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Thermochemistry

Thermochemistry. S tudy of the transfer of energy as heat V irtually every chemical reaction is accompanied by a change in energy. Chemical reactions usually absorb or release energy as heat.

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Thermochemistry

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  1. Thermochemistry • Study of the transfer of energy as heat • Virtually every chemical reaction is accompanied by a change in energy. • Chemical reactions usually absorb or release energy as heat. • Thermochemistry: is the study of the transfers of energy as heat that accompany chemical reactions and physical changes.

  2. The direction of energy transfer is determined by the temperature differences between the objects within a system. The energy is transferred as heat from the hotter brass bar to the cooler water. • This energy transfer will continue until the bar and the water reach the same temperature.

  3. Thermochemistry • Temperature: average kinetic energy of the particles in a sample of matter • K=273.15 + °C • 1 Joule= 1N·m=1kg·m2/s2 • Joule:is the SI unit of heat as well as all other forms of energy.

  4. Specific Heat • Specific heat: amount of energy required to raise the temperature of one gram of a substance by 1 °C • cp=specific heat • q=energy lost or gained (in J) • m=mass of sample (in g) • ΔT=change in temperature (in °C or K) • Energy transferred as heat always moves spontaneously from matter at a higher temperature to matter at a lower temperature. • Heat can be thought of as the energy transferred between samples of matter because of a difference in their temperatures. q=mcDT

  5. Page 520 You need ALL these for HW • Other cp • Steel = 0.452 • P = 0.80 • Concrete = 0.75 • Wood = 2.00 • Ethanol = 2.5 Write these Additional 5 down in your notes. You need them for HW, too!

  6. Packet Pg 4 0.5 kg Al DT = 7°C Cp= q=mcDT

  7. Specific Heat WS Answers • 3,100 J • 15,000 J • 4,520 J • 6,270,000 J • 2.5, ethanol • 0.8, P • 0.25kg • 14kg • 22C or 22K • -1C or -1K • steel

  8. Enthalpy ΔH = H(products)-H (reactants) • An enthalpy change is the amount of energy absorbed or lost by a system as heat during a process at constant pressure. • The energy absorbed or released as heat during a chemical reaction at constant pressure is represented by ∆H. • The H is the symbol for a quantity called enthalpy.

  9. ΔH = H(products)-H (reactants) 2H2 (g) + O2 (g) 2H2O (g) DH=-483.6 kJ/mol 2H2O (g)  2H2 (g) + O2 (g)DH= +483.6 kJ/mol exothermic endothermic

  10. If R>P then the reaction is exothermic • If R<P then the reaction is endothermic EA curves and Reaction Types

  11. Activation Energy CH 16.2 Pg 572

  12. Activation energy • EA= activation energy • The energy required to begin a reaction

  13. If R>P then the reaction is exothermic • If R<P then the reaction is endothermic EA curves and Reaction Types

  14. Catalyst • Lowers the EA of a reaction, while not being consumed • Will remain After the reaction

  15. Reaction Rate 16

  16. Collision Theory • Reaction rate is a function of the collisions between molecules • For a chemical reaction to occur, molecules must collide with the correct • Orientation • Energy

  17. Rate-Influencing Factors

  18. Nature of Reactants • Inert or stable atoms or molecules are less likely to react simply because of their nature

  19. Surface Area • Increased surface area allows for a greater reaction rate since more material is able to come into contact with other reactants

  20. Temperature • Increased temperature increases the likelihood of collisions • It also increases the energy of the collisions

  21. Concentration • Increased concentration increases the likelihood of collisions

  22. Concentration

  23. Catalysts • Catalysts lower the energy required to get a reaction going • They are not used up in the process of the reaction

  24. Chemical Equilibrium Chapter 17

  25. Reversible reactions • Chemical reactions in which products can reform into reactants • Endothermic in one direction, exothermic in the other direction • 2HgO2Hg + O2 • A + BC + D

  26. Equilibrium • Occurs when the forward reaction and the reverse reaction occur at the same rate • Le Châtelier’s Principle: equilibrium is shifted in the direction that tends to release the stress on the system

  27. Shift in equilibrium

  28. Change in Pressure 3H2+ 1 N22 NH3 + 92kJ • (a) H2, N2, and NH3 are in equilibrium within a closed system. • (b) Addition of more N2 causes a stress on the initial equilibrium. • (c) The new equilibrium position for this system.

  29. Change in Concentration • A + B  C + D • By adding A or B, equilibrium will shift to increasing C and D • By adding C or D, equilibrium will shift to increasing A and B

  30. Change in Temperature Recall that: • Endothermic (-) • – energy is absorbed • - Temperature decreases • Exothermic (+) • - energy is released • - temperature increases Energy is a Reactant!! Energy is a Product!!

  31. Change in Temperature • Different temperatures can cause an equilibrium system to shift and seek a new equilibrium position.

  32. Reactions that go to completion • Form a Gas 2. Form a Precipitate 3. Form a Slightly Ionized Product

  33. Form a gas • Often move toward completion because the gas is naturally removed as it diffuses away unless it is trapped • H2CO3  H2O + CO2

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