Topic 8 and 13: Energetics I and II Flashcards
Standard enthalpy of a reaction
The enthalpy change of a reaction when completed under standard conditions of 100kPa and 298K
Enthalpy change of formation
The enthalpy change when one mole of a substance is formed from its constituent elements in standard state under standard conditions
Enthalpy change of combustion
The enthalpy change when one mole of a substance burns completely with excess oxygen under standard conditions.
Enthalpy change of neutralisation
The enthalpy change to form one mole of water when an acid and alkali reacts under standard conditions
Problems with the calorimetry experiment
Heat loss to the surroundings from lack of insulation
Not complete combustion –> less exothermic for incomplete combustion
Evaporation of fuel after weighing
Experiment not carried out under standard conditions
What is Hess’ law?
Enthalpy change for any reaction is independent of the route at which the reaction takes place, provided that the initial and final conditions for products and reactants are the same.
How to quickly calculate standard enthalpy of combustion ?
ΔHf of products - ΔHf of reactants
Why should Hess Cycle be used instead of experiments?
- Cannot measure temperature change of solid substances (also for decomposition)
- Unable to know how much water to add exactly
Definition of Bond enthalpy
The energy needed to break one mole of covalent bonds in gaseous molecules under standard conditions for a certain reaction
Definition of mean (average) bond enthalpy
The energy needed to break one mole of covalent bonds in gaseous molecules under standard conditions, averaged from bond enthalpies in range of similar molecules
Limitations of calculating with mean bond enthalpy
- For gaseous atoms, so enthalpy of vaporisation not included
- Not specific to the molecules and reaction involved
Definition of lattice energy
The energy change when one mole of ionic solid is formed from its gaseous ions
Always endothermic
Measure of ionic bond strength
Definition of enthalpy change of atomisation
The energy change to form one mole of gaseous atoms from its element in its standard state under standard conditions
Definition of enthalpy of electron affinity
The enthalpy change to add one mole of electron to one mole of gaseous atoms to form one mole of gaseous 1- ions
Definition of enthalpy change of solution
The enthalpy change when one mole of solid is dissolved in sufficient solvent to give an infinitely dilute solution
