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Heats of Reactions

Heats of Reactions. Energy change as a function of molar quantities. The change in enthalpy between reactants and products in a reaction is called the HEAT of reaction, represented by the symbol D H Heat of reaction = Heat content of products - Heat content of reactants

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Heats of Reactions

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  1. Heats of Reactions Energy change as a function of molar quantities

  2. The change in enthalpy between reactants and products in a reaction is called the HEAT of reaction, represented by the symbol DH • Heat of reaction = Heat content of products - Heat content of reactants • Enthalpy change = Enthalpy of products - Enthalpy of reactantsDH = H (products) — H (reactants) Describing enthalpy changes using chemical reactions

  3. this expression shows that DH for an exothermic reaction must benegative.DH = H (products) – H (reactants) • Reactant energy > Product energy • DH = a small number – a big numbereg; 15 – 25 = – 10 (negative) • The atoms have lost (gone DOWN in) energy

  4. this expression also shows that DH for an endothermic reaction must be positive. DH = H (products) – H (reactants) Product energy > Reactant energy DH = a big number – a small numbereg; 25 – 15 = + 10 (positive) The atoms have gained (gone UP in) energy

  5. The negative sign indicates the reaction is exothermic. • This value of DH is for the production of 2 mol of water. If 4 moles were produced, the value for DH would be twice = -483.6 kJ • If the reaction is reversed, the sign of DH would be reversed. • DH does depend on the state of matter. The enthalpy change would be different for the formation of liquid water instead of gaseous water. 2 H2 (g) + O2 (g)  2H2O (g) DH= -483.6 kJ

  6. 2NaHCO3 (s)  Na2CO3(s) + H2O(l) + CO2 (g) DH = +91.6 kJ • Calculate DH if 4.5 mol of NaHCO3react • If Ea is 201.6 kJ draw the reaction profile diagram for the reaction Sample Problem

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