CHE 112 (KOTZ)

1. CHE 112 (KOTZ) Chapter 5 Energy & Chemical Reactions

2. Che 112 Course Introduction • Prerequisites • Course Policy and Grading • Homework • Shopping List • Lab Start up

3. Chapter 5 Overview • Energy, work and heat • 1st Law of Thermodynamics • Enthalpy, H

4. Energy and Heat Definitions:Energy is the capacity to do work or transfer heat. Heat is the form of energy that flows between 2 objects because of their difference in temperature. • KE = ½ m v 2 examples • PE (composition or location)examples • KE ↔PE interconvertedLAW OF CONSERVATION OF ENERGY

5. Energy Units 1 calorie DEFINED 1000 cal = 1 kilocalorie = 1 kcal 1 kcal = 1 Calorie (a food “calorie”) But we use the unit called the JOULE 1 cal = exactly 4.184 joules

6. Systems • System vs surroundings • Three types of systems open - closed - isolated –

7. Specific Heat Capacity, C • C = q / m ΔT mass dependent • Algebra q = ? know: specific C H2O(l) = 4.18 J/g 0C or 1 cal /g 0C(can use Celsius or Kelvin; ΔT is same)

8. Lab Determination of Specific C of a Metal • Assume isolated system • Thermal equilibrium or q metal = - q waterNB: textbook & lab manual do not agree!!specific heat capacity = specific heat C textbook = SH manual textbook uses K; manual uses 0C See problem #17 page 243

9. Heating Curves, review • Heat of fusion, ∆Hffor water 80 cal/g • Heat of vaporization, ∆Hvapfor water 540 cal/g • See other values Appendix D (table 12) See Heating Curve for water p. 219

10. Internal Energy, U • U defined: total energy withinsystem KE and PE molecular motion, KE bonding, PE • Heat, “q” transferred energy (joules) • Work, w = - P Δ V • State functions

11. 1st Law Δ U = q + w Sign Conventions [see summary table p. 224] + energy absorbed by system; U increases when:q > 0 Endo w > 0 compression - energy lost by system; U decreases when:q < 0 Exo w< 0 expansion NOTES: work = f x d ( where f = ma ) work and energy have SAME units, joules!

12. energy transfer in (endothermic), +q energy transfer out (exothermic), -q w transfer in (+w) w transfer out (-w) SYSTEM ∆U = q + w

13. Terms • Adiabatic [ thermal insulation], q =0 • Exothermic , q < 0 • Endothermic, q > 0

14. Che Reactions • @ V = constant w = 0 So, ΔU = qv • @ P = constant ΔU = qp + w = qp - P ΔV • Or qp = ΔU + P ΔV

15. Enthalpy, Hdefined as H = U + PV • So, ΔH = ΔU + PΔV • Thus ΔH = qp extensive a state function qp = ΔH, unique value ; “heat of rxn” • Recall, exo and endo • General Enthalpy Diagrams (Energy Profiles) (skip energy level diagrams-textbook)

16. ΔH and Stoichiometry Example: Given H2 + Cl2 → 2 HCl ΔHrxn = - 184 KJfind ΔHrxn if a) 12.8 g H 2 react b) 6.25 mol. HCl form

17. Hess’ Law of Heat Summation A + B → C + D ΔH1 C + B → E + F ΔH2 ___________________________________ A + 2B → D + E + F ΔHrxnΔHrxn = ΔH1 +ΔH2 You see ex.5.8 page 235

18. Standard Enthaply of FormationΔfH0 • Define: standard state • “standard heat of formation” • ΔfH0 = 0 for elements • See table Appx.L ΔHrxn0 = ∑ ΔfH0prods - ∑ ΔfH0reactnts [using molar coeffs ]see ex. 5.9 page 238

19. End Ch 5 • You may skip sec 5.8 • Homework questions? logoff