Thermodynamics Flashcards
Standard enthalpy of atomisation
The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
First ionisation energy
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge.
First electron affinity
The standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge. Exothermic as energy is given out when an electron is attracted to the positively charged nucleus of an atom.
Second electron affinity
The enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form a mole of ions each with two negative charges. Endothermic as energy must be put in to overcome the repulsion between an electron and a negatively charged ion.
Lattice enthalpy of formation
Standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
Lattice enthalpy of dissociation
The standard enthalpy change when one mole of solid ionic compound dissociated into its gaseous ions.
Enthalpy of hydration
Standard enthalpy change when water molecules surround one mole of gaseous ions.
Enthalpy of solution
Standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart not to interact with each other.
Mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds.
How to construct a Born-Haber cycle for lattice enthalpy of dissociation or formation.
Arrows go up for endothermic and down for exothermic. Need state symbols and electrons when required. Start with elements in standard states which have 0 enthalpy by definition. Add atomisation enthalpy of metal then non-metal. Add first ionisation energy of metal then more if needed. Add first electron affinity of non-metal (down) then more if needed (up). Uses same electrons. Add enthalpy of formation of the salt (down) at start. The unknown is lattice dissociation (up) or formation (down).
Trends in lattice enthalpy
Larger ions lead to smaller lattice enthalpies because the opposite charges do not approach each other as closely. Larger charges lead to greater lattice enthalpies as the electrostatic force of attraction between the opposite charges is greater.
Which compounds are more likely to form from the same elements?
The ones with more negative lattice enthalpy of formation (more exothermic).
How do ionic solids get dissolved in water?
The lattice must be completely broken up which requires an input of energy (lattice enthalpy). The separate ions are solvated by the solvent molecules (water). These cluster around the ions so that the positive ions are surrounded by the negative ends of the dipole of the water molecules and the negative ions are surrounded by the positive end of the dipoles. This is called hydration. The enthalpy change is more negative for larger charges and less negative for bigger ions.
How do ions in ionic compounds become polarised
If the ions have a large enough charge and the positive ions is small and the negative ion is large. The small cation can approach closely to the electron clouds of the anion and distort them by attracting them towards it. The large size of the anion means it’s electrons are further from the nucleus so it is easy to distort. The distortion means there are more electrons than expected concentration between the two nuclei and represents a degree of electron sharing.
What does a difference between experimental and theoretical values of lattice enthalpy indicate?
That the compound has some covalent character and the ions are not perfectly spherical.
