5.5 ENTHALPY AND ENTROPY Flashcards
What is lattice enthlapy of formation?
Lattice enthlapy of formation is the enthalpy change when 1 mole of a solid ionic compound is formed from it’s gaseous ions under standard conditions.
What is the enthalpy change of 1st ionisation energy?
The enthalpy change of 1st ionisation energy is the enthalpy change when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms.
What effect does the size of charge have on ionic bonding?
The size of the charge on an ion affects the strength of the ionic bond. The bigger the charge on an ion, the stronger the electrostatic attraction between ions. Therefore more energy is required to overcome these forces and so they have a high melting and boiling point.
e.g KCl has a melting point of 770°c (K+ and Cl-), CaO has a melting point of 2572°c (Ca2+ and O2-)
What effect does ionic radius have on ionic bonding?
The ionic radius/size of ion affects the strength of the ionic bond. The smaller the ion, the stronger the electrostatic forces of attraction between ions. Smaller ions can pack together more closely and more energy is required to overcome the stronger forces. So, melting and boiling points are higher.
e.g NaCl has a melting point of 801°c (Na+ and Cl-), KCl has a melting point of 770°c (K+ and Cl-).
What is charge density?
Ions that have a smaller ionic radius and higher charge have a high charge density.
Why is the second electron affinity of oxygen endothermic when the first is exothermic?
The second electron affinity of oxygen endothermic when the first is exothermic because energy is needed to add an electron to a negative ion. O- will repel the negative electron.
What is entropy?
Entropy is the measure of disorder in a system. It is the number of ways energy can be shared out between particles. The more disorder there is, the higher the level of entropy.
What are the levels of entropy?
Entropy levels:
Solids have the lowest level of disorder, particles are arranged neatly in rows. Liquids and then gases are more disordered. Gases have the highest entropy.
How does number of particles affect enthalpy?
Number of particles affects enthalpy change. If a reaction is in the same state but more moles are produced, then entropy increases. There are more ways energy can be distributed.
Spontaneous reactions.
A reaction can be spontaneous (feasible) even if it is enthalpically unfavourable (e.g endothermic). A reaction will tend towards more disorder and hence increase entropy. Increasing entropy is energetically favourable and some reactions that are enthalpically unfavourable (endothermic) can still spontaneously react if changes in entropy overcome changes in enthalpy.
How is entropy change calculated between reactants and products?
Entropy change = entropy products - entropy reactants
How is Gibbs free energy calculated?
Free energy = enthalpy change - temperature (K) x entropy change
What are the units for entropy?
Units for entropy (S) are JK-1mol-1
What are the units of free energy change?
Units of free energy change are JK-1mol-1
What does Gibbs free energy state about feasible reactions?
If Gibbs free energy is negative or zero, a reaction is feasible in theory.
What effect does temperature have on free energy?
Temperature on free energy:
negative enthalpy + positive entropy = gibbs always negative, always feasible reaction
positive enthalpy + negative entropy = gibbs always positive, never feasible reaction
negative enthalpy + negative entropy = gibbs negative at lower temps, feasible at lower temps
positive enthalpy + positive entropy = gibbs negative at higher temps, feasible at higher temps
What is the standard enthalpy change of solution?
Standard enthalpy change of solution is the enthalpy change that takes places when one mole of compound is completely dissolved in water under standard conditions.
born-haber:
lattice enthlapy one pathway, hydration of pos ion, hydration of neg ion and solution other pathway
endo = enthalpy change of solution pos
exo = enthalpy change of solution neg
What is the enthalpy of hydration?
The enthlapy of hydration is the enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions.
Why cant lattice enthalpy be measured experimentally?
Lattice enthalpy cant be measured exponentially:
- difficult to get gaseous ions without them reacting
- difficult to vaporise ions and measure the enthalpy change at the same time
