unit 4 section 1 thermodynamics

Question 1

what is lattice formation enthalpy

Answer 1

the enthalpy change when 1 mole of a solid ionic crytstal lattice is formed from its gaseous ions.

Question 2

what is lattice dissociation enthalpy

Answer 2

the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions

Question 3

what is bond dissociation enthalpy

Answer 3

the enthalpy change when all the bonds of the same type in 1 mole of gaseous ions are broken

Question 4

what is the enthalpy change of atomisation of an element

Answer 4

the enthalpy change when 1 mole of gaseous atoms are formed from its elements under standard stated and conditions

Question 5

what is the enthalpy change of hydration

Answer 5

the enthalpy change when 1 mole of aqueous ions is formed from its gaseous atoms

Question 6

what is the enthalpy change of solution

Answer 6

the enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.

Question 7

how do you work out the theoretical lattice enthalpy

Answer 7

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

Question 8

why is the experimental enthalpy normally different to the theoretical enthalpy

Answer 8

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.

Question 9

why does water dissolve ionic lattices

Answer 9

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.

Question 10

what is entropy

Answer 10

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

Question 11

what happens to the entropy as you go from a solid to a liquid to a gas

Answer 11

the entropy increases as the particles are becoming more disordered

Question 12

what is the equation for calculating overall entropy of a reaction

Answer 12

ΔS = S (productst) - S (reactants)

it is just the difference in entropies

Question 13

what is free energy change ΔG

Answer 13

ΔG - it is the measure used to predict if a reaction is feasible

Question 14

what is a feasible reaction

Answer 14

it is a reaction that once started it will go until completion without any energy being supplied to it.

Question 15

what is the formula for free energy change

Answer 15

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)

Question 16

what does it means when ΔG is 0

Answer 16

it means the reaction is just feasible/the system is in equilibrium

Question 17

what does ΔG < 0 mean

Answer 17

it means the reaction is feasible and is spontaneous

Question 18

what does ΔG > 0 mean

Answer 18

it means the reaction is not feasible and is not spontaneous

Question 19

how do you make and use a free energy graph

Answer 19

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

Question 20

what is enthalpy change

Answer 20

it is the heat energy of a substance at constant pressure.

Question 21

explain why the enthalpy change of hydration decreases as you go down the group

Answer 21

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

Question 22

what is the route for the born harber cycle

Answer 22

=>enthalpy of atomisation
=>ionisation enthalply
=>electron affinity
=>lattice enthalpy of formation
<=enthalpy of formation (this sign is pointing in the opposite direction)

Question 23

how do you solve the born harber cycle

Answer 23

you add up all the values and put it equal to zero and then you solve for the missing value

Question 24

how do you do the enthalply of solution hydration

Answer 24

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)