unit 4 section 1 thermodynamics Flashcards
what is lattice formation enthalpy
the enthalpy change when 1 mole of a solid ionic crytstal lattice is formed from its gaseous ions.
what is lattice dissociation enthalpy
the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions
what is bond dissociation enthalpy
the enthalpy change when all the bonds of the same type in 1 mole of gaseous ions are broken
what is the enthalpy change of atomisation of an element
the enthalpy change when 1 mole of gaseous atoms are formed from its elements under standard stated and conditions
what is the enthalpy change of hydration
the enthalpy change when 1 mole of aqueous ions is formed from its gaseous atoms
what is the enthalpy change of solution
the enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.
how do you work out the theoretical lattice enthalpy
use the purely ionic model of a lattice( assumes that all the ions are spherical and have their charges spread out evenly ) to do calculations
why is the experimental enthalpy normally different to the theoretical enthalpy
this is because the ionic compounds have some covalent character. the positive and negative ions in a lattice aren’t exactly spherical. positive ions polarise neighbouring ions to different extents- more polarisation the more covalent bonding there will be.
why does water dissolve ionic lattices
the water can form bonds with the ions because it is a polar molecule. the oxygen in the water is more electronegative so it pulls the bonding electrons towards itself creating a dipole. therefor the positive ions from the lattice form weak bonds with the partial negative charge on oxygen and the negative ions from the lattice form weak bonds with the partial positive charge of the hydrogen.this pulls the ions away from the lattice causing the lattice to dissolve.
what is entropy
it is the measure of the amount ways that a particle can be arranged and the number of ways the energy can be shared out between the particles
what happens to the entropy as you go from a solid to a liquid to a gas
the entropy increases as the particles are becoming more disordered
what is the equation for calculating overall entropy of a reaction
ΔS = S (productst) - S (reactants)
it is just the difference in entropies
what is free energy change ΔG
ΔG - it is the measure used to predict if a reaction is feasible
what is a feasible reaction
it is a reaction that once started it will go until completion without any energy being supplied to it.
what is the formula for free energy change
ΔG = ΔH - TΔS
ΔG - free energy change (kj mol-1)
ΔH - enthalpy change (j mol-1)
T - temperature (in K )
ΔS - entropy change ( kj K-3 mol-1)
what does it means when ΔG is 0
it means the reaction is just feasible/the system is in equilibrium
what does ΔG < 0 mean
it means the reaction is feasible and is spontaneous
what does ΔG > 0 mean
it means the reaction is not feasible and is not spontaneous
how do you make and use a free energy graph
you do ΔG(y-axis) by temperature(x-axis)
the ΔG by temp graph is a straight line you can use it in the form y=mx +c for ΔG = ( -ΔS x T ) + ΔH
what is enthalpy change
it is the heat energy of a substance at constant pressure.
explain why the enthalpy change of hydration decreases as you go down the group
this is because the size of the ion increases as you go down the group and the electrostatic forces of attraction between the metal ion and the partial negative charge of the oxygen decreases
what is the route for the born harber cycle
=>enthalpy of atomisation
=>ionisation enthalply
=>electron affinity
=>lattice enthalpy of formation
<=enthalpy of formation (this sign is pointing in the opposite direction)
how do you solve the born harber cycle
you add up all the values and put it equal to zero and then you solve for the missing value
how do you do the enthalply of solution hydration
gaseous ions(top)
<=lattice dissociation enthalpy ( ionic to gas )
mole of ionic compound (middle)
<=enthalpy of solution (ionic to aqueous )
aqueous ions (bottom)
to go from the gasous ions to the aqueous ions it is the (hydration enthalpies of ions)
