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Topic: Thermodynamics

Do Now: packet p.1. Topic: Thermodynamics. Every physical or chemical change is accompanied by energy change. Energy released = _________________ Energy absorbed = _______________. exothermic. endothermic. G L S.

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Topic: Thermodynamics

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  1. Do Now: packet p.1 Topic: Thermodynamics

  2. Every physical or chemical change is accompanied by energy change • Energy released = _________________ • Energy absorbed = _______________ exothermic endothermic G L S

  3. Thermodynamicsis the study of entropy and enthalpy changes that accompany chemical reactions Thermodynamics Tells us if a reaction will occur

  4. The total amount of energy a substance contains depends on many factors, some of which are not totally understood…it’s impossible to know the total heat content of a substance.So scientists measure ΔH H = enthalpy = the heat content of a system at constant pressure. It describes chemical PE stored in matter ΔH = enthalpy (heat) of reaction ΔH = Hproducts – Hreactants--the difference between the enthalpy(heat) of the substances that exist at the end of the reaction and the enthalpy(heat) of the substances that exist at the beginning of the reaction

  5. Thermochemical Equations • balanced chemical equation • shows physical state of all reactants & products • gives energy change • It can be written 2 ways • energy term can be written as reactant or product OR • H is given right after equation

  6. 4Fe(s) + 3O2(g)  2Fe2O3(s) + 1625 kJ OR 4Fe(s) + 3O2(g)  2Fe2O3(s) H = -1625 kJ NH4NO3(s) + 27 kJ  NH4+(aq) + NO3-(aq) OR NH4NO3(s)  NH4+(aq) + NO3-(aq) H = 27 kJ

  7. If ΔH is negative… - ΔH = exothermic • Hproducts< Hreactants • PE of Products < PE of Reactant • Example: 4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJ • ΔH = -1625kJ

  8. If ΔH is positive… + ΔH = endothermic • Hproducts> Hreactants • PE of Products >PE of Reactant Example: NH4NO3(s) + 27 kJ  NH4+(aq) + NO3-(aq) • ΔH = +27kJ

  9. Since energy is conversed…the system changes in one direction and the surrounding have to change in the opposite direction Universe Environment System A B A. Reaction is Exothermic, environment gets _________________ warmer B. Reaction is Endothermic, environment gets _________________ colder

  10. For any reaction occurring at constant pressure ΔH = Q • Q = mCT • Q = Energy change • m = mass of water • C = specific heat of water • T = temperature change = Tf – Ti

  11. Since there are different types of reactions, you have various ΔH’s • ΔHcomb = enthalpy (heat) of combustion – the enthalpy change for the complete bunring of one mole of the substance • ΔHformation = enthalpy (heat) of formation – the enthalpy change for the formation of a compound from its constituent elements • ΔHsolution = enthalpy (heat) of solution – the enthalpy change when 1 mole of an ionic substance is dissolved in water. Look at Table I: Heats of Reaction Lets label the various types from above

  12. Table I • Rxns 1-6: combustion rxns -H = heat of combustion • Rxns 7-18: formation (synthesis) rxns • Substance is formed from its elements • H = heat of formation • Rxns 19-24: dissolving equations • H = heat of solution

  13. Many other processes other than chemical reactions absorb or release energylike, Changes of state Hvaporization = molar heat of vaporization = amount of heat required to vaporize one mole of a liquid Hfusion = molar heat of fusion = amount of heat required to melt one moles of a solid

  14. Energy depends on amount • Remember – it takes more energy to heat up water in the ocean than to make a cup of tea

  15. CH4(g) + 2O2(g)  CO2(g) + 2H2O (l) H = -890.4 kJ 1 mole of methane + 2 mole of oxygen → 1 mole of carbon dioxide gas & 2 moles of liquid water reaction is ____________ (negative sign for ΔH) exothermic 890.4 kJ energy released per mole of CH4(g) burned What would happen if we had 2 moles of methane? Twice as much energy would be released 2 x 890.4 kJ = 1780.8 kJ will be released

  16. Reactions: Energy depends on direction too! • N2(g) + 3H2(g)  2NH3(g) H = -91.8 kJ • 2NH3(g)  N2(g) + 3H2(g) H = _______ If reverse equation, reverse sign of H 91.8 kJ

  17. If we can’t calc. H for one individual substance how are we able to calc. ΔH???!

  18. FYI: Hess’s Law(not on regents) Can add 2 or more equations by adding the H’s Enables you to calculate H for # of rxns Say you’re interested in 2S(s) + 3O2(g)  2SO3(g)

  19. 2S(s) + 3O2(g)  2SO3(g) 2 2 2 x (2) Have H’s for the following: a) S(s) + O2(g)  SO2(g) H = -297 kJ b) 2SO3(g)  2SO2(g) + O2(g) H = 198 kJ 2SO2(g) + O2(g)  2SO3(g) H = -198 kJ H = -594KJ + (- 198kJ) H = -792 kJ

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