Physical 2 - Chapter 17: Thermodynamics Flashcards
What is Hess’ law?
The enthalpy change for a chemical reaction is always the same, whatever route is taken from reactants to products
What are the standard conditions of a reaction?
100kPa + 298K
Define the standard molar enthalpy of formation
The enthalpy change when one mole of a compound is formed from it’s constituent elements under standard conditions, all reactant and products in their standard states
Define the standard molar enthalpy change of combustion
The enthalpy change when one mole of substance is completely burnt in oxygen
Define the standard enthalpy of atomisation
The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in it’s standard state in standard conditions
1/2 Br₂(l) -> Br(g)
This is 1/2 mole of bromine molecules forming 1 mole of gaseous bromine atoms
Define 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
Define the second ionisation energy
The standard enthalpy change when a mole of electrons in lost from a mole of singly positively charged ions
Why are ionisation enthalpies always positive?
Energy has to be put in to pull an electron away from the attraction of the positively charged nucleus of an atom
Define 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
Why are first electron affinities always negative?
Energy is given out when an electron is attracted to the positively charged nucleus of an atom
Define 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 ions each with 2 negative charges
What are second electron affinities always positive?
Energy must be put in to overcome the repulsion between an electron and a negatively charged ion
Define lattice enthalpy of formation
The standard enthalpy change when one mole of solid ionic compound is formed from it’s gaseous ions
Define lattice enthalpy of dissociation
The standard enthalpy change when one mole of solid ionic compound dissociates into it’s gaseous ions
Define enthalpy of hydration
The standard enthalpy change when water molecules surround one mole of gaseous ions
Define enthalpy of solution
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
In other words, it’s breaking of the lattice and then hydrating it.
If the change in enthalpy is positive, it will not be feasible unless it is very small and entropy can overcome to difference
Define mean bond enthalpy
The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form 2 free radicals, averaged over a range of compounds
How does the size of the ion affect it’s lattice enthalpy and why?
Bigger ion = Smaller lattice enthalpy
The opposite charges do not approach each other as closely when the ions are larger
How does the size of the ion affect the enthalpy of hydration and why?
Bigger ion = Smaller enthalpy of hydration
The opposite charges do not approach each other as closely when the ions are larger
How does the charge of an ion affect it’s enthalpy of hydration and why?
Bigger charge = Larger enthalpy of hydration
Ions with more charge give out more energy when they come together
How does the charge of an ion affect it’s lattice enthalpy and why?
Bigger charge = Larger lattice enthalpy
Ions with more charge give out more energy when they come together
Which factors increase polarisation for a positive ion (cation)?
Small size = Higher charge density = Pulls on electron cloud more effectively = More polarisation
High charge = Stronger attraction on electrons of anion = Pulls on electron cloud more effectively = More polarisation
Which factors increase polarisation for a negative ion (anion)?
Large size = Electrons are further = Less attraction to anion = Easier to pull by cation
High charge = [WIP]
What is entropy?
I hate this definition
It is the randomness of a system and is a measure of disorder.
Gases are more disordered than liquids, which are more disordered than solids because of their arrangement of particles
