Thermodynamics Chapters 16 and 13

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

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!

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

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

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!

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)

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

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

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

Measuring Heat Calorimeter Insulated container to measure heat Really you measure the temperature change and then calculate heat http://www.chm.davidson.edu/java/calorimetry/calorimetry.html http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/simDownload/index4.html#thermoChem http://www.tutorvista.com/content/chemistry/chemistry-iv/thermodynamics/bomb-calorimeter.php

Calorimeter Coffee cup Calorimeter http://highered.mcgraw- hill.com/sites/9834092339/student_ view0/chapter48/bomb_calorimeter .html Coffee cup Calorimeter

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

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

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

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

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

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

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

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

Entropy Explained – Microstates and Macrostates

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)

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)

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)

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)

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

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)

Energy and Heat Phase Changes…

…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

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!

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

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

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