Chapter 22: Lattice Enthalpy, Entropy And Free Energy Flashcards
Definition of Lattice Enthalpy
The enthalpy when one mole of an ionic compound is formed from its gaseous ions under standard conditions.
Write the lattice enthalpy equation for KCl
K^+(g) + Cl^-(g) ——> KCl (s)
Is lattice enthalpy endothermic or exothermic and why?
Exothermic, because it involves the ionic bond formation between the gaseous ions.
Definition of Standard Enthalpy Change of Formation.
The enthalpy change when one mole of a compound is formed from its elements under standard conditions with all the reactants and products under standard conditions.
It is exothermic, because bonds are formed between the elements.
Write an equation for the standard enthalpy change of formation of NaCl.
Na(g) + 1/2Cl2 ——-> NaCl (s)
Definition of the Standard Enthalpy Change of Atomisation
The enthalpy change that occurs when one mole of gaseous atoms are formed from its element in its standard states under standard conditions.
It is endothermic because energy is required to break the bonds to form the gaseous atoms.
Write equations for the standard enthalpy change of atomisation for Na and Cl
- Na(s) ——> Na(g)
- 1/2Cl2 (g) —- -> Cl (g)
Definition of first ionisation energy
The enthalpy change when one electron is removed from each atom of one mole of gaseous atoms to form 1 mole of gaseous +1 ions
This is endothermic, because energy is required to overcome the electrostatic attraction between the negatively charged electron and the positively charged nucleus.
Write an equation for the first ionisation energy for K.
K(g) ——> K^+(g) + e^-
Definition of First Electron Affinity.
The enthalpy change that occurs when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.
The first electron affinity is exothermic because the electron is attracted to the positive nuclear so energy is released.
The second electron affinity is endothermic because a negatively charged electron is being added to a negative ion so energy is needed to overcome the repulsion.
Write an equation for the first and second electron affinity of Ca
Ca(g) ——> Ca^+ (g)
Ca^+ (g) ——> Ca^2+ (g)
Definition of the standard enthalpy change of solution
The enthalpy change that occurs when 1 mole of a solute dissolves in a solvent.
Can be either endothermic or exothermic.
Write and equation for the enthalpy change of solution for NaCl.
Na^+Cl^- (s) + aq ——> Na^+ (aq) + Cl^-(aq)
Draw the attraction of water molecules around Na+ and Cl-
(check textbook)
Experimental determination of enthalpy change of solution
- Weigh mass of sample
- pour a known volume of water into a plastic cup (e.g 25cm3).
- Measure the temperature of the water.
- Tip all the sample into the water and stir with the thermometer until all the sample has dissolved and the temperature no longer changes.
- Record the final temperature.
Calculation for the experimental determination of the enthalpy change of solution
- Equation: Q=MC(delta)T
- use mass of solution (mass of water+ mass of sample)
- Specific heat capacity: 4.18
-Change in temperature: final temp of solution- initial temp of water.
Definition of enthalpy change of hydration
The enthalpy change that accompanies the dissolving of gaseous ions to form one mole of aqueous ions.
Exothermic
Write the equations for the enthalpy change of hydration for Na+ and Cl-
Na+ (g) + aq ——> Na+ (aq)
Cl- (g) + aq ——> Cl- (aq)
Factors affecting Lattice enthalpy
Ionic Size:
- ionic radius increases
- attraction between ions decreases
- lattice enthalpy is less negative
- melting point decreases.
Ionic Charge:
- ionic charge increases
- attraction between ions increases
- lattice enthalpy becomes more negative
- melting point increases.
The more exothermic the lattice enthalpy value, the more likely it is for the compound to have a higher melting point.
Factors affecting lattice enthalpy change of hydration
Ionic size:
- ionic radius increases
- attraction between ions and water molecules decreases
- hydration enthalpy is less negative
Ionic charge:
- ionic charge increases
- attraction with water molecules increases
- hydration enthalpy becomes more negative.
If the enthalpy of hydration is greater than lattice enthalpy, the compound should dissolve. But some compounds with endothermic enthalpies are still soluble- it depends of feasibility and entropy.
Definition of entropy
The dispersal of energy within the chemicals making up a system. Units: JK-1mol-1.
Changes of state and entropy
Entropy will increase when a species changes from a solid to liquid to gas as there is more disorder- the randomness of the particles increase so the energy is more spread out and entropy increases.
Predicting entropy changes
- at 0 kelvin, there is no energy in a system.
- positive entropy change = more disorder
- negative entropy change= less disorder
Standard entropies
- always positive value
- at 298k and 100Kpa
