Topic 8: Energetics I Flashcards
Standard enthalpy change of formation
Enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions
What are standard conditions
298K and 100kPa
First ionisation energy
The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous unipositive cations
Second ionisation energy
The energy required to remove one mole of electrons from one mole of gaseous unipositive cations to form one mole of gaseous 2+ cations
Lattice enthalpy of formation
The energy released when one mole of an ionic compound is formed from its ions in the gaseous state.
First electron affinity
The energy released when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous uninegative anions.
Second electron affinity
The energy required for one mole of gaseous uninegative anions to gain one mole of electrons to from one mole of gaseous 2- anions.
Standard enthalpy change of atomisation of an element
The enthalpy change when one mole of gaseous atoms is formed from its element in standard state under standard conditions
Enthalpy change of atomisation of a compound
Enthalpy change when one mole of gaseous atoms are formed from a species in its standard state
i.e. 1/2 H_2 (g) -> H(g)
What is Hess’s Law
The total enthalpy change of a reaction is always the same, regardless of the reaction route taken.
What is the assumption which makes theoretical lattice enthalpies possible?
Ionic compounds contain no covalent character - they are purely ionic
What does it mean if the experimental and theoretical lattice enthalpies are very close?
The ionic compound contains very little covalent character (almost none)
What makes a cation polarising?
Small ionic radius and high charge - high charge density
What makes an anion easily polarised?
High charge and large ionic radius
What is the formula for predicting ΔHr using bond enthalpies?
ΔHr = ΣBE(reactants) - ΣBE(products)
Bond dissociation enthalpy
The energy required to break one mole of bonds are broken in the gaseous state to form gaseous atoms
What is the relationship between enthalpy of atomisation and bond enthalpy for diatomic gases?
Bond enthalpy = enthalpy of atomisation
For diatomic molecules, the enthalpy of atomization is equal to the enthalpy of bond dissociation
What does a smaller bond enthalpy mean?
Weaker bond
What is the definition of standard bond enthalpy values
Average of the dissocation energies of a specific bond in different molecules
Why are the ΔHr values a bit off when you use average bond enthalpies?
The strength of the bond also depends on the environment which means it is not actually the bond enthalpy of the specific bond involved in the reaction.
What is enthalpy change
The heat energy change measured at constant pressure
What is the standard enthalpy of combustion
The enthalpy change when one mole of a substance in its standard state reacts completely with oxygen under standard conditions
What is enthalpy of neutralisation
The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water.
What is average bond enthalpy
The amount of energy required to break one mole of bonds of a particular type between two atoms in gaseous state, averaged over different molecules.
