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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 .).

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Work on a system at constant pressure

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  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.

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