DF - energetics *03 Flashcards
Thermodynamics first law?
energy can be neither created nor destroyed but it can be converted from one form to another
What is enthalpy
a measure of the heat content of a substance at constant pressure
enthalpy can’t be measured directly
What are the standard conditions?
Pressure = 100kPa (1atm)
Temperature = 298K (25 degrees)
All solutions have a concentration of 1 moldm-3
Enthalpy change of reaction
enthalpy change of a stated reaction
enthalpy change of combustion
enthalpy change when one mole of a substance undergoes complete combustion
enthalpy change of formation
enthalpy change when one mole of a substance is formed from its elements
enthalpy of neutralisation
the enthalpy change when one mole of water is formed form the reaction of an acid and alkali
what is the enthalpy of elements in their standard conditions
zero
bond dissociation enthalpy
energy required to break one mole of gaseous bond to form one mole of gaseous atoms
why are mean values used
because bond strength depends on environment
- the stronger the attraction between the atoms, the higher the bond enthalpy and the shorter the bond length.
what is the bond enthalpy for diatomic gases
twice the enthalpy of atomisation
what does higher bond enthalpy mean?
stronger bonds
Hess’s Law
enthalpy change is independent of the path taken
what is hess’s law used for
To calculate changes that cant be measured directly.
e.g combustion. formation or lattice enthalpy
exothermic reaction
a reaction that releases more energy from making bonds than the energy required (taken in) to break bonds
e. g combustion - CH4 (g) + 2O2 (g) –> CO2 (g) + 2H2O (l)
e. g respiration