Reactivity 1.2—Energy cycles in reactions SL Flashcards
Bond enthalpy, H
The energy required to break one mole of chemical bonds in the gaseous state.
Bond dissociation energy, E
The energy required to break one mole of chemical bonds in the gaseous state.
Average bond enthalpy
The enthalpy change when one mole of bonds are broken in the gaseous state averaged for the same bond in similar compounds.
How do you calculate the enthalpy change of a reaction using average bond enthalpy data?
The enthalpy change (ΔH) is calculated by subtracting the total bond enthalpies of the bonds formed from the total bond enthalpies of the bonds broken in the reactants. Formula: ΔH=Σ(bond enthalpies of bonds broken)−Σ(bond enthalpies of bonds formed).
Why are bond enthalpy values considered average values, and why might they differ from experimentally measured values?
Bond enthalpy values are averages because the energy required to break a bond varies depending on the molecule’s environment. They provide a general value rather than an exact measure for a specific molecule, as different molecules with the same type of bond can have different bond energies due to structural or surrounding atom influences.
State function
A thermodynamic value is independent of the path taken to reach that specific value. For example: enthalpy.
Hess’s law
Law that states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same.
