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Enthalpy (H) and Change in Enthalpy ( Δ H )

Enthalpy (H) and Change in Enthalpy ( Δ H ). Enthalpy (H) is a function that accounts for the heat flow into or out of a system in a process that occurs at constant pressure. Represented as Δ H rxn or Δ H. Δ H is also called as heat of reaction. D H = H (products) – H (reactants).

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Enthalpy (H) and Change in Enthalpy ( Δ H )

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  1. Enthalpy (H) and Change in Enthalpy (ΔH) • Enthalpy (H) is a function that accounts for the heat flow into or out of a system in a process that occurs at constant pressure. • Represented as ΔHrxn or ΔH. • ΔH is also called as heat of reaction. DH= H (products) – H (reactants) DH = qp

  2. Sign of ΔH for Exothermic Reactions • Enthalpy of reactants > Enthalpy of products Hinitial > Hfinal , ΔH < 0 (negative) • The system transfers heat to the surroundings • Energy is released • Heat energy appears as a product in a chemical equation CH4(g) + 2O2(g)  CO2(g) + 2H2O (l) + 890 kJ • Energy of the reaction is also given as ΔH with a negative value CH4(g) + 2O2(g) CO2(g) + 2H2O (l) ΔH = - 890 kJ

  3. Sign of ΔH for Endothermic Reactions • Enthalpy of products > Enthalpy of reactants Hfinal > HinitialΔH > 0 (positive) • The system absorbs heat from the surroundings • Energy appears as a reactant in a chemical equation N2(g) + O2 (g) +183 kJ → 2 NO(g) • Energy of the reaction is also given as ΔH with a positive value N2(g) + O2 (g) → 2 NO(g) ΔH = +183 kJ

  4. Practice: Sign of ΔH and Endothermic and Endothermic Reactions • A chemical reaction that absorbs heat from the surroundings is said to be __________ (exothermic/endothermic) and has a __________ (positive/negative) ΔH at constant pressure. 2. The reaction 4 Al(s) + 3 O2(g)  2 Al2O3(s) ΔH° = -3351 kJ is __________ (exothermic/endothermic), and therefore heat is __________(absorbed/released) by the reaction.

  5. = -11.9 KJ Note: The negative sign (exothermic) indicates that 11.9 kJ of energy is given off by the system into the surroundings. 1 mol CO -163 KJ 8.18 g CO 28.01 g CO 4 mol CO Example: Calculation of the heat transferred for a given amount of reactant using ΔH 1. Consider the following reaction: Ni(s) + 4CO(g)  Ni(CO)4(s)ΔH = -163 kJ Calculate the heat produced when 8.18 g of CO react with an excess of Ni to give Ni(CO)4 in a constant pressure system

  6. Practice: calculation of the the heat transferred for a given amount of reactant and vice-versa using ΔH 2. The heat of combustion of ethylene gas (C2H4) is -1.39 x103 kJ/mol. How many kilojoules of heat is evolved when 17.7 g of ethylene gas burns? C2H4 + 3O2 -----> 2H2O + 2CO2 (Ans: -879 kJ)

  7. Practice: calculation of the the heat transferred for a given amount of reactant and vice-versa using ΔH 1. Consider the following reaction: 2H2(g) + O2(g)  2H2O(l) ΔH = -572 kJ a) Is this reaction exothermic or endothermic? b) How many grams of H2O are produced during an enthalpy change of -429 kJ? (Ans: 27.0 g)

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