Enthalpy And Entrophy Flashcards
Lattice enthalpy (3)
Enthalpy change when 1 mol of an ionic compound is formed from its gaseous ions under STP.
Exothermic process as forming bonds releases energy.
Higher lattice enthalpy means that the strength of the ionic bonds in the compound is stronger.
Four processes that indirectly fine the lattice enthalpy of formation.
Enthalpy of formation.
Enthalpy of atomisation.
Ionisation energies.
Electron affinity energies.
Enthalpy of solution. (3)
Energy produced when water forms bonds with ions.
When there is energy left over, it is released as heat therefore the reaction is exothermic.
When there is not enough energy, it is taken from the surroundings: endothermic.
Two processes to find lattice enthalpy from solution. (2)
Enthalpy change of solution: a Mole of a compound dissolved in water under STP
Enthalpy change of hydration: a mole of aqueous ions formed from their gaseous ions under STP- exothermic process
Factors that affect the value of lattice enthalpy. (2)
Ionic size; smaller ions have greater electrostatic force of attraction so can pack more closely.
Therefore smaller ions= more exothermic LE
Ionic charge: Greater charge, increase electrostatic force of attraction. Smaller ionic radius, so greater attraction between oppositely charged ions.
Greater charge= more exothermic LE
Factors that affects Enthalpy of hydration. (2)
Ionic size: smaller ions have greater electrostatic force, therefore it attracts water more closely.
Ionic charge: increased charge attracts water more easily due to stronger force of attraction due to smaller atomic radii.
Entropy (3)
Measure of disorder in a system: dispersal of energy.
Values of entropy are always positive as particles are always in motion.
Solids have the lowest entropy whereas gases have the highest entropy.
How are entropy changes predicted. (3)
If the system becomes more disordered then the entropy change (delta S) will be more positive.
If the system become less disordered then, entropy change will be more negative.
When there is an increase in gaseous molecules, entropy change increases.
How is the enthalpy change of reaction calculated.
(Sum of the standard entropies of products)- (Sum of standard entropies of reactants)
Free energy (3)
Overall change in energy during a reaction.
Change in free energy is measured through the Gibbs’ equation:
= Enthalpy change- [Temperature (K) x Entropy change]
Describe the condition for feasibility of a reaction. (2)
There must be a decrease in free energy in order for a reaction to be feasible.
Change in free energy < 0
Why may the Gibbs’ equation not be reliable
Some reactions may have a very slow rate, does not mean that it is not feasible.
Why is it difficult to measure lattice enthalpy directly?
Difficult to vapourise ions and measure the enthalpy change at the same time
