Enthalpies Flashcards
What is Hess’ law of constant heat summation?
It states that enthalpy changes are independent of the reaction pathway - as enthalpy is a state function.
Reactants and products must have same conditions
What is the enthalpy change of formation?
It’s the enthalpy change that accompanies the formation of 1 mol of compound in its standard state from its constituent elements in their standard states.
What’s the enthalpy change of formation of an element?
0 KJ/mol
How do you calculate the enthalpy change of reaction?
∆Enthalpy = sum of products - sum of reactants
(Sum of enthalpy change)
Give an example of the enthalpy change of vaporisation
H2O(l) —> H2O(g)
Give an example of the enthalpy change of sublimation
H2O(s) —> H2O(g)
Give an example of the enthalpy change of bond dissociation
Cl2(g) —> 2Cl(g)
Give an example of the enthalpy change of atomisation
Na(s) —> Na(g)
Give an example of the enthalpy change of ionisation
Na(g) —> Na+(g) + e-
Give an example of the enthalpy change of hydration
Na+(g) —> Na+(aq)
Give an example of the enthalpy change of lattice dissociation
NaCl(s) —> Na+(g) + Cl-(g)
Define lattice dissociation enthalpy
Is the enthalpy change that accompanies the dissociation of an ionic solid into gaseous ions
AB(s) —> A+(g) + B-(g)
Is lattice dissociation enthalpy exothermic or endothermic?
Lattice dissociation enthalpy is ALWAYS endothermic
- and hence always positive
What’s the reverse of lattice dissociation?
Lattice formation - hence always exothermic
How do you find the value of lattice dissociation enthalpy?
Construct a Hess cycle that separates ionic solid into its elements
Assuming ∆Enthalpy are known, the sum will be equal to the lattice dissociation enthalpy.
What is d0 in the Coulomb energy (Vc) equation?
d0 = equilibrium bond length
What is d* in the Coulomb energy (Vc) equation?
d* = 34.5pm
(1pm = 1 x 10^12)
Why does NaCl form an ionic solid and not a diatomic molecule?
Because ionic bonding is non-directional - opposite ions attract and surround each other as much as possible, the inverse is the same for same charge ions
This gives a crystalline compound with a regular periodic structure
What is the Madelung constant and what does it mean?
The Madelung constant A, expresses the relative stability of an ionic lattice of a given structure type to that of a single ion pair
What is the relative stability of a single ion pair AB in terms of the Madelung constant?
1
What is the equilibrium distance equal to the sum of?
d0 = the sum of ionic radii
d0 = rA + rB
What’s the benefit of using the Born-Mayer equation instead of Born-Haber cycles to find enthalpies?
The Born-Mayer equation gives a more accurate result since it is based on a simple ionic model that uses tabulated radii
Born-Haber cycles use enthalpy changes which are prone to experimental errors - less accurate
What do larger/smaller ions result in?
Larger ions result in lower enthalpy
Smaller ions result in higher enthalpy
What also increases the energy released when the ionic structure is broken?
A greater difference in electronegativity = a greater energy released
(Greater energy required to separate the strongly bound ions)
What lattice enthalpy relationship can be used to drive chemical reactions?
The inverse relationship between lattice enthalpy and the sum of the ionic radii means that combinations of small ions have disproportionally higher lattice enthalpies
Why does the thermal stability of carbonates increase down group 2?
Because the metal oxides corresponding with the metal carbonates (MgCO3 —> MgO) are much are stable due to the ions being much smaller
Thermal stability of carbonates increases because the inverse is true as well - less stable oxide = more stable carbonate
