Thermodynamics Flashcards
Enthalpy changes + perfect ionic model
Exothermic processes
Bond forming
– enthalpy change
Endothermic processes
Bond breaking
+ enthalpy change
Enthalpy change of formation
+ formation of NaCl
The enthalpy change when 1 mole of a substance is formed from its component elements in their standard states
Na(s) + 1/2Cl2(g) –> NaCl(s)
Enthalpy change of lattice formation + Formation of NaCl
The enthalpy change when 1 mole of an ionic compound is formed from its component ions in a gaseous state
Na+(g) + Cl-(g) –> NaCl(s)
What factors affect strength of enthalpy change of lattice formation
Charge on ion
Size of ion
greater charge + smaller ion = more exothermic
Charge > size
What energy change is lattice formation
Exothermic, forms ionic bond
Enthalpy change of atomisation + atomisation of Na + Cl
The enthalpy change when 1 mole of gaseous atoms are FORMED
Na(s) –> Na(g)
1/2Cl2(g) –> Cl(g)
What type of energy change is atomisation
Endothermic, breaking bonds of attractions
1st ionisation energy
+ 1st ionisation energy of Na
The enthalpy change to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
Na(g) –> Na+(g) + e-
What type of energy change is ionisation energy
Endothermic, overcoming electrostatic bond between nucleus and outer electron
What factors affect strength of ionisation energy
Nuclear charge, size of ion, electron shielding
Bond dissociation energy + Bond dissociation energy of Cl2
The enthalpy change when 1 mole of covalent bonds are broken in the gaseous state
Cl2(g) –> 2Cl(g)
1st electron affinity + 1st electron affinity of Cl
The enthalpy change of to add 1 mole of electrons to 1 mole of gaseous atoms to make 1 mole of gaseous ions with a 1- charge
Cl(g) + e- –> Cl-(g)
What type of energy change is 1st electron affinity
Exothermic, forming electrostatic attraction between nucleus and ion
2nd electron affinity + 2nd electron affinity of O-
The enthalpy change of to add 1 mole of electrons to 1 mole of gaseous atoms with a 1- charge to make 1 mole of gaseous ions with a 2- charge
O-(g) + e- –> O^2-(g)
What type of energy change is 2nd electron affinity
Endothermic, although a bond is being formed, negatively charged electrons are being added to a negatively charged ion which causes forces of repulsion, requiring energy to form the bond
What are perfect ionic compounds
Ionic compounds with :
Perfect spheres
No covalent character
What is meant by covalent character?
Where the electron clouds in an ionic bond overlap, forming a weak covalent bond
If covalent character is present in an ionic bond, what will happen to the lattice enthalpy?
It will become more exothermic
What 2 factors affect the strength of an ionic bond?
Ionic charge
Ionic radius
What does theroretical lattice enthalpy assume?
That the ionic compounds resemble the perfect ionic model
What does it mean if the theoretical and experimental value of lattice enthalpy are similar?
Compound obeys the perfect ionic model
What does it mean if the theoretical and experimental value of lattice enthalpy are not similar?
The bonding is stronger and so contains covalent character, does not obey perfect ionic model
Why do some ionic compounds contain covalent character?
If the positve ion is polarising, the electron cloud is pulled from the negative ion, distorting the ion and overlapping clouds
What factors increase the polarising ability of the + ion in an ionic compound?
Small ionic radius
Large ionic charge
What factors increase the polarisability of the - ion in an ionic compound?
Larger ionic radius
Low ionic charge
When considering factors affecting the polarising of an ionic bond, is the charge or radius of the ions more significant?
Charge is a more significant factor to consider.
What is entropy?
The measure of disorder in a system
What factors affect the entropy of a system
States - Gas>liquid>solid (gas causes greater entropy)
Greater Mr = greater entropy
Number of species/ more species = greater entropy
How is entropy change calculated?
Entropy of products - reactants
What is the gibbs free - energy equation?
ΔG = ΔH - TΔS
T must be in kelvin
ΔS must be KJ K-1 mol-1
What does ΔG mean + its units
If the value is 0 or below, a reaction is feasible at that temperature
Kjmol^-1
Why may some reactions not occur even if the reaction is deemed feasible?
Activation energy is too high
How can you use a graph to calculate gibbs free energy?
ΔG = ΔH - T ΔS
Y = -mx + c
ΔG = - ΔST + ΔH
Where -ΔS = gradient
What is enthalpy of solution + Enthalpy of solution of MgCl2
The enthalpy change when 1 mole of an ionic substance is completely dissolved in water to infinite dilution
MgCl2(s) –> Mg^2+(aq) + 2Cl-(aq)
What is enthalpy of hydration + enthalpy of hydration of Mg^2+
The enthalpy change when 1 mole of gaseous ions is diluted to 1 mole of aqueous ions
Mg^2+(g) –> Mg^2+(aq)
What is lattice dissociation enthalpy + equation using MgCl2
The enthalpy change when 1 mole of an ionic solid is converted into gaseous ions
MgCl2(s) –> Mg^2+(g) + 2Cl-(g)
What is the formula to find enthalpy change of solution?
ΔH solution = ΔH L.D + ΔH hydration of both ions
What does it mean if the enthalpy change of L.D is more than hydration?
Solution enthalpy will be endothermic
What does it mean if the enthalpy change of L.D is less than hydration
Solution enthalpy will be exothermic
What factors can affect hydration enthalpy?
Size of ion - smaller means more exothermic
Charge of ion - greater means more exothermic
What factors can affect lattice dissociation
SIze of ion - smaller means more endothermic (stronger electrostatic forces need more energy to overcome)
Charge of ion - larger means more endothermic (requires more energy to overcome)
Why are ionic compounds with very endothermic lattice dissociation enthalpies insoluble?
Lots of energy needed to break the electrostatic forces.
What is the nature of attraction when talking about enthalpy of hydration
The forces between the partial charges on the water molecule and the ions
How does the enthalpy of solution affect the solubility of the compound?
If enthalpy of solution exothermic, solubility decreases as temperature increases
If enthalpy of solution is endothermic, solubility increases with temperature
