Presentation on theme: "Molar Enthalpy - DH It is impossible to measure all the energy associated with each chemical reaction. Every molecule has kinetic energy (due to movement:"— Presentation transcript:
1Molar Enthalpy - DHIt 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).
2Therefore 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.
3Types 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)
4Representing Enthalpy Changes Four different ways:Heat term in the chemical equationEg: 2H2(g) + O2(g) 2H2O(g) kJDH valueEg: 2H2(g) + O2(g) 2H2O(g) DHo = kJNegative values indicate exothermic reactionsPositive values indicate endothermic reactions
5Standard molar values of enthalpy Eg: H2(g) + 1/2O2(g) H2O(g) DHo = kJ/molReaction is carried out under standard conditions SATP for 1 mole of specific product made or reactant usedEnergy DiagramsEg: