Chater 23. Chemical Energetics Flashcards
Define lattice energy
The energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions
What are the factors affecting lattice energy, explain
-The magnitude of lattice energy increases as the attractive force between the ions in a crystal increases
-The larger the charge on the ions the more exothermic the lattice energy
-The smaller the ionic radii the more exothermic the lattice energy because the opposite charges approach each other more closely than when larger
Define standard enthalpy of atomization
The enthalpy change when 1 mole of gaseous atoms are formed from the element in its standard state
Define the first and second ionisation energy
- The enthalpy change when one mole of gaseous atoms is converted to one mole of gaseous ions with a single positive charge
- The enthalpy change when one mole of gaseous ions with a single positive charge is converted to one mole of gaseous ions with a double positive charge
Define first and second electron affinity
- The enthalpy change when one mole of gaseous atoms is converted to a mole of gaseous ions with a single negative charge
- The enthalpy change when one mole of gaseous ions with a single negative charge is converted to a mole of gaseous ions with a double negative charge
Explain the trends in electron affinity
- The general trend is for the values to get less negative as we descend a group, this is because an electron is being added to a shell that has shielding. Electron affinity is a measure of the attraction between the nucleus and an electron hence there will be less attraction
- Fluorine is an exception because it’s small size means the electron feels the repulsion from the two electrons in the first shell hence the charge is partly cancelled
- The second electron affinity is positive because work has to be done in overcoming the repulsion between the electron and the negative ion
Define enthalpy change of solution
The enthalpy change when 1 mole of an ionic substance dissolves in water to give a very dilute solution
Define enthalpy change of hydration
The enthalpy change when one mole of gaseous ions dissolve in water to give a very dilute solution
Define entropy
The number of possible arrangements of particles and their energy in a given system
Explain the trends of entropy and entropy changes
- Entropies increase with temperature partly because at higher temperatures particles spread out and randomness increases and partly because there are more quanta of energy distributed through the system
- Entropy increases as particles dissolve and move further away from each other
- Entropy increases in a reaction where more gas molecules are being produced
- A positive entropy change implies an increase in order
Formula for entropy change
Δ S = Summation of the entropies of the products - Summation of the entropies of reactants
How to calculate Gibbs free energy change, Δ G and its implications
Δ G = Δ H - T Δ S
- when ΔG is negative, the reaction is feasible
Note
Remember to check your units when calculating ΔG
What happens when ΔG is zero?
In a closed system an equilibrium will be formed where both products and reactants are present
What is the difference between thermodynamically stable and kinetically stable?
Thermodynamically stable means the substance’s ΔG is positive and it is not spontaneous while kinetically stable means its rate of reaction is very low.
