Thermodynamics 2 Flashcards
What do you call it when you measure heat change at constant pressure
Enthalpy change
Define standard molar enthalpy of formation /\fH
The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products being in their standard states.
Define standard molar enthalpy change of combustion /\cH
The enthalpy change when one moles of substance is completely burnt in oxygen.
Define standard enthalpy change of atomisation /\atH
The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
Define first ionisation energy IE
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge.
Define first electron affinity /\eaH
The standard enthalpy change when a mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge.
Define second ionisation energy
Second ionisation energy is the loss of a mole of electrons from a mole of singly positively charged ions.
Define second electron affinity /\eaH
The enthalpy change when a mole of electrons is added to a mole of gaseous ions each with a single negative charge to form ions each with two negative charges.
Are first and second electron affinities positive or negative enthalpy changes
1) first electron affinity is a negative enthalpy change
2) second electron affinity is positive.
Define lattice enthalpy of formation /\LH
The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
Is lattice enthalpy of formation a positive or negative enthalpy change and why
-Lattice enthalpy of formation is always negative.
- This is because when a lattice forms, new bonds are formed which causes energy to be given out as it is exothermic.
Define lattice enthalpy of dissociation
Lattice enthalpy of dissociation is the standard enthalpy change when one mole of solid ionic compound dissociated into its gaseous ions.
How are the lattice dissociation enthalpy and lattice enthalpy of formation of a compound linked.
- They have the same numerical value.
- Lattice enthalpy of formation is the negative version and lattice dissociation is positive.
Define enthalpy of hydration /\hydH
The standard enthalpy change when water molecules surround one mole of gaseous ions.
Define enthalpy of solution /\solH
The 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.
Define 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.
Describe the enthalpy changes involved in forming a compound from its constituent elements under’t standard conditions, all products and reactants being in their standard states.
1) Atomisation
2) Ionisation
3) Electron affinity
4) Lattice formation
Why might an exam question with a Born-Haber cycle have more stages than you think
- It may show the same stage for each compound as separate.
- eg. You could write the atomisation energy for sodium and chlorine together as one step or you could have the atomisation of sodium first, then atomisation of chlorine next as a separate step.
Describe Hess’ law
Hess’ law states that the total energy (or enthalpy) change for a chemical reaction is the same whatever route is taken, provided that the initial and final conditions are the same.
