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Energy of Rxns

Energy of Rxns. Endothermic Rxn: must absorb nrg to occur Ex: using gas stove to fry an egg; cold pack Exothermic Rxn: releases nrg Ex: burning methane for a stove; hot pack. Calorimeter.

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Energy of Rxns

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  1. Energy of Rxns • Endothermic Rxn: must absorb nrg to occur Ex: using gas stove to fry an egg; cold pack • Exothermic Rxn: releases nrg Ex: burning methane for a stove; hot pack

  2. Calorimeter • Insulated device used to measure the amount of heat absorbed or released during a chemical rxn or physical process • Can be used to measure the specific heat for metals, food, etc. • Object being measured generally is surrounded by water and the change in temperature of the water is used to calculate heat released or absorbed.

  3. Try It • A piece of metal with a mass of 4.68 g absorbs 256 J of heat when its temperature is increased by 182o C. What is the specific heat of the metal? Could the metal be one of the alkaline earth metals listed in table 16-3 (p. 497)? 2. If 335 g water at 65.5o C loses 9750 J of heat, what is the final temperature of the water?

  4. Thermochemistry • Study of heat changes that accompany chemical rxns and phase changes • Chemical System – the specific part of the universe that contains the rxn or process you wish to study • Surroundings – everything in the universe other than the system

  5. Thermochemistry • The total energy of the universe must equal the energy of the system plus the energy of the surroundings: Esystem + Esurroundings = Euniverse Exothermic: Esystem decreases, Esurroundings must increase Endothermic: Esystem increases, Esurroundings must decrease

  6. Which is endothermic ? Which is exothermic? 4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625kJ EXOTHERMIC 27kJ + NH4NO3(s)  NH4+(aq)+ NO3-(aq) ENDOTHERMIC

  7. Enthalpy • ENTHALPY is the heat content of a system at a constant pressure. • Almost impossible to know the total heat content of a substance. • But chemists are usually more interested in the CHANGES in energy or enthalpy (heat) of rxn than what the total energy of a system is • Hrxn = Hproducts - Hreactants

  8. + and – values of enthalpy • Exothermic rxns lose heat so the products have less heat than the reactants; so exothermic rxns = negative enthalpy 4Fe(s) + 3O2(g) 2Fe2O3(s)ΔHrxn = -1625 kJ • Endothermic rxns gain heat so the products have mores heat than the reactants; so endothermic rxns = positive enthalpy NH4NO3(s)  NH4+(aq)+ NO3-(aq) ΔHrxn = 27 kJ

  9. + and – values of enthalpy • What would a graph of each of the previous rxns look like? • Remember that we used q = heat gained or lost in a rxn. If a rxn occurs at constant pressure than, q = ΔHrxn (all of our rxns will be at constant pressure).

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