Chapter 22 - Lattice enthalpy Flashcards
What is lattice enthalpy the measure of ?
It is a measure of the strength of ionic bonding in a giant ionic lattice
Define lattice enthalpy
It is the enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
What type of energy change is lattice enthalpy ?
- It is an exothermic change
- The value will always be negative
Does lattice enthalpy involve bond making or bond breaking ?
- Bond making
- Ionic bond formation from separate gaseous ions
What is the Born-Haber cycle ?
It is a type of energy cycle that allows you to calculate lattice enthalpy indirectly
How many routes are there in the Born-Haber cycle ?
There are 2 routes in the Born-Haber cycle
What are the three different processes in route 1 ?
- Formation of gaseous atoms
- Formation of gaseous ions
- Lattice formation
What does the formation of gaseous atoms involve ?
- Changing the elements in their standard states into gaseous atoms
- This change is endothermic as it involves bond breaking
What does the formation of gaseous ions involve ?
- Changing the gaseous atoms into positive and negative gaseous ions
- Overall this change is endothermic
What does the process of lattice formation involve ?
- Changing the gaseous ions into the solid ionic lattice
- This is the lattice enthalpy and is exothermic
What is route 2 ?
- It converts the elements in their standard states directly to the ionic lattice
- There is only one enthalpy change
What is the enthalpy change for route 2 ?
- Enthalpy change of formation
- This is an exothermic enthalpy change
What is the standard enthalpy change of formation ?
- It is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
- With all reactants and products in their standard states
What is the standard enthalpy change of atomisation ?
It is the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
What kind of process is the standard enthalpy change of atomisation ?
- It is an endothermic process
- This is because bonds are broken to form gaseous atoms
What is the first ionisation energy ?
It is the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions
Is the first ionisation energy exothermic or endothermic ?
Endothermic because energy is required to overcome the attraction between a negative electron and the positive nucleus
What is electron affinity ?
It is the opposite of ionisation energy
What does electron affinity measure ?
It measures the energy needed to gain electrons
What is the first electron affinity ?
The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
Is the first electron affinity exothermic or endothermic ?
It is exothermic because the electron being added is attracted towards the nucleus
How do you determine lattice enthalpies ?
Using the Born-Haber cycle
When are successive electron affinities required ?
When an anion has a greater charge than 1-, such as O2-
Are second electron affinities exothermic or endothermic ?
They are endothermic
Why are second electron affinities endothermic ?
- A second electron is being gained by a negative ion, which repels the electron away
Define standard enthalpy change of solution
The enthalpy change that takes place when one mole of a solute dissolves in a solvent.
What mass should be used in enthalpy calculations ?
The mass of everything in the solution
What processes take place when a solid ionic compound dissolves in water?
- Ionic lattice breaks up
- Water molecules are attracted to and surround the ions
What are the energy changes involved in these processes of a solid ionic compound dissolving in water?
- Ionic lattice is broken up forming separate gaseous ions
- Separate gaseous ions interact with polar water molecules to form hydrated aqueous ions
- Called enthalpy change of hydration
Define enthalpy change of hydration
The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.
What can the values of the enthalpy change of solution be ?
Exothermic and endothermic
What are the general properties of ionic compounds ?
- High melting and boiling points
- Soluble in polar solvents
- Conduct electricity when molten or in aqueous solution
Do all ionic compounds follow the general properties ?
No
Describe the effect of ionic size on lattice enthalpy and melting point
- Ionic radius increases
- Attraction between ions decreases
- Lattice enthalpy becomes less negative
- Melting point decreases
Describe the effect of ionic charge on lattice enthalpy and melting point
- Ionic charge increases
- Attraction between ions increases
- Lattice enthalpy becomes more negative
- Melting point increases
Describe the effect of ionic size on hydration enthalpy
- Ionic radius increases
- Attraction between ion and water molecules decreases
- Hydration energy less negative
Describe the effect of ionic charge on hydration enthalpy
- Ionic charge increases
- Attraction with water molecules increases
- Hydration energy becomes more negative
Why is hydration enthalpy an inaccurate method of predicting solubility?
Many compounds with endothermic enthalpy changes of solution are soluble.
Define entropy
A measure of the dispersal of energy within the chemicals in a chemical system
What are the unit for entropy ?
J/K/mol
How does entropy relate to physical states?
In order of size : gases > liquids > solids, for entropy values
How are entropy, the dispersal of energy and disorder related ?
The greater the entropy, the larger the dispersal of energy and the greater the disorder
What is a system said to be if it is very random ?
- Energy is spread out more
- Entropy change is positive
What is a system said to be if it is not very random ?
- Energy is concentrated
- Entropy change is negative
When does entropy generally increase ?
During changes of state because the particles become more random
What is entropy like when there is an increase in gaseous molecules in the reaction ?
It is positive
What is entropy like when there is a decrease in gaseous molecules in the reaction ?
It is negative
Define standard entropy
- The entropy of one mole of a substance under standard conditions
- It is always positive
How is an entropy change calculated for a reaction?
Change in entropy is the sum of the entropies of the products minus the sum of the entropies of the reactants
How is an entropy change calculated for a reaction?
Change in entropy is the sum of the entropies of the products minus the sum of the entropies of the reactants
What is feasibility?
Whether a reaction is able to happen and is energetically feasible (or spontaneous).
What is the equation for free energy change?
ΔG = ΔH - TΔS
What is the requirement for the free energy change such that a reaction is feasible?
- ΔG < 0
- A decrease in free energy.
Even though a reaction may have a negative value for ΔG, why might the reaction not take place at room temperature?
High activation energy resulting in a very slow rate.
