Topic 13: Energetics II 1️⃣ Flashcards
Define standard lattice enthalpy change
Energy change when 1 mole of the ionic solid is formed from its gaseous ions
@standard conditions
Which 2 factors affect the magnitude of lattice energy ?
- magnitudes of the charges on the ions
- the sum of the ionic radii
As the ionic radius of the ions increases, what is the effect on the lattice energy ?
Lattice energy becomes less exothermic
As the ionic charge of the ions increases, what is the effect on the lattice energy ?
Lattice energy gets more exothermic
Define the standard enthalpy change of atomisation
Energy change measured at a standard conditions when 1 mole of gaseous atoms is formed from an element in its standard state
Define 1st electron affinity
Energy change when each atom in 1 mole of gaseous atoms gains an electron to form one mole of gaseous 1- ions
What are the directions of the arrows representing the following in Born Harber Cycles?
- enthalpy change of formation
- enthalpy change of atomisation
- enthalpy change of ionisation
- electron affinity
- lattice enthalpy
Enthalpy change of formation = downwards
Enthalpy change of atomisation = upwards
Enthalpy change of ionisation = upwards
Electron affinity = downwards/ upwards
Lattice enthalpy = downwards
What does the experimental lattice energy refer to
The value of the lattice energy calculated using the born haber cycle
What 3 assumptions must be made before calculating theoretical lattice energy
- the ions are all in contact with one another
- the ions are perfectly spherical
- ion charge is evenly distributed
The greater the percentage difference between the experimental and theoretical values…
The father away from the ‘ionic’ side of the scale the lattice is.
ie. the lattice has more covalent character
Describe & explain what we mean by covalent character of a lattice
A region where electrons are existing in an area of orbital overlap.
why?
In an ionic lattice the cation will attract the electrons of the anion.
If the electrons are pulled towards the cation, the anion is said to be polarised because the even distribution of its electron density has been distorted
What are the 2 factors that change the extent to which an anion is polarised ? (Fajan’s rules)
- high charge and small size of the cation
- high charge and large size of anion
If the experimental and theoretical lattice energy values are similar what do we say about the lattice?
The lattice is nearly 100% ionic
Define the enthalpy change of solution
Energy change when 1 mole of ionic solid completely dissolves in water (to form an infinitely dilute solution)
Eg. Eg. NaCl(s) —-> Na+ (aq) + Cl- (aq)
Define the enthalpy change of hydration
Energy change when 1 mole of gaseous ions are completely hydrated by water (to form an infinitely dilute solution)
Eg. Na+ (g) —-> Na+ (aq) & Cl- (g) —-> Cl- (aq)
What factors affect the magnitude of hydration enthalpy ?
Charge of ions: greater charge = stronger interaction with water molecules = more negative hydration value
Size of ions: larger ion = weaker attraction with water molecule = less negative hydration value
Describe the enthalpy diagram that describes the relationship between solution, hydration and lattice enthalpies
———Ions (g) ————
|. lattice. |
|. |
Solid compound. |hydration
|. solution. |
|. |
Ions (aq) ——————
Define entropy
A property of matter that is associated with the degree of disorder, or degree of randomness, of the particles
Define a spontaneous process
One that takes place without continuous intervention from us
The greater the degree of disorder…
The greater the entropy
What is the 2nd law of thermodynamics ?
In a spontaneous process, the total entropy increases
What is total entropy ?
Give the equation
Defined as the sum of the entropy change of the system and the entropy change of the surroundings
ΔS(total) = ΔS(system) + ΔS(surroundings)
What is the symbol for entropy
S
How can we use ΔS(total) to determine whether a reaction is spontaneous or not ?
If ΔS(total) is positive the reaction is spontaneous
How can you calculate the entropy change of system (ΔS(system))
ΔS(system) = ∑S(products) - ∑S(reactants)
How would you calculate the entropy change of the surroundings (ΔS(surroundings))
. ΔH
ΔS(surroundings) = - —-
. T
T is temp in Kelvin
ΔS(total) will be positive in what 3 circumstances?
- both ΔS(surroundings) & ΔS(system) are positive
- ΔS(surroundings) is positive & ΔS(system) is negative but magnitude of ΔS(surroundings) > ΔS(system)
- ΔS(surroundings) is negative and ΔS(system) is positive but magnitude of ΔS(surroundings) < ΔS(system)
Role of temp on entropy
Entropy increases when heat is applied however with a greater degree if heat is applied to a cold object compared to an object that is already hot
Give the equation for Gibbs energy, ΔG
ΔG = ΔH - T ΔS(system)
T is temp in Kelvin
How can we determine whether or not a reaction is feasible using Gibbs ?
If ΔG is positive = reaction is not feasible
If ΔG is negative = reaction is feasible
If ΔG is 0 = reaction is in equilibrium
Give the equation that relates ΔG and the equilibrium constant K
ΔG = - RTlnK
rearranged to…
K = e ^ (- (ΔG ÷ RT))
When using the Gibbs/K equation if ΔG is negative…
K will be greater than 1 = products are favoured
When using the Gibbs/K equation, if ΔG is positive…
K has a value of less than 1 = reactants are favoured
Reactions can be thermodynamically feasible and yet still not react if…
The activation energy is not met
How can we use K and Gibbs to predict solubility of salts
If K is large the equilibrium shifts to the products = soluble
If K is small the equilibrium shifts to the reactants = sparingly soluble
