Energetics Flashcards
Enthalpy change meaning
Heat energy change measured under conditions of constant pressure
Exothermic profile
Reactants above products, arrow facing down
Endothermic profile
Reactants below products, arrow facing up
Standard conditions(temp., pressure, conc.)
298K 100kPa 1mol/dm3
Standard enthalpy of combustion meaning
Enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions and with all chemicals in standard states
Standard Enthalpy change of formation meaning
Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions and all chemicals in their standard states
Standard Enthalpy of combustion equation
Thermal energy(kJ)/no. Of moles
Why are published values of standard Enthalpy of combustion higher than what you get?(4)
- some fuel may evaporate
- some heat energy not transferred to water, but rather to air
- fuel may be incompletely combusted, releases less energy
- experiment may not have been carried out under standard conditions
Why is standard Enthalpy sometimes 0
When it’s an element only
Hess’s law
If a reaction can be carried out by 2 different pathways, total Enthalpy change will be the same, provided that the starting and final conditions are the same for both pathways
Average bond Enthalpy
Energy needed to break one mole of a specific bond in a molecule in the gaseous state
Why do we get a different value for average bond enthalpy vs. Hess’ law?
Mean bond enthalpies are mean values calculated across a range of compounds containing that bond where as Hess’s law find the actual values of the bond enthalpies in that reaction.
