thermodynamics past paper questions

Question 1

bond enthalpy and enthalpy of atomisation

Answer 1

if the question asks you to find the bond enthalpy; work out the bond association then x 2

Question 2

Suggest why process F (1st electron affinity) exothermic

Answer 2

the forces of attraction between the added electron and positive nuclues AND

energy is released while the added electron gets closer to the positive nucleus

Question 3

what is lattice enthalpy of dissociation

this applies to formation

Answer 3

Enthalpy change when 1 mol of an (ionic) compound/lattice (under standard
conditions)
Allow heat energy change
1
Is dissociated/broken/separated into its (component) ions
1
The ions being in the gaseous state (at infinite separation)

Question 4

explain the meaning of the term electron affinity

Answer 4

enthalpy change when one mole of gaseous atoms from one mole of gaseous negative ions with a single charge

Question 5

state the meaning of the symbol ΔH

Answer 5

enthalpy change

Question 6

in terms of behaviour explain why L2 is longer than L1

Answer 6

L2 represents boiling while L1 represents melting

there is a bigger chAnge in disorder for L2 than L1

Question 7

why is a reaction feasible

Answer 7

state the equation for Gibbs free energy

if G <= 0then the reaction is feasible

Question 8

why does this equation occur:

H20(l) → H2O(g) is feasible

Answer 8

in the reaction of evaporation of water, it is going from a liquid to a gas

a gas has a higher entropy value than a liquid as it is more disordered (there are more ways in which it can arrange its particles)

Therefore the entropy change is positive

therefore TS >H

Question 9

define the term electron affinity for chlorine

Answer 9

it is the entlapy change when 1 mole of gaseous chlorIDE ions are formed from chlorine atoms

with both the ions and the atoms in gaseous states

Question 10

explain why there is a difference between the hydration enthalpies of the magnesium and sodium ions

(-1920 for magnesium and sodium is -406)

Answer 10

magnesium is smaller with a higher charge

water is polar

therefore magnesium attracts water molecules more strongly
(requires less energy)

Question 11

explain why your answer to part c is different from the lattice enthalpy of dissociation for magnesium chloride

Answer 11

magnesium is smaller with the same charge

so it attracts the Cl-ions more strongly

therefore mgcl2 has stronger ionic bonding

Question 12

explain why the theoretical enthalpy of lattice dissociation for silver fluoride is different from the experimental value

Answer 12

the experimental value considers the covalent interactions

while the theoretical value assumes the perfectly ionic model which only assumes ionic interactions

Question 13

the theoretical value of lattice dissociation for silver chloride is 770kjmol-1

explain why this value is less than the value for silver fluoride

Answer 13

chloride ions are large than fluoride ions

therefore, the attraction between Ag+ and Cl- is weaker

Question 14

suggest why hydration of the chloride ion is an exothermic reaction

Answer 14

water is polar/ contains Hδ+

(Chloride ion) attracts (the H in) water molecules

Question 15

what is the enthalpy of solution

Answer 15

the enthalpy of solution is the enthalpy change when a mole of an ionic solid is dissolved in the minimum amount of solvent for no further enthalpy change to be observed upon further dilution

Question 16

by describing the nature of the attractive forces involved, explain why the value for the enthalpy change of hydration for the chloride ion is more negative than that for the bromide ion

Answer 16

Chloride ion is smaller than the bromide ion

water is polar and has a δ+ on the H

The negative chloride ions therefore have stronger forces of attraction between the δ+ on the H in the water molecule compared to the bromide ion

Question 17

explain why the free energy for the dissolving of potassium chloride in water is negative even though the enthalpy change is positive

Answer 17

1. state if the overall change is negative or positive
   e. g. the entropy change is positive
2. explain why:
   there are more disorder/ more moles
3. therefore TΔS >ΔH

Question 18

explain why the dissolving of silver fluoride is always spontaneous

Answer 18

the entropy change is always negative and the enthalpy change is always positive

therefore ΔG is always negative (so the reaction is feasible no matter the temperature)