A - Thermodynamics Flashcards
What is enthalpy change?
The heat energy transferred in a reaction at constant pressure.
What are standard conditions?
298K and 100kPa.
Why do exothermic reactions have a negative enthalpy change?
Because heat energy is given out.
Why do endothermic reactions have a positive enthalpy change?
Because heat energy is absorbed.
Define enthalpy change of formation.
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Define bond dissociation enthalpy.
The enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules are broken.
Define enthalpy change of atomisation of an element.
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.
Define enthalpy change of atomisation of a compound.
The enthalpy change when one mole of a compound in its standard state is converted to gaseous atoms.
Define first ionisation energy.
The enthalpy change when one mole of gaseous 1+ ions is formed from one mole of gaseous atoms.
Define second ionisation energy.
The enthalpy change when one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions.
Define first electron affinity.
The enthalpy change when one mole of gaseous 1- ions is formed from one mole of gaseous atoms.
Define second electron affinity.
The enthalpy change when one mole of gaseous 2- ions is formed from one mole of gaseous 1- ions.
Define enthalpy change of hydration.
The enthalpy change when one mole of aqueous ions is formed from one mole of gaseous ions.
Define enthalpy change of solution.
The enthalpy change when one mole of solute is dissolved in enough solvent that no further enthalpy change occurs on further dilution.
How can lattice enthalpy be defined?
Either as enthalpy of lattice dissociation or enthalpy of lattice formation.
Define lattice enthalpy of formation.
The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions.
Define lattice enthalpy of dissociation.
The enthalpy change when one mole of a solid ionic compound is completely dissociates into its gaseous ions under standard conditions.
What is Hess’ law?
Says that the total enthalpy change of a reaction is independent of the route taken.
What is the purely ionic model of a lattice?
It assumes that all the ions in a lattice are spherical, and it’s their charge evenly distributed around them.
Why is the experimental value of lattice enthalpy often different from the theoretical value based on the purely ionic model?
As most ionic compounds have some covalent character.
The positive and negative ions in a lattice aren’t usually exactly spherical. Positive ions polarise neighbouring negative ions to different extents, and the more polarisation there is, the more covalent the bonding will be.
What two things happen when a solid ionic lattice dissolves in water?
- The bonds between the ions break to give free ions (this is endothermic).
- Bonds between the ions and water are made (this is exothermic).
The enthalpy change of solution is the overall effect on the enthalpy of these two steps.
Why are water molecules able to bond to ions?
Because oxygen is more electronegative than hydrogen, so draws electrons towards itself, creating a dipole.
The dipole means the positively charged hydrogen atoms can form bonds with negative ions and negatively charged oxygen atoms can form bonds with positive ions.
Why do soluble substances tend to have exothermic enthalpies of solution?
Because substances generally only dissolve if the energy release is roughly the same, or greater than the energy taken in.
What is entropy, S?
A measure of disorder.
It’s a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles.
The more disordered particles are, the higher their entropy is.
