Chapter 22 - Enthalpy and Entropy Flashcards
What is lattice enthalpy, is it exo or endo?
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
e.g. K+ (g) + Cl- (g) → KCl (s)
EXOTHERMIC - ionic bond formation
What is the standard enthalpy of formation?
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
What is the standard enthalpy change of atomisation? Is it exo or endo?
The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
e.g. Na (s) → Na(g) or 1/2Cl2 (g) → Cl (g)
ENDOTHERMIC
Because bonds are broken to form gaseous atoms
What is first ionisation energy? Is it exo or endo?
The enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
ENDOTHERMIC - energy required to overcome attraction between-ve electron and +ve nucleus
What is first electron affinity? Is it exo or endo?
The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
EXOTHERMIC - electron being added is attracted in towards nucleus
How can you construct a Born Haber cycle?
Route 1 - formation of lattice from elements
Route 2 - formation of gaseous atoms, gaseous ions and then lattice formation from gaseous ions
Why are second electron affinities endothermic?
A second electron is being gained by a negative ion, which repels the electron away. Therefore energy must be put in to force the -ve electron onto the negative ion
What is the standard enthalpy change of solution?
The enthalpy change that takes place when one mol of a solute dissolves in a solvent (WATER).
How can you experimentally determine the enthalpy change of solution?
- weigh out set amount of ionic solid
- put 25cm3 of water in polystyrene cup with lid
- measure starting temp of water
- quickly tip in all of ionic solid and stir
- record final temp when all of solid has dissolved and temp remains constant.
THEN USE Q = MCΔT m = the mass of the water + the mass of the salt)
What two processes take place when a solid ionic compound dissolves in water?
- ionic lattice breaks up
- water molecules are attracted to and surround ions
What 2 types of energy change are involved in the dissolving process?
- ionic lattice broken up (opposite energy change of lattice enthalpy)
- gaseous ions interact with polar water molecules to form hydrated aq ions (enthalpy change of hydration)
What is the enthalpy change of hydration?
The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
e.g. Na+ (g) + aq → Na+ (aq)
What affects the cycle to calculate the enthalpy change of solution?
Whether the enthalpy change of solution is endothermic or exothermic
ENDO: arrow up
EXO: arrow down
What are the general properties of ionic compounds?
- high melting and boiling points
- soluble in polar solvents
- conduct electricity when molten or in aq solution
What is the effect on lattice enthalpy of ionic size/radius increasing?
As ionic radius increases, attraction between ions decreases. This means that lattice energy is less negative and melting point decreases
What is the effect on lattice enthalpy of ionic charge increasing?
As ionic charge increases, attraction between ions increases. Therefore lattice energy becomes more negative and melting point increases
What is the effect of ionic size and charge across Period 3?
→ (2 supporting forces) increasing charge and decreasing size gives more attraction
← (2 opposing forces) increasing charge gives more attraction but increasing size gives less attraction
What can metal oxides like MgO be used for and why?
Used as a protective coating for the inside of furnaces and refractories as they are stable and have very exothermic lattice enthalpies and very high melting points
How does increasing ionic radius affect hydration enthalpies?
As ionic radius increases, the attraction between ion and water molecules decreases so hydration energy becomes less negative
How does increasing ionic charge affect hydration enthalpies?
As ionic charge increases, the attraction with water molecules increases and hydration energy becomes more negative
What must happen for an ionic compound to dissolve in water, what can we use to predict this?
The attraction between ions in the ionic lattice must be overcome
If sum of hydration enthalpies is larger than magnitude of lattice enthalpy, the overall enthalpy change (solution) will be exo and the compound SHOULD dissolve (but doesn’t always)
What is entropy?
The dispersal of energy within the chemicals making up the chemical system
How does entropy vary with solids, liquids and gases?
Solids have the smallest entropies, liquids have greater entropies, and gases have the greatest entropy
What are the units for entropy?
J K-1 mol-1
The greater than entropy value, the greater that energy is spread out per Kelvin per mole
