Lattice Enthalpy and Free Energy Flashcards
Definition of second ionisation energy
The Enthalpy change accompanying the removal of an electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
How does enthalpy change of hydration vary across a period (of cations) ?
Charge on the cation gets larger across the period and the ions get smaller, the ionic charge is concentrated into a smaller volume so charge density increases. Attraction and bonding to water molecules will be stronger, more energy released as the hydrated ion is formed.
Standard entropy change of reaction definition
The entropy change that accompanies a reaction in the molar quantities expresses in a chemical equation under standard conditions, all reactants and products being in their standard states
Relationship between lattice enthalpy and electrostatic attraction
Lattice enthalpy is a measure of the strength of the electrostatic forces of attraction between the oppositely charged ions. The more energy released, so the more exothermic the lattice enthalpy, the stronger the electrostatic attraction
Why might feasible reactions not actually occur?
Activation energy not been reached
Features of lattice enthalpy
-Measuring it
Cannot be measured directly as it is impossible to form one mole of an ionic lattice from gaseous ions
What are the factors which affect electrostatic attraction?
Ionic size
Ionic charge
What does it mean if
a) Entropy change of reaction is positive
b) it is negative
a) The change has made the system more random
b) The change has made the system more ordered
Features of lattice enthalpy
- Strength of ionic lattice
A large negative value for lattice enthalpy shows that there are strong electrostatic forces of attraction between the oppositely charge ions
Entropy definition
S - the quantitative measure of the degree of disorder in a system
Definition of Enthalpy change of hydration
The Enthalpy change that takes place when one mole of isolated gaseous ions is dissolved in water forming one mole of aqueous ions under standard conditions
Units of entropy change
kJ mol^-1
Definition of second electron affinity
The Enthalpy change that accompanies the addition of one electron to each ion in one mole of gaseous 1- ions to form one mole of gaseous 2- ions
In what conditions do many exothermic reactions take place spontaneously?
At room temp.
Enthalpy content of chemical system decreases during the reaction, excess energy is released into the surroundings, increasing stability
Definition of standard Enthalpy change of atomisation?
Enthalpy change which takes place when one mole of gaseous atoms is formed from its element in its standard state
What factors are taken into account when looking at ionic size?
Charge and radius
Units of enthalpy change
kJ mol -1
Definition of standard Enthalpy change of solution
The Enthalpy change that takes place when one mole of a compound is completely dissolved in water under standard conditions
What does feasible mean?
Consideration of enthalpy and entropy change means it is energetically possible for a reaction to take place
Free energy change definition
(Delta G)
= enthalpy change - temp. x entropy change
It is the balance between the enthalpy, entropy and temperature for a process.
A process can take place spontaneously when DG is less than 0
Requirements for an endothermic reaction to take place spontaneously
Entropy change must be positive and temperature must be high enough so that when multiplied by entropy change, it is greater than enthalpy change
Definition of lattice Enthalpy
The Enthalpy change that accompanies the formation of 1 mole of an ionic compound from its gaseous ions under standard conditions
How does lattice enthalpy change as the charge density on decreases?
As the charge density gets smaller, the electrostatic attraction between the ions decreases, lattice enthalpy becomes less exothermic
Definition of first electron affinity
The Enthalpy change accompanying the addition of an electron to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
Definition of first ionisation energy
The Enthalpy change accompanying the removal of an electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
How does enthalpy change of hydration vary down group 1 metal ions and why?
The cations get larger down the group, ionic charge is spread over a larger volume and so charge density decreases. Reduced attraction to waer molecules, weaker bonds formed. Therefore, less energy would be expected to be released as the hydrated ion is formed
What is a spontaneous process
A process which proceeds on its own, leading to lower energy and increased stability
What is the relationship between the charge on the ion, the size of the ion and the charge density?
Charge density is directly proportional to the charge on the ion. It is inversely proportional to the size of the ion.
How does enthalpy change of hydration vary down the group 7 halogen ions?
Ion gets larger so its ionic charge is spread through larger volume and electron density decreases. Less attraction to water molecules, weaker bonds formed. Therefore less energy released as hydrated ion is formed.
Features of lattice enthalpy
- Sign
It is exothermic and has a negative sign because energy is given out when ionic bonds are being formed from gaseous ions
What is the sign of S
Always positive as all substances possess some degree od disorder because particles are always in constant motion
How does lattice enthalpy change as the ion increases in size?
Ionic charge is spread through a larger volume, hence the charge density decreases. The electrostatic attraction between the ions decreases and so the lattice enthalpy becomes less exothermic
How does lattice enthalpy change as the size of an ion decreases?
The ionic charge is concentrated into a smaller volume and so charge density increases. This means the electrostatic attraction between the ions increases and so lattice enthalpy also increases
