1. Chapter 17 Thermochemistry

2. Basics Thermochemistry Thermochemistry –Study of heat changes in a chemical reaction Calorimeter Calorimeter –Instrument used to measure heat changes (actually measure temp changes because heat cannot be measured directly!) –Styrofoam cup, “real” calorimeter (metal) Temperature (T) Temperature (T) –Measure of average KE of sample –Increased KE = Increased temp Heat Heat –Measure of TOTAL KE of sample –Depends on the speed of the particles, the number of particles (the size or mass), and the type of particles in an object. Specific Heat (s or c) Specific Heat (s or c) –Amount of energy needed to raise 1g of sample 1°C –A constant; a given value per substance  C H2O = 4.186 J/g°C

3. Calorimetry Calculations Q = msΔT Q = msΔT –Q = heat –m = mass –s = specific heat –ΔT = change in temp Basic Heat Example Problems Basic Heat Example Problems –Calculate the amount of heat needed to increase the temperature of 250g water from 20°C to 56°C. –Calculate the specific heat of copper given that 204.75J of energy raises the temperature of 15g of copper from 25°C to 60°C.

4. Calorimetry Calculations When using a calorimeter to calculate heat you use the energy transfer between item in calorimeter and the calorimeter itself: When using a calorimeter to calculate heat you use the energy transfer between item in calorimeter and the calorimeter itself: –Heat lost = heat gained –msΔT (item) = -(ms ΔT (calorimeter) + ms ΔT (water) )

5. Calorimetry Calculations 1. A 25 g sample of a metal is heated in a boiling water bath to 99.3°C. A calorimeter of ms value of 5.32 J/C is used. 105mL of water (specific heat = 4.184 J/g°C) at 22.7°C is placed in the calorimeter. The metal is added to the calorimeter. The final temperature achieved is 44.8°C. What is the specific heat of the metal?

6. Heat of Reaction Heat of Reaction (Q) Heat of Reaction (Q) –The amount of energy gained or lost during a chemical equation Thermochemical Equation Thermochemical Equation –A reaction with heat information listed Enthalpy (H) Enthalpy (H) –Measure of heat of reaction; usually per mol of substance tested

7. Heat of Reaction ΔH = change in enthalpy ΔH = change in enthalpy ΔH = ΔH products – ΔH reactants ΔH = ΔH products – ΔH reactants If ΔH is negative = exothermic If ΔH is negative = exothermic If ΔH is positive = endothermic If ΔH is positive = endothermic Example: Example: –2H 2 O (g)  2H 2 (g) + O 2 (g) ΔH =+483.6kJ  Positive ΔH means endothermic rxn – needs energy

8. Heat of Reaction Stability of products vs. reactants Stability of products vs. reactants –If products have higher energy = endothermic –If reactant have higher energy = exothermic ENDO – PUT IN ENERGY EXO – ENERGY GIVEN OFF

9. Heat of Reaction Heat of Formation (ΔH f ) Heat of Formation (ΔH f ) –Energy change when forming 1mol of a compound from elements in most stable form Heat of Combustion (ΔH combus ) Heat of Combustion (ΔH combus ) –Energy change during burning in O 2 Heat of Reaction (ΔH rxn ) Heat of Reaction (ΔH rxn ) – -ΔH are ‘favored’ (expect to happen) –Exothermic reactions

10. Calculating ΔH ΔH rxn = [Σ(coefficient)(ΔH f products)] – [Σ(Coefficient)(ΔH f reactants)] ΔH rxn = [Σ(coefficient)(ΔH f products)] – [Σ(Coefficient)(ΔH f reactants)] ∑ = sum of∑ = sum of Practice problems (using ΔH charts) Practice problems (using ΔH charts)

11. Hess’ Law Hess’ Law Hess’ Law –One big reaction that is the sum of several smaller reactions –Rules:  Flip a reaction = change sign of ΔH  Multiply by a coefficient = multiply ΔH

12. Hess’ Law Practice Problems Practice Problems

13. Driving Forces for Reactions Enthalpy (ΔH) Enthalpy (ΔH) –Exothermic (-) are favored Entropy (ΔS) Entropy (ΔS) –Disorder –Gas – high entropy –More disorder (+) are favored

14. Entropy Practice Problems (Using ΔS charts) Practice Problems (Using ΔS charts)

15. Free Energy Gibb’s Free Energy (ΔG) Gibb’s Free Energy (ΔG) –Available energy –Is the FINAL answer whether a reaction will occur or not –ΔG = ΔH – TΔS  Units must match!! –Temps in Kelvin!!  If ΔG is negative  Reaction does occur spontaneously  If ΔG is positive  Reaction does not occur spontaneously

16. Free Energy Calculations (Using equation) Calculations (Using equation)

17. Free Energy Calculations (Using ΔG charts) Calculations (Using ΔG charts)

18. Summary -ΔH are favored -ΔH are favored +ΔS are favored +ΔS are favored -ΔG are favored -ΔG are favored