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)
