Presentation on theme: "Molar Enthalpy - H It is impossible to measure all the energy associated with each chemical reaction. Every molecule has kinetic energy (due to movement:"— Presentation transcript:
Molar Enthalpy - H 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 ( H). H = H products – H reactants (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: H o fus – heat of fusion (melting solid to liquid) H o vap - heat of vapourization (boiling liquid to gas) H o comb - heat of combustion (burning substance) H o sol - heat of solution (dissolving substance) H o neut - heat of neutralization (neutralizing acid/base) H o f -heat of formation (making substances from its elements) H o r -heat of reaction (energy from that specific reaction)
Representing Enthalpy Changes Four different ways: Heat term in the chemical equation Eg: 2H 2(g) + O 2(g) 2H 2 O (g) + 571.6 kJ H value Eg: 2H 2(g) + O 2(g) 2H 2 O (g) H o = -571.6 kJ Negative values indicate exothermic reactions Positive values indicate endothermic reactions
Standard molar values of enthalpy Eg: H 2(g) + 1/2O 2(g) H 2 O (g) H o = -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: