Energetics Flashcards
Why do more stable species have a lower potential energy than unstable species?
The lower the chemical potential energy of a given chemical species, the more stable it is.
In what reactions are the products more stable than reactants?
Exothermic
In what reactions are the reactants more stable than products?
Endothermic
Enthalpy Change
Heat energy change measured under conditions of constant pressure.
Standard Enthalpy Change
Heat energy change measured under standard conditions
ΔH =
Heat energy change/no. of moles
Standard enthalpy of formation
Enthalpy change when one mole of that substance is formed from the most stable allotropes of its elements in their standard states under standard conditions.
Standard enthalpy of combustion
Enthalpy change when one mole of that substance is burned in an excess of oxygen under standard conditions.
Specific heat capacity
The amount of heat required to heat 1g of a substance by 1K
Bond dissociation enthalpy
The energy required to separate completely the atoms in one mole of covalent bonds
Why do mean bond enthalpies only give you approximate values for enthalpy changes?
Mean bond enthalpies are calculated from different reactions which may vary slightly. They also do not take intermolecular forces into account.
Atomisation energy
The energy required to separate completely the atoms in one mole of the substance
Hess’ Law
The enthalpy change for a chemical reaction depends only on the initial and final states and is independent of the path followed
Hess’ Law equation
∆H = Σ[∆Hf(products)] – Σ[∆Hf(reactants)]
or
∆H = Σ[∆Hc(reactants)] – Σ[∆Hc(products)]
