8P- Thermodynamics Flashcards
What are the two types of lattice enthalpy
Enthalpy of lattice dissociation
Enthalpy of lattice formation
Define enthalpy of lattice formation
Enthalpy change when 1 mol of a solid ionic compound is formed its constituent ions in the gas phase
Define enthalpy of lattice dissociation
Enthalpy change when 1mol of solid ionic compound is broken up into its constituent ions in the gas phase
Define enthalpy of formation
Enthalpy change when 1 mol of substance is formed from its constituent elements with all substances in their standard states
Define 1st ionisation energy/enthalpy
Enthalpy change when 1mol of electrons is removed from 1mol of gaseous atoms to form 1mol of gaseous ions
Define 2nd ionisation energy/enthalpy
Enthalpy change when 1mol of electrons is removed from 1mol of gaseous 1+ ions to form 1 mol of gaseous 2+ ions
Define enthalpy of atomisation
Enthalpy change when 1mol of gaseous atoms is produced from an element in its standard state
What’s mean bond enthalpy
Enthalpy change when 1mol of gaseous molecules each breaks a covalent bond forming 2 free radicals, averaged over a range of compounds
What’s first electron affinity
Enthalpy change when 1mol of gaseous atoms is converted to 1mol of gaseous ions with a -ve charge
Eg O + e- —> O-
Define second electron affinity
Enthalpy change when 1mol of electrons is added to 1mol of gaseous ions with a -ve charge to form ions with a 2- charge
Eg O- + e- —> O 2-
Define enthalpy of solution
Enthalpy change when 1mol of solute dissolves completely in sufficient solvent to form a solution where ions are separated and thus do not interact with each other
Define enthalpy of hydration
Enthalpy change when 1 mol of gaseous ions are converted into aqueous ions
What’s a theoretical lattice enthalpy
An enthalpy calculated from the charge in an ion and their distance apart
(Fake value)
What’s a born haber lattice enthalpy
(Real/experimental value)
Experimental value calculated from a born haber cycle
What’s a perfect ionic model
Ions are 100% ionic and spherical, the interaction is purely electrostatic
What’s covalent character
A compound tends towards a giant covalent structure, the electron cloud is distorted (becomes polarised), the lattice is therefore stronger than a 100% ionic structure
Born haber value is larger than the theoretical (fake) value
The more covalent character a structure shows the larger the difference in the two enthalpy values
What’s a feasible/spontaneous process
A reaction which will occur on its own without any external influence (eg diffusion)
Why is enthalpy change ^H not sufficient to explain feasible change
Exothermic reactions (-ve ^H) result in products which are more thermodynamically stable than reactants
This is a factor as to why reactions can be spontaneous
Endothermic (+ve ^H) reactions can also be spontaneous but there is no explanation for this (in terms of ^H)
What’s entropy change
The randomness of a system
Products more disordered than reactants, +ve entropy
^S
Why is ^S better to explain feasible change than ^H
^S takes into account physical and chemical changes
What has 0 entropy
Perfect crystals at absolute zero (0K)
How to find entropy qualitatively
Use balanced chemical equations
Entropy will increase (+ve ^S) if
Solid reactant turn to liquid/gaseous products
There are more mol of products than mol of reactants
The opposite occurs for entropy (-ve^S) to decrease
How to calculate entropy change from absolute entropy values (quantitatively)
^S = sum of ^S products - sum of ^S reactants
Use values given
Units J/K/mol
What’s gibbs free energy change
Give the equation
Combines enthalpy and entropy to determine the feasibility of a reaction
^G (gibbs free energy) = ^H (enthalpy) - T(temp)x^S(entropy)
Reactions with +ve entropy and -ve enthalpy make ^G -ve or 0 making the reaction feasible
But reactions with -ve ^G and a high activation energy may not occur
How to calculate ^S and ^H for use in the gibbs eq
Check if enthalpy is always formation
^S
^S of products - ^S of reactants
Measured in J/K
Convert to Kj by dividing by 1000
^H formation
^H products - ^H reactants
What’s ^G for changes of state
0
At melting point equal existence of solid and liquid, there is equilibrium
^H= T^S
Effects of T on ^G
If ^S is +ve increasing temp makes ^G more -ve, more feasible reaction
If ^S if -ve increasing temp makes ^G more +ve, less feasible reaction
If ^S is close to 0, -T^S will be the same, no change to ^G
How to calculate ^S and ^H from a graph of ^G and T
^G = ^H -T^S
Y= mx + c
Gradient = -^S (-ve entropy don’t forget)
Y intercept = ^H
