Molar Enthalpy - DH It is impossible to measure all the energy associated with each chemical reaction. Every molecule has kinetic energy (due to movement:

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Presentation transcript:

Molar Enthalpy - DH It is impossible to measure all the energy associated with each chemical reaction. Every molecule has kinetic energy (due to movement: translational, vibrational, rotational, and even centripetal of electrons around nucleus) and potential energy (stored within bonds arising from electrostatic and nuclear attractions of electrons & protons).

Therefore it is easier to measure the change in heat content of a reaction, known as enthalpy (DH). DH = Hproducts – Hreactants (where H is really q) The amount of heat generated by a reaction will depend on the amount of substance being reacted (m). So it is more convenient to express the energy from a reaction on a mole basis. Molar enthalpy = amount of energy from a chemical reaction in terms of number of moles of chemical reacted or produced.

Types of Molar Enthalpies Many reactions have specific meanings: DHofus – heat of fusion (melting solid to liquid) DHovap - heat of vapourization (boiling liquid to gas) DHocomb- heat of combustion (burning substance) DHosol- heat of solution (dissolving substance) DHoneut- heat of neutralization (neutralizing acid/base) DHof-heat of formation (making substances from its elements) DHor-heat of reaction (energy from that specific reaction)

Representing Enthalpy Changes Four different ways: Heat term in the chemical equation Eg: 2H2(g) + O2(g)  2H2O(g) + 571.6 kJ DH value Eg: 2H2(g) + O2(g)  2H2O(g) DHo = -571.6 kJ Negative values indicate exothermic reactions Positive values indicate endothermic reactions

Standard molar values of enthalpy Eg: H2(g) + 1/2O2(g)  H2O(g) DHo = -285.8 kJ/mol Reaction is carried out under standard conditions SATP for 1 mole of specific product made or reactant used Energy Diagrams Eg: