1 / 18

Energetics

Energetics. Topic 5. Energy & Chemistry. 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. System and Surroundings. SYSTEM The object under study SURROUNDINGS

lucia
Download Presentation

Energetics

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. Energetics Topic 5

  2. Energy & Chemistry 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.

  3. System and Surroundings • SYSTEM • The object under study • SURROUNDINGS • Everything outside the system

  4. Directionality of Energy Transfer • Energy transfer as heat is always from a hotter object to a cooler one. • EXOthermic: energy transfers from SYSTEM to SURROUNDINGS.

  5. Directionality of Energy Transfer • Energy transfer at heat is always from a hotter object to a cooler one. • ENDOthermic: heat transfers from SURROUNDINGSto theSYSTEM.

  6. James Joule 1818-1889 UNITS OF ENERGY 1 calorie = heat required to raise temp. of 1.00 g of H2O by 1.0 oC. 1000 cal = 1 kcal 1 kcal = 1 Calorie (a food “calorie”) But we use the unit called the JOULE 1 cal = exactly 4.184 joules

  7. Which has the larger heat capacity? HEAT CAPACITY The heat required to raise an object’s T by 1 ˚C.

  8. Specific Heat Capacity How much energy is transferred due to T difference? The heat (q) “lost” or “gained” is related to a) sample mass b) change in T and • specific heat capacity (is the amount of heat needed to raise the temperature of 1 gram of a substance 1°C.) q C = ------------ m x Δ T

  9. heat gain/lose = q = (specific heat)(mass)(∆T) Specific Heat Capacityexample: If 25.0 g of Al cools from 310 oC to 37 oC, what amount of energy (J) has been transferred by the Al? where ∆T = Tfinal - Tinitial q = (0.897 J/g•K)(25.0 g)(37 - 310)K q = - 6120 J Notice that the negative sign on q signals heat “lost by” or transferred OUT of Al.

  10. ENTHALPY Most chemical reactions occur at constant Pressure, Heat transferred at constant P = qp qp = ∆Hwhere H = enthalpy ∆H = change in heat contentof the system ∆H = Hfinal - Hinitial

  11. ENTHALPY ∆H = Hfinal - Hinitial If Hfinal > Hinitial then ∆H is positive Process is ENDOTHERMIC If Hfinal < Hinitial then ∆H is negative Process is EXOTHERMIC

  12. Thermochemical Equations • Energy amount can be written as a reactant or product: CaO + H2O  Ca(OH)2 + 65.2 kJ or CaO + H2O  Ca(OH)2 ∆H = -65.2kJ ____________________________________ 2NaHCO3 + 85kJ  Na2CO3 + H2O +CO2 or 2NaHCO3  Na2CO3 + H2O + CO2ΔH = 85kJ

  13. Example: 2NaHCO3 + 85kJ  Na2CO3 + H2O +CO2 Calculate the amount of heat required to decompose 2.24 mol NaHCO3 85 kJ ΔH = 2.24 mol NaHCO3 x ---------------------- 2 mol NaHCO3 ΔH = 95 kJ

  14. Standard Enthalpy Values Most ∆H values are labeled ∆Ho which is enthalpy measured under standard conditions P = 100 kPa Concentration = 1 mol/L T = usually 25 oC with all species in standard states (STP) Examples: C = graphite and O2 = gas Standard enthalpy of formation of a substance is the enthalpy change when 1 mole of a substance is formed from its elements in their standard states under standard conditions. ΔH° = ΔH° (products) - ΔH° (reactants)

  15. Example Making liquid H2O from H2 + O2 involves twoexothermic steps. H2 + O2 gas H2O vapor Liquid H2O

  16. Example Consider the formation of water H2(g) + 1/2 O2(g)  H2O(g) + 241.8 kJ Exothermic reaction — energy is a “product” and∆H = – 241.8 kJ

  17. Example Making H2O from H2 involves two steps. H2(g) + 1/2 O2(g)  H2O(g) + 242 kJ H2O(g)  H2O(l) + 44 kJ -------------------------------------------------------- H2(g) + 1/2 O2(g)  H2O(liq) + 286 kJ Example of HESS’S LAW— If a rxn is the sum of 2 or more others, the net ∆H is the sum of the ∆H’s of the other rxns.

  18. Hess’s Law & Energy Level Diagrams Forming H2O can occur in a single step or in a two steps. ∆rHtotal is the same no matter which path is followed. NOTE: ∆rH stands for the enthalpy change for a reaction, r

More Related