1 / 10

Work on a system at constant pressure

Work on a system at constant pressure. For work to be done, the system must expand (Work is done ON the surroundings BY the system; the quantity of work will be negative . ) or contract (Work is done BY the surroundings ON the system; the quantity of work will be positive .).

miyo
Télécharger la présentation

Work on a system at constant pressure

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. Work on a system at constant pressure • For work to be done, the system must expand(Work is done ON the surroundings BY the system; the quantity of work will be negative.)or contract(Work is done BY the surroundings ON the system; the quantity of work will be positive.). • At constant pressure, the work done on the system will be P ΔV.

  2. Constant pressure Gas pushes out piston Turn arm to add Mg Bubbles of H2 HCl solution

  3. Internal Energy E and heat • Recall E = q + w ; if no work is done on or by the system, w = 0. Then E = q. Therefore for a process with NO volume change, E = q.

  4. Enthalpy H and heat q • The heat absorbed by a system at constant pressure is equal to the change in enthalpy ΔH. ΔH = Δ(E+ PV) ΔH = ΔE + Δ(PV); constant pressure, so ΔH = ΔE + P ΔV; but ΔE = q + w, so ΔH = q + w + P ΔV; but w = P ΔV, so ΔH = q

  5. Enthalpy as a chemical reaction characterization • ΔH (and thus heat) is proportional to the quantity of material reacted. • ΔH is equal in magnitude, and opposite in sign, to ΔH for the reverse reaction. • ΔH depends not just on the chemical identity, but on the state of the reactants and products.

  6. Heat Capacity and Specific Heat • Heat capacity – property of an object • Amount of heat (energy) required to raise the object’s temperature 1K. • Molar heat capacity – amount of energy required to raise the temperature of one mole of a compound 1K. • Specific heat – amount of energy required to raise the temperature of one gram of a compound 1K.

  7. Sample exercise 5.6 • How much heat is needed to warm 250 g of water (about 1 cup) from 22C (about room temperature) to near its boiling point, 98C? The specific heat of water is 4.18 J∙g-1∙K-1.

  8. Calorimetry • Constant pressure ΔH = q • Constant volume (bomb calorimetry) Need heat capacity of the calorimeter

  9. Sample exercise 5.7 • When a student mixes 50 ml of 1.0 M HCl and 50 ml of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0C to • 27.5C. Calculate the enthalpy change for the reaction, assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J∙g1∙K1.

  10. Practice exercise below sample exercise 5.8 • A 0.5865-g sample of lactic acid (HC3H5O3) is burned in a calorimeter whose heat capacity os 4.812 kJ/ C. The temperature increases from 23.10 C to 24.95 C. Calculate the heat of combustion of (a) lactic acid per gram and (b) per mole.

More Related