everything else Universe = system + surroundings Helps explain conservation of energy It may seem like energy is lost or gained, just remember it is all part of the bigger picture…energy can be gained from or lost to surroundings

15 Enthalpy and Heat Enthalpy is the heat absorbed (or lost) at constant pressure Can’t really tell enthalpy but can measure the change in enthalpy—DH DHrx=Hf – Hi In a reaction the products are the final and the reactants are initial so: DHrx=Hproducts - Hreactants

16 What does it mean?? If DH is a negative number…
The energy of the reactants is greater than the energy of the products Energy is “left over” EXOTHERMIC REACTION

17 C + O2 ® CO2 + 395 kJ C + O2 Energy -395kJ CO2 Reactants Products

18 What does it mean?? If DH is a positive number…
Energy of the products is greater than the energy of the reactants Energy is required (needs to be added or absorbed) ENDOTHERMIC REACTION

19 CaCO3 + 176 kJ ® CaO + CO2 CaCO3 ® CaO + CO2 CaO + CO2 Energy +176 kJ
Reactants Products

20 Endothermic vs. Exothermic
endo = enter more energy to make products than given by the reactants Positive DH Absorbs heat (feels cold) Or have to add heat (burner or hot plate) Energy written with the reactants EXOTHERMIC Exo = exit More energy released by reactants than required by products Negative DH Releases heat (feels hot) Energy written with the products

21 Entropy Measure disorder or randomness of the particles in a system
Law of Disorder- tendency toward disorder or randomness (that explains my desk!)

22 Entropy Explained – Microstates and Macrostates

23 Changes in state: Predicting Disorder
Solids are the most ordered, gases the least As an object melts to become a liquid and then a gas—gets more disordered Entropy increases H2O (l)  H2O (g)

24 Dissolving a gas in a solvent
Predicting Disorder Dissolving a gas in a solvent Gas particles can’t move around as much so they are more ordered Entropy decreases O2(g)  O2 (aq)

25 Number of Gas particles
Predicting Disorder Number of Gas particles If more gas particles are in the products, then there is less order Entropy increases 2SO3(g)  2SO2 (g) + O2(g) 2 gas particles in the reactants (2 SO3) and 3 gas particles in the products (2 SO2 plus 1 O2)

26 Dissolving a solid or a liquid
Predicting Disorder Dissolving a solid or a liquid Entropy increases Solid and liquid particles move around more NaCl(s)  NaCl(aq)

27 Increasing temperature
Predicting Disorder Increasing temperature As temp increases particles move more Entropy increases

28 What does this have to do with reactions??
Reactions can occur on their own (spontaneous) or with help or not at all (nonspontaneous) For a reaction to be spontaneous, it must move towards less energy (enthalpy) AND/OR more disorder (entropy)

29 Energy and Heat Phase Changes…

30 …and more math… …and more constants from your reference tables!!!
More uses of H… …and more math… …and more constants from your reference tables!!! DHf = Heat of fusion Amount of heat required to melt ice DHv = Heat of vaporization Amount of heat required to boil water Q=mHf –use to determine amount of energy to melt a certain amount of ice Q=mHv –use to determine amount of energy to boil a certain amount of water

31 Practice… Q=mHf Q=? m=58g Hf=334 J/g (from your ref. tab.)
How much heat is absorbed by 58g of water as it melts? Q=? m=58g Hf=334 J/g (from your ref. tab.) Q=mHf Q=19372J…that easy!

32 More Practice… Q=mHv Q=? m=13g Hv=2260J/g (from your ref. tab.)
How much energy is required to boil 13g of water? Q=mHv Q=? m=13g Hv=2260J/g (from your ref. tab.) Q=(13g)(2260J/g) Q=29380J

33 More About Energy: Heating and Cooling Curves
Shows Or energy Gas - KE  Boiling - PE  Liquid - KE  Melting - PE  Solid - KE 

34 Calculating Q Use Q=mCDT when temp is changing
Use Q=mHf at the melting point Use Q=mHv at the boiling point


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