✅13 - Energetics II Flashcards
What is standard lattice energy?
The energy change when one mole of ionic solid is formed from its gaseous ions under tankard conditions.
What is the equation for standard lattice energy for NaCl?
Na+(g) + Cl-(g) —-> NaCl(s)
What are the factors affecting the magnitude of lattice enthalpy?
The charge on the ion
The amount of cation-anion interactions in the molecule
The distance between the centres of the cations and the anions
What is the standard enthlapy change of atomisation?
The enthlapy change when one mole of gaseous atoms is formed from an element in its standard state.
What is the equation for the atomisation of Na?
Na(s) —-> Na(g)
What is electron affinity?
The first energy change is the change when each atom in one mole of atoms in the gaseous state gains an electron to form a -1 ion
What is the equation for the first electron affinity of Chlorine?
Cl(g) + e- —-> Cl-
Which enthlapy change goes on the bottom of the Born Haber cycle?
The enthlapy change of formation
What is experimental lattice energy?
That calculated from a Born-Haber cycle
What is theoretical lattice energy?
The value found from X-ray crystallography
What are the assumptions made with theoretical lattice energy?
The ions are in contact with one another
The ions are perfectly spherical
The charge on each ion is evenly distributed around the centre so that each ion can be considered as point charges
What does an agreement between theoretical and experimental lattice energies indicate?
That the ionic model is good for the compound, so it has good ionic character.
What does a significant difference between vales for experimental and theoretical lattice energies indicate?
That the compound has significant covalent character
When as an ion polarised?
When the electrons are pulled towards the cation and the distribution of electron density is distorted
What factors increase polarisation?
A high charge and small size of the cation
A high charge and large size of the anion
Which ions are polarised most easily?
Anions with a large charge and large size
What is the enthalpy change of solution?
The energy change when one mole of an ionic solid dissolves in water to form an infinitely dilute solution
Upon dilution, how do the ions move?
They move apart (endothermic) and become more hydrated (exothermic)
How is the value for enthalpy change of solution found?
The value cannot be found experimentally so is found by a process of extrapolation.
What is the point of infinite dilution?
The point where further dilution has no measurable effect on the enthalpy change of solution.
What is the enthalpy change of hydration?
The enthalpy change when one mole of an ion in a gaseous state is completely hydrated in water
In practice, when has complete hydration occurred?
When the solution formed is of infinite dilution
What is ion-dipole interaction?
The interaction as a result of the attraction between the delta - oxygen atom of the water molecule and the cation.
What are the factors affecting the magnitude of hydration enthalpy?
The charge on the ion
The ionic radius
How does ionic radius affect hydration enthalpy?
As the ions get larger,the attraction between them and the water molecule decreases and the enthalpy of hydration decreases
How does charge affect hydration enthalpy?
The higher the charge, the more negative the value
What makes the products of a reaction energetically stable?
If they have less energy then the reactants
What is a spontaneous process?
One which takes place without continuous intervention from us.
What is entropy?
A property of matter that is associated with the degree of disorder, or degree of randomness, of the particles
What is the second law of thermodynamics?
In a spontaneous reaction, the total entropy increases.
What is the total entropy change?
The entropy change of the system and the entropy change of the surroundings
What is the system in a chemical reaction?
The species taking part in the reaction
What must the total entropy change be fro a reaction to be spontaneous?
Positive
What is the equation for the entropy change of the system?
S(products) - S(reactants)
What is the equation for entropy change of the surroundings?
-ΔH / T
What will ΔSsurroundings be for an exothermic reaction?
Positive
What would ΔSsurroundings be for an endothermic reaction?
Negative, so the entropy decreases
Ice has a lower entropy than water so ΔS system is…
…negative
If a process is exothermic, ΔS surroundings is…
…Positive
How does an increase in the number of moles change the entropy?
Increasing moles increases the entropy
What changes take place when ionic solids dissolve in water?
The lattice is broken down
The ions become hydrated
Is the hydration of ions endothermic or exothermic?
Exothermic
How is the solubility of an ionic salt determined?
By its entropy change
If a salt is soluble, is entropy positive or negative?
Positive
What is the expression for Gibbs energy?
∆G = ∆H - T x ∆S system
Is ∆G positive or negative if the reaction is spontaneous?
Negative
If ∆G = 0, the reaction is…
…in equilibrium
When will ∆G be negative?
∆H < 0 and ∆S > 0
∆H < 0 and ∆H > T ∆S
∆H > 0 and ∆H < T ∆S
When is ∆G positive?
∆H > 0 and ∆S < 0
∆H > 0 and ∆H > T ∆S
∆H < 0 and ∆H < T ∆S
What is the equation for the relationship between K and ∆G?
∆G = -RTlnK
How can K be calculated using ∆G?
K = e^(-∆G/RT)
If ∆G is negative, K is…
… greater than 1, so the products are favoured
If ∆G is positive, K is…
…less than 1, so the reactants are favoured
If ∆G is positive, can all the reactants be converted into products?
No, it is impossible
How can K be calculated from temperature?
lnK = -∆H/RT + ∆S system/R
If the reaction is exothermic and temperature is increased, what happens to K?
It decreases, reaction shifts to the left
If the reaction is endothermic and temperature increases, what happens to K?
It increases, equilibrium shifts to the right
Why may a reaction not take place spontaneously even if ∆G is negative?
If the activation energy is very high and cannot be reached spontaneously, the reactants are kinetically stable
When is a salt soluble in terms of ∆Gsol?
When ∆Gsol is negative, and the products are favoured at equilibrium
What is the solubility product?
The equilibrium constant, Ksp
How are Ksp and ∆solG linked?
∆solG = -RTlnKsp
How are ∆solG, ∆solH and ∆S system linked?
∆solG = ∆solH - T ∆S system
When will Ksp indicate that a salt is soluble?
When it has a large value, so equilibrium lies to the right and products are favoured.
If a salt is sparingly soluble, what will its values be?
∆solG will be positive and Ksp will be very small
What sign does ∆G have for a strong acid?
-
What sign does ∆G have for a weak acid?
+
