Thermodynamics Flashcards
define enthalpy change
the heat energy change at constant pressure
define standard enthalpy of formation
the enthalpy change when one mole of a substance is formed from its elements under standard conditions with all substances in their standard states
define standard enthalpy of combustion
the enthalpy change when one mole of a substance is completely combusted in excess oxygen under standard conditions with all substances in their standard states
define mean bond enthalpy
the enthalpy change when one mole of a given covalent bond is broken averaged over a range of compounds
define standard enthalpy of atomisation
the enthalpy change for the formation of one mole of gaseous atoms from the element in its standard state
define bond dissociation enthalpy
the enthalpy change to break the bond in one mole of gaseous molecules to form gaseous atoms
define first ionisation energy
the enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to form one mole of gaseous 1+ ions
define second ionisation energy
the enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions
define lattice formation enthalpy
the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions
define first electron affinity
the enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous 1- ions
define second electron affinity
the enthalpy change when one mole of electrons is added to one mole of 1- ions to form one mole of gaseous 2- ions
why is the second electron affinity of any ion always endothermic
due to the repulsion between the negative ion and negative electron, energy is required to overcome it
why is the second ionisation of an ion greater than the first
it is more difficult to remove an electron from a positive ion due to the stronger electrostatic attraction
define lattice dissociation enthalpy
the enthalpy change to separate one mole of solid ionic compound into its gaseous ions
how to calculate the standard enthalpy of atomisation for diatomic substances
bond dissociation enthalpy/2
why is the first electron affinity exothermic
due to the electrons attraction to the nucleus
what does Bend-Mex mean
- breaking bonds is endothermic
- making bonds is exothermic
how does the strength of attraction correlate with the lattice formation enthalpy
- the stronger the attractions, the more exothermic the lattice formation enthalpy is
how does the strength of attraction correlate with the lattice dissociation enthalpy
- the stronger the attractions, the more endothermic the lattice dissociation enthalpy is
how does ion charge affect lattice enthalpy
- the higher the charge on the ion, the smaller the ionic radius will be meaning they are closer together in the lattice leading to a stronger ionic bond
- stronger attraction means more energy is given out on the formation and more energy is needed to dissociate the lattice
how does the size of ion affect lattice enthalpy
- the smaller the ionic radius the closer together they are in the lattice leading to a stronger ionic bond
- stronger attraction means more energy is given out on the formation and more energy is needed to dissociate the lattice
what is the perfect ionic model
- a model that assumes all ions are perfect spheres with purely ionic attraction
how can you use the theoretical and experimental lattice enthalpy values to predict the ionic shape of a compound
- if the experimental value is the same as the theoretical there is no covalent character and it is the perfect ionic model
- if the experimental value differs from the theoretical it is due to the covalent character
how is a covalent character formed
- the cation is much smaller than the anion and/or is highly charged
- the cation attracts the anion’s electrons causing the anion to become distorted and more covalent i.e. polarised
how does a covalent character affect the strength of an ionic bond
- with a covalent character, the ionic bond is stronger
how does polarisation affect the experimental enthalpy of lattice formation
the experimental enthalpy of lattice formation will be more exothermic than the theoretical value
how does polarisation affect the experimental enthalpy of lattice dissociation
stronger ionic bond therefore more endothermic
what does the enthalpy of solution mean
- when one mole of solid dissolves in water to form aqueous ions
what is enthalpy of hydration
- the enthalpy change when one mole of gaseous ions becomes aqueous ions
why is the enthalpy of hydration exothermic
Due to the attraction between…
• the positive ion and the partially negative O of water
• the negative ion and the partially positive H of water.
how to calculate enthalpy of solution
enthalpy of lattice dissociation + enthalpy of hydration
what affects the value of the enthalpy of hydration
- the stronger the attraction between the ion and part of the water molecule, the more exothermic the enthalpy of hydration
what is entropy
- a measure of disorder in a system
which factors affect entropy
- the state of the substances ( gas has most disorder, solid has least disorder)
- the number of particles/moles ( more moles = greater disorder)
how to calculate entropy change
- entropy change = ∑entropy of products - ∑entropy of reactants
how to calculate gibbs free energy
- ΔG = ΔH - TΔS
how to know if a reaction is feasible at a given temperature
- if the gibbs free energy is less than or equal to 0, reaction is feasible
- if the gibbs free energy is more than 0, reaction is not feasible
