18 Enthalpy & Hess's Law

18 Enthalpy & Hess's Law - Which has the larger...

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1 1 Which has the larger heat capacity? A) 1.25 kg of water, c s = 4.19 J g -1 o C -1 B) 3.25 kg of cooking oil, c s = 1.97 J g -1 o C -1 Heat capacity has units of J o C -1 so multiply the specific heat capacity by the mass. Water: 1250 g x 4.19 g -1 o C -1 = 5240 J o C -1 Oil: 3250 g x 1.97 J g -1 o C -1 = 6400 J o C -1 2 Water: 1250 g x 4.19 J g -1 o C -1 = 5240 J / o C Oil: 3250 g x 1.97 J g -1 o C -1 = 6400 J / o C So if these amounts of water and cooking oil at 20 o C are heated at the same rate, which will reach 100 o C first? If equal masses of water and cooking oil at 20 o C are heated at the same rate which will reach 100 o C first? A) Water B) Cooking Oil
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2 3 Correction: F of C p. 90 . 4 SmartWork Homework Comes out every MWF after each class. Check every MWF! Generally due as follows (due dates on SW) M W (or F) W F (or M) F W But may be extended as indicated. Friday Class Topic Changed Example Problems Thermochemistry & Calorimetry Enthalpy Hess’s Law
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3 5 Enthalpy Change ( H) Chemical reactions can use (absorb) or release energy and do work. Example: an automobile Burns gasoline. Work (w) is done – the car moves Energy (q) is released – engine gets hot E gasoline = q + w E gasoline = H gasoline - P V V = V final – V initial H gasoline = E gasoline + P V rearrange 6 Units of P V In common chemistry units P measured in atmospheres (atm)
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