Energetics I & II

Question 1

Lattice Energy/Enthalpy

Answer 1

The amount of energy released when one mole of a solid ionic lattice is formed from its ions in the gaseous state

(lattice formation)

Always negative

Question 2

Enthalpy Change of Atomisation

Answer 2

Enthalpy change when an element is converted into one mole of free gaseous atoms

If the element is a solid, the enthalpy of atomisation is the same as the enthalpy of sublimation

If the element is a diatomic molecule, the atomisation energy is half the bond dissociation energy

Question 3

Electron Affinity

Answer 3

The energy change per mole when an electron is added to a gaseous atom to form a gaseous anion

Question 4

Ionisation Enthalpy

Answer 4

Amount of energy required to remove completely an electron from one mole of gaseous atoms to form one mole of gaseous +1 cations

Question 5

Bond Dissociation Enthalpy

Answer 5

The amount of energy required per mole to break a covalent bond in the gas phase forming two gas-phase free radicals

The mean bond enthalpy is an average of the bond dissociation enthalpy measured in a variety of different compounds

Question 6

Lattice Enthalpy and Ionic Lattices

Answer 6

Lattice enthlapy is a measure of the strength of the electrostatic force between the ions in an ionic solid.

• the greater the lattice enthalpy, the stronger the electrostatic forces
• the forces are only completely broken when the ions are present as gaseous ions, that are so far apart that there is negligable attraction between them

Question 7

Factors affecting the strength of an Ionic Bond

Answer 7

Ionic Radius

• Smaller ions pack closer together so the electrostatic forces of attraction are much stronger so the lattice enthalpy is more exothermic
• The larger the ions, the further apart they are in the ionic lattice, so the electrostatic forces are weaker
• smaller radius = more exothermic LE

Ionic Charge

• As the size of the charge increases, the electrostatic forces of attraction get stronger
• As the charge increases,thedecreasein ionic radius produces agreater attractionbetween the positive and negative ions, bringing the ions in the latticecloser together, producingstronger electrostatic attractions
• greater charge = more exothermic LE

Question 8

Enthalpy Change of Solution

Answer 8

The enthalpy change when one mole of a compound dissolves to form a solution of infinite dilution under standard conditions

Question 9

Enthalpy Change of Hydration

Answer 9

The enthalpy change when one mole of isolated gas ions is dissolved in water to give and infinitely dilute solution under standard conditions

It is an exothermic process that is affected by ionic radius and ionic charge

• as ionic radius decreases, the enthalpy of hydration becomes more exothermic as small ions will approach closer to the water molecules, to the small ions form stronger electrostatic forces of attraction so more energy is released when hydrated
• as the charge on an ion increases, the ion formsstronger forces of attractionwith the water molecules somore energyis released on hydration of ions with ahigher charge

Question 10

Polarisation of Ions

Answer 10

Polarisation is caused by the distortion of a negative ion by a positive ion

When a polarising effect is placed on an anion, it can distort the charge cloud, but the Born-Haber lattice enthalpy and theoretical values will differ

The polarising power of a cation increases when:

• the positive ion has a smaller ionic radius
• the positive ion has a larger charge
• charge density

The polarisability of an anion depends on:

• its size - the bigger the ion, the more easily it is distorted

Question 11

Theoretical VS Experimental

Answer 11

Theoretical lattice enthalpies assume a perfect ionic model when the ions are 100% ionic and spherical and the attractions are purely electrostatic

The born-haber lattice enthalpy is the real experimental value

When 100% ionic, the ions are spherical and the experimental and theoretical values are the same, however, when a polarising effect is placed on an anion, there will be some covalence and the values will differ.

Question 12

Constructing Born Haber Cycles

Answer 12

1. construct cycle using lattice enthalpy and enthalpy change of solution
2. add in the hydration enthalpies for both ions

Question 13

Entropy of Surroundings

Answer 13

∆S_surr = -∆H/T

Question 14

Total Entropy Change

Answer 14

∆S_tot = ∆S_sys + ∆S_surr

Question 15

Entropy of System

Answer 15

∆S_sys = ΣS_products - ΣS_reactants

Question 16

Free Energy Change

Answer 16

∆G = ∆H - T∆S_system

The free energy change is the balance between enthalpy, entropy and temperature for a process

For all spontaneous processes, ∆G must be negative

The total entropy of the universe always tends to increase, it never goes down

Question 17

∆G Feasability

Answer 17

Sometimes a reaction in which ∆G is negative may not occur if it is not observed at the stated temperature.

The activation energy for the reaction may be too high and so the rate of reaction is so slow that the reaction is not observed

∆G must be negative for a reaction to be feasible

Question 18

Entropy

Answer 18

Entropy is a measure of the disorder of a system

The natural direction of change is increasing total entropy (positive entropy change)

Since some endothermic reactions can occur at room temperature, enthalpy changes alone do not control whether reactions occur

Question 19

Enthalpy Change of Formation

Answer 19

The enthalpy change when one mole of a compound is formed from its elements under standard conditions with the element and the compound in their standard states

Question 20

Enthalpy Change of Reaction

Answer 20

The energy transferred when the molar quantities of reactants as stated in the equation react under standard conditions

Question 21

Enthalpy Change of Combustion

Answer 21

The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions

Question 22

Enthalpy Change of Neutralisation

Answer 22

The enthalpy change when the acid and the alkali in the equation for the reaction neutralise each other under standard conditions to form one mole of water

Question 23

Hess’ Law of Combustion

Answer 23

Question 24

Hess’ Law Formation

Answer 24

Question 25

Bond Enthalpy

Answer 25

Bond enthalpy is the energy required to break one mole of gaseous bonds (endothermic)

If the molecule has several bonds rather than just 1, we refer to mean bond enthalpy.

E.g. with methane, a different amount of energy would be needed to break each of the four C-H bonds (every time a hydrogen is broken off the strength of the bond changes)