5.2 Energy Flashcards
Define lattice enthalpy
enthalpy change associated with the formation of one mole of an ionic lattice from its gaseous ions under standard conditions K+(g) +Cl- –> KCl(s)
Define enthalpy change of formation
The enthalpy change associated with the formation of one mole of a compound from its constituent elements in their standard conditions
How do lattice enthalpy values indicated the relative strength of ionic bonds
More exothermic lattice enthalpy values mean stronger ionic bonds and higher melting and boiling point
It also indicates smaller ions and larger charges
What can we use to measure lattice enthalpy
Born - Haber cycles because we can’t measure lattice enthalpy directly
What are the key features of a born haber cycle
continuous cycle
includes on step that shows the formation of one mole of the solid ionic lattice from the gaseous atoms
the remaining steps show intermediate changes
the lattice enthalpy can be calculated by applying hess’ law
Define the enthalpy change of atomisation
The enthalpy change associated when one mole of gaseous atoms is formed from its element in its standard state
K(s) –> K(g)
Define first ionisation energy
When one mole of gaseous 1+ ions is formed from its gaseous atoms
K (g)- e –> K+(g)
Define second ionisation energy
When one mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions
Mg+(g) - e- –> Mg2+(g)
Define the first electron affinity
when one mole of gaseous 1- ions is formed from gaseous atoms
Cl (g)+ e- –> Cl- (g)
Define second electron affinity
When one mole of gaseous 2- ions is formed from one mole of gaseous 1- ions
Describe the steps in a born haber cycle to determine lattice enthalpy of sodium chloride
atomisation of sodium
atomisation of chlorine
ionisation of sodium
electron affinity of chlorine
What happens when a solid dissolves
The ionic lattice breaks down
the free ions become a part of the solution (hydration )
Define the standard enthalpy of solution
The enthalpy change that take place when one mole of a solute is completely dissolved in water under standard conditions
KCl (s) = + aq –> K+ (aq) + Cl- (aq)
What is the enthalpy change associated with the breakdown of an ionic lattice
Lattice enthalpy of dissociation
How do lattice enthalpies differ energetically
Association is exothermic
Dissociation is endothermic
What dictates the size of the lattice enthalpy
the size of the ions involved
the charge on the ions
How does ionic radius affect lattice enthalpy
smaller ions can get closer together and will attract one another more strongly and give rise to a more exothermic lattice enthalpy values
How does ionic charge affect lattice enthalpy
Ions with higher charges cause greater electrostatic attraction and in turn more exothermic lattice enthalpy values
hat ions have the most negative lattice enthalpy values
The smallest most highly charged ions will give rise to the most exothermic lattice enthalpies as they can pack closer to the oppositely charged ions with a higher attraction
Define the standard enthalpy of hydration
The enthalpy change that takes place when dissolving one mole of gaseous ions in water under standard conditions
K+ (g) –> K+(aq)
What occurs during hydration
The positive ions will be attracted to the slightly negative oxygen
The negative ions will be attracted to the slightly positive hydrogen
the water molecules will completely surround the ions
What kind of process is hydration
Exothermic
What dictates enthalpy of hydration
Ionic radius
ionic charge
How does ionic radius affect enthalpy of hydration
ions with a smaller radii are able to get closer to the water molecule and are able to attract them more strongly so on hydration more energy is released and becomes more exothermic
How does ionic charge affect enthalpy of hydration
The higher the charge on an ion the greater the attraction it will have on the water molecule giving a more exothermic value
define entropy
the quantitative measure of the degree of disorder in a system
Define standard entropy
The entropy content of one mole of a substance under standard conditions
define standard entropy change of a reaction
the entropy change that accompanies a reaction in the molar quantities expressed in the chemical reaction under standard conditions
what are the units for entropy
J/K/mol
what figure is entropy
Always a positive figure
When is entropy 0
At 0K in perfect crystals
What state has the highest entropy
gas
Why do gases have a higher entropy
more spreading out of energy and more random arrangement of molecules
How is entropy affected by temperature
entropy increases as temperature increases
How does entropy change when dissolving ionic solids
the reaction results in the products having more disorder
Ions can spread out and the positions of the ions are more disordered within the lattice
How does the number of gas molecules affect entropy
An increase in the number of gas molecules causes an increase in entroy
How do you calculate entropy change
entropy of products - entropy of reactants
How do you calculate total change in entropy
Entropy change of the system + entropy change of the surroundings
What do positive and negative entropy changes indicate
+ve - the change made the system more random
-ve - made the system more ordered
What is the Gibbs equation
ΔG = ΔH-TΔS
What must the value of ΔG must be for a reaction to occur spontaneously
negative
What do we assume Δ G is when calculating free energy
0
What are the limitations of using ΔG
whether or not a reaction happens also depends on factors such as
activation energy and the reaction might be very slow
What is a half cell
comprises of an element in its two oxidation states
The simplest has a metal placed in its aqueous sloution of ions
What is a hydrogen half cell
Contains hydrogen gas in contact with hydrogen ions
Why is a platinum electrode used in a hydrogen half cell
No solid piece of metal can act as the electrode except platinum as it is inert and does not react at all
Its sole purpose is to be in contact with both the H2 and H+ ions and allow transfer of electrons in and out of the half cell via a connecting wire
What are the standard conditions in a half cell
298k
100kPa
1mol/dm
What is the standard electrode potential of a half cell
the e.m.f. of a half cell compared to a standard hydrogen half cell measured at 298k with solution concentrations of 1mol/dm at 100kPa
what are metal ion/ metal ion half cells
This type of cells contain ions of the same element in different oxidation states for example Fe 3+ and Fe2+
this type of half cell requires a platinum electrode
Define equimolar
solution containing ions at same concentrations
How do we measure standard electrode potentials using a hydrogen half cell
the tendency for different half cells to accept or release electrons is measured as an electromotive force which is measured in volts
The hydrogen half cell can be used as a reference as it has an emf of 0V
What is a salt bridge
This connects the two solutions and allows ions carrying charge to be transferred between the half cells
Filter paper soaked in KNO3 or NH4NO3
What is the relationship between the E value and the metal
The more negative the value the greater the tendency towards the half cell undergoing oxidation (loss of electrons) so the reaction shifts to the left
Fe2+ + 2e- ⇌ Fe
whereas Cl2 +2e- ⇌ 2Cl- is positive so it is more likely to be reduced and shifts to the right
How do you make a cell from two half cells
Two half cells with different electrode potentials are joined using a wire and salt bridge
One half cell releases electrons and the other gains electrons
How would you determine which half cell is being reduced and oxidised
the half cell with the more negative value releases electrons more readily
How is a reading aquired in a cell
The negative terminal releases electrons into the wire and they flow to the other electrode
The reading of the voltmeter measures the difference between electrode potentials of the half cells,
The bigger the value the further away from the equilibrium position the reaction moves
How would you calculate E of the cell
E of positive terminal - E of negative terminal
What can we find out from reactions by using standard electrode potentials
a species will react with the species below it
