Thermodynamics Flashcards
Standard Enthalpy Change
Change in heat energy under standard conditions, i.e. 298K and 100kPa
Standard Enthalpy of Formation
Enthalpy change when one mole of a compound is formed from its constituent elements all in their standard states under standard conditions
Standard Enthalpy of Combustion
Enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions
Enthalpy of Atomisation
Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state
First Ionisation Energy
Enthalpy change when one mole of gaseous atoms each lose an electron to form one mole of gaseous ions with a single positive charge
First Electron Affinity
Enthalpy change when one mole of gaseous atoms each acquire an electron to form one mole of gaseous ions with a single negative charge
Lattice Enthalpy of Formation
Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions infinitely far apart
Lattice Enthalpy of Dissociation
Enthalpy change to separate one mole of solid ionic compound into its gaseous ions infinitely far apart
Enthalpy of Hydration
Enthalpy change when one mole of gaseous ions form aqueous ions
Enthalpy of Solution
Enthalpy change when one mole of an ionic compound completely dissolves in sufficient water to form an infinitely dilute solution
Mean Bond Enthalpy
Enthalpy change when breaking one mole of a given bond in a molecule of a gaseous species (averaged over different molecules)
Entropy
A measure of disorder of a system
Feasible/Spontaneous
- reactions that occur on their own accord
- free energy is zero or negative
Gibbs Free Energy
Thermodynamic quantity that combines enthalpy and entropy under a constant temperature and pressure to determine the spontaneity of a reaction (direction it takes)
First Law of Thermodynamics
Energy can be neither created nor destroyed, only converted into different forms
Second Law of Thermodynamics
Entropy of an isolated system not at equilibrium will tend to increase over time
Third Law of Thermodynamics
Entropy of a perfect crystal at absolute zero is zero
Suggest why ionic model and actual lattice enthalpies are different
- Ionic model assumes ions are spherical and arranged in a lattice
- Calculated value is more negative suggesting stronger attraction
- Compound has covalent character
Suggest what can be deduced from comparisons between theoretical and actual lattice enthalpies
Similar = Purely Ionic Different = Covalent Character
Which types of ionic compounds have covalent character
- high charge
- small positive ion
- large negative ion
= negative ion polarised > more sharing > covalent character
Suggest why successive ionisation energies get larger
- negative electron is removed from a positive ion
- requires more energy than removing from an atom
- removing electrons from orbitals closer to the nucleus require much more energy
Suggest why first electron affinity is negative but second is not
- first electron affinity is exothermic since energy is released when an electron is added to an atom due to attraction
- second electron affinity is endothermic since energy is required to overcome repulsion from negative ion
How to determine Enthalpy of Solution
- difficult to measure directly
- calculated using lattice dissociation enthalpy and enthalpy of hydration
Suggest why Enthalpy of Hydration is always negative
- (hydrogen) bonds are always formed between ions and water
- bond making is exothermic