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Energy

Energy. The capacity to do work or to produce heat. Law of Conservation of Energy. Energy can be converted from one form to another but can neither be created nor destroyed. ( E universe is constant). The Two Types of Energy.

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Energy

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  1. Energy • The capacity to do work or to produce heat.

  2. Law of Conservation of Energy • Energy can be converted from one form to another but can neither be created nor destroyed. • (Euniverse is constant)

  3. The Two Types of Energy • Potential: due to position or composition - can be converted to work PE = mgh • Kinetic: due to motion of the object • KE = 1/2 mv2 • (m = mass, v = velocity)

  4. State Function • Depends only on the present state of the system - not how it arrived there. • It is independent of pathway.

  5. System and Surroundings • System: That on which we focus attention • Surroundings: Everything else in the universe • Universe = System + Surroundings

  6. First Law • First Law of Thermodynamics: The energy of the universe is constant.

  7. First Law • E = q + w • E = change in system’s internal energy • q = heat • w = work

  8. Temperature v. Heat • Temperature reflects random motions of particles, therefore related to kinetic energy of the system. • Heat involves a transfer of energy between 2 objects due to a temperature difference

  9. Exo and Endothermic • Heat exchange accompanies chemical reactions. • Exothermic: Heat flows out of the system (to the surroundings). • Endothermic: Heat flows into the system (from the surroundings).

  10. Expansion Work • work = force distance • since pressure = force / area, • work = pressure volume • wsystem = PV

  11. Heat Capacity

  12. Some Heat Exchange Terms • specific heat capacity heat capacity per gram = J/°C g or J/K g • molar heat capacity heat capacity per mole = J/°C mol or J/K mol

  13. Bomb Calorimeter

  14. Hess’s Law • Reactants  Products • The change in enthalpy is the same whether the reaction takes place in one step or a series of steps.

  15. Calculations via Hess’s Law • 1. If a reaction is reversed, H is also reversed. • N2(g) + O2(g)  2NO(g) H = 180 kJ • 2NO(g)  N2(g) + O2(g) H = 180 kJ • 2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer. • 6NO(g) 3N2(g) + 3O2(g) H = 540 kJ

  16. Standard States • Compound • For a gas, pressure is exactly 1 atmosphere. • For a solution, concentration is exactly 1 molar. • Pure substance (liquid or solid), it is the pure liquid or solid. • Element • The form [N2(g), K(s)] in which it exists at 1 atm and 25°C.

  17. Change in Enthalpy • Can be calculated from enthalpies of formation of reactants and products. • Hrxn° = npHf(products) nrHf(reactants)

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