Enthalpy Flashcards
What is the definition of lattice enthalpy?
The enthalpy change when one mole of an ionic solid is formed from its gaseous ions in standard conditions
Give an example of a lattice enthalpy reaction
Na+ (g) + Cl- (g) -> NaCl (s)
What factors result in a greater magnitude of lattice enthalpy?
- Higher charges on the ions
- Greater number of cation-anion interactions
- Smaller distance between the centres of the two ions (inter-ionic distance)
What must all lattice enthalpy values be, and why?
All values are negative, as it is a formation reaction meaning energy is released
What is the definition of standard enthalpy change of atomisation?
The enthalpy change when one mole of GASEOUS ATOMS is formed from an element in its standard state
Give two examples of an atomisation reaction
C (s) -> C (g)
1/2H2 (g) -> H (g)
What is important to remember in atomisation?
The product must be gaseous
It is one mole of atoms, not one mole of molecules (e.g. not one mole of H2 but one mole of H)
What is the first electron affinity of an element?
The enthalpy change when each atom in one mole of gaseous atoms gains an electron to form a 1- ion
Give an example of an electron affinity reaction
Cl (g) + e- -> Cl- (g)
Is first electron affinity exothermic or endothermic?
Exothermic (negative value)
What is the exception to first electron affinity being exothermic?
For noble gases, first electron affinity is endothermic
Is second electron affinity exothermic or endothermic?
Endothermic (positive value)
Why is second electron affinity endothermic?
As energy has to be put in to overcome the forces of repulsion from the already negative ion
What is the definition of standard enthalpy of neutralisation?
The enthalpy change when an acid and an alkali react to form 1 mole of water, under standard conditions
What is the definition of standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions
What is the definition of standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions
Give an example of standard enthalpy of formation
Na (s) + 1/2Cl2 (g) -> NaCl (s)
What assumptions are made when find theoretical lattice enthalpy?
- The ions are in contact with one another
- The ions are perfectly spherical
- The charge on each ion is evenly distributed around
What causes differences between theoretical and experimental lattice energy?
Covalent character in the ionic bonding
What causes covalent character?
The polarisation of the anion by the cation
What is the effect of covalent character in ionic bonding on the value of lattice enthalpy?
Greater covalent character = more negative experimental value compared to the theoretical value
Why does the cation polarise the anion?
The positive cation will attract electrons from the anion, resulting in a distortion of the electron density of the anion. Some electrons will be drawn closer to the cation (covalent character)
What two cation factors increase the level of polarisation?
The cation will have a greater polarising effect when it is smaller, and has a higher charge (i.e. has a higher charge density
What two anion factors increase the level of polarisation?
The anion will be more easily polarised when it is larger, and has a higher charge
