1 / 23

Thermodynamics

Thermodynamics. Every physical or chemical change is accompanied by energy change. Thermodynamics: branch of chemistry that studies energy changes s pecifically: changes in heat energy. Thermodynamics. Tells us if a reaction will occur 2 considerations: e nthalpy (heat energy)

gaston
Download Presentation

Thermodynamics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Thermodynamics

  2. Every physical or chemical change is accompanied by energy change • Thermodynamics: branch of chemistry that studies energy changes • specifically: changes in heat energy

  3. Thermodynamics • Tells us ifa reaction will occur • 2 considerations: • enthalpy (heat energy) • entropy (chaos/randomness)

  4. Enthalpy, H • enthalpy: heat content of system at constant pressure • symbol = H

  5. Changes in Enthalpy are measurable cannot measure enthalpy content of system directly can measure changes in enthalpy! symbol = H H = Hfinal – Hinitial = Hproducts - Hreactants

  6. Net gain in energy • Endothermic Process: energy absorbed • Hfinal > Hinitial so Hfinal – Hinitial results in positive value H is positive

  7. Net loss in energy • Exothermic Process: energy released • HfinalHinitial so Hfinal – Hinitial results in negative value H is negative [ see footnote to table I]

  8. Energy of universe is conserved Universe Environment System A B Which arrow represents an endothermic change? ? exothermic change B A energy can move between system and the environment

  9. Change in Energy choose how measure energy change • depends on how set up experiment • monitor the system • monitor the environment* * usually easier

  10. change in temperature! Energy lost = Energy gained • how do you know energy has moved? • can measure energy gained or lost by environment • equals energy lost or gained by system

  11. water is the environment! cup is the universe! reaction is carried out in water in styrofoam cup temperature of water is monitored source

  12. Q = mCT • Q = energy change • m = mass of water • c = specific heat of water • T = temperature change = Tf – Ti

  13. Different kinds of H’s H of dissolving: heat of solution H of phase change: heat of fusion/heat of vaporization H of reaction: heat of reaction • categorized by rxn type

  14. Table I: Heats of Reaction • rxns #1-6: combustion rxnsH: heat of combustion • rxns #7-18: formation reactions • substance is formed from its elements H: heat of formation • rxns #19-24: dissolving equations H: heat of solution

  15. Energy depends on amount • remember – it takes more energy to heat up water in bathtub than to make a cup of tea

  16. CH4(g) + 2O2(g)  CO2(g) + 2H2O (l)H = -890.4 kJ 1 mole of methane + 2 moles of oxygen → 1 mole of carbon dioxide gas & 2 moles of liquid water reaction is exothermic (negative sign for ΔH) 890.4 kJ energy released per mole of CH4(g) burned

  17. Energy depends on amount • burn 2 moles of CH4(g) with 4 moles of O2(g), get 2 times as much energy out • Stoichiometry! • (2)(890.4 kJ) = 1780.8 kJ is released

  18. Gas PE Liquid Solid Phase Change: Energy depends on direction Up is endothermic Down is exothermic melting/fusion boiling/ vaporization sublimation condensation freezing deposition

  19. Reactions: Energy depends on direction too! • N2(g) + 3H2(g)  2NH3(g) H = -91.8 kJ • 2NH3(g)  N2(g) + 3H2(g) H = 91.8 kJ If look at reverse reaction, then need to reverse sign of H

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

  21. Exothermic Exothermic Rxn: energy = product 4Fe(s) + 3O2(g)  2Fe2O3(s) H = -1625 kJ OR 4Fe(s) + 3O2(g)  2Fe2O3(s) + 1625 kJ

  22. Endothermic Rxn: energy = reactant NH4NO3(s)  NH4+(aq) + NO3-(aq) H = 27 kJ OR NH4NO3(s) + 27 kJ  NH4+(aq) + NO3-(aq)

  23. Changes of State H2O(s)  H2O(l) Hfusion = 333.6 J/g at 0oC H2O(l)  H2O(s) H = -333.6 J/g at 0oC energy is absorbedwhen water melts & energy is released when water freezes! H2O(l)  H2O(g) Hvapor = 2260 J/g at 100oC H2O(g)  H2O(l) H = -2260 J/g at 100oC energy is absorbedwhen water evaporates & energy is released when water condenses!

More Related