1.8 Thermodynamics

Question 1

State the meaning of the term enthalpy change.

Answer 1

Heat energy change at a constant pressure

Question 2

Define lattice enthalpy of dissociation

Answer 2

the enthalpy change needed to convert 1 mole of solid ionic compound into its gaseous ions under standard conditions

Question 3

Equation for lattice dissociation of CaCl2 (s)

Answer 3

CaCl2 (s) -> Ca (s) + Cl2 (g)

Question 4

Magnesium chloride dissolves in water.
Give an equation, including state symbols, to represent the process that occurs when the enthalpy of solution of magnesium chloride is measured.
b) define enthalpy of solution

Answer 4

MgCl2 (s) -> Mg^2+ (aq) + 2Cl^- (aq)
b) the enthalpy change when one mole of of ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions.

Question 5

The enthalpy of hydration of Ca2+(g) is –1650 kJmol^1
Suggest why this value is less exothermic than that of Mg2+(g)

Answer 5

Ca^2+ is a bigger ion as it has more shielding and a larger ionic radius
- Has a weaker attraction to the delta negative oxygen in H2O

Question 6

Explain why the standard entropy value for carbon dioxide is greater than
that for carbon.

Answer 6

CO2 is more disordered than Carbon

Question 7

What is the entropy gibbs calculation

Answer 7

Delta G = ( - T x Delta S ) + Delta H

Question 8

which compounds will have the largest enthalpy change values between MgO, BaCl2 and MgCl2

Answer 8

MgO greater than MgCl2,
as Oxygen has a greater charge than chlorine, -2 vs -1
MgCl2 is greater than BaCl2 because Mg has less shielding than Ba so MgCl2 is smaller and has a stronger electrostatic attraction to outermost electrons.

Question 9

State why there is a difference between the theoretical and experimental
values.

Answer 9

experimental takes covalent character into account.

Question 10

Explain why the enthalpy of hydration becomes less exothermic from Li+
to K+

Answer 10

size of ion increases as charge stays same
less electrostatic attraction to delta negative oxygen in H2O

Question 11

Define enthalpy of formation
b) show enthalpy of formation of CsI caseium iodide

Answer 11

a) the standard enthalpy of formation is change in enthalpy when one mole of a substance is created under its standard conditions from its pure elements.
Cs (s) + 0.5 I2 (s) -> CsI (s)

Question 12

The enthalpy of lattice formation for caesium iodide in Table 1 is a value
obtained by experiment which was +107 kJmol^-1.
The value obtained by calculation using the perfect ionic model is −582 kJmol−1
Deduce what these values indicate about the bonding in caesium iodide

Answer 12

Caesium iodide is almost perfectly ionic

Question 13

Lattice enthalpy values can be obtained from Born–Haber cycles and by
calculations based on a perfect ionic model.
Which compound shows the greatest percentage difference between these two
values?
A CsF
B CsI
C LiF
D LiI

Answer 13

D
Look for compound with most covalent character
look for smallest positive ion & largest negative ion

Question 14

Define the term enthalpy of lattice formation.

Answer 14

Enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions

Question 15

A calculation of the enthalpy of lattice formation of silver iodide based on a
perfect ionic model gives a smaller numerical value than the value in the born - harbor cycle
Explain this difference.

Answer 15

silver iodide contains covalent character
forces holding lattice together are stronger

Question 16

Identify a reagent that could be used to indicate the presence of iodide ions in an aqueous solution and describe the observation made.

Answer 16

AgNO3, yellow ppt
Cl2 or Br2, brown solution / black solid formed

Question 17

Anhydrous magnesium chloride, MgCl2, can absorb water to form the hydrated
salt MgCl2.4H2O
MgCl2(s) + 4H2O(l) ⟶ MgCl2.4H2O(s)
Suggest one reason why the enthalpy change for this reaction cannot be determined directly by calorimetry

Answer 17

Not possible to prevent some dissolving
it is soluble and it dissolves

Question 18

Describe how you would carry out an experiment to determine the enthalpy
of solution of anhydrous magnesium chloride.
You should use about 0.8 g of anhydrous magnesium chloride.
Explain how your results could be used to calculate the enthalpy of solution.

Answer 18

• measure water with appropriate apparatus
• pour solution into insulated container / polystyrene cup
• add known mass of MgCl2
• weigh polystyrene cup before and after mass is added and find difference to find mass of MgCl2
• Record initial temperature and continue to measure temperature at suitable intervals of every 5 minutes until trend is seen
• Plot a graph of temperature against time
• extrapolate lines to when solid is added to find initial and final value of temperature
• final temperature - initial = change in temperature
• use of equation q=mc delta t

Question 19

A reaction is exothermic and has a negative entropy change.
Which statement is correct?
A The reaction is always feasible
B The reaction is feasible above a certain
temperature
C The reaction is feasible below a certain
temperature
D The reaction is never feasible

Answer 19

C

Question 20

Define the term electron affinity for chlorine.

Answer 20

the enthalpy change for formation of one mole of chloride ions from chlorine atoms

Question 21

Explain why there is a difference between the hydration enthalpies of the Mg^2+ and Na^+ ions.

Answer 21

• magnesium is a smaller ion and has a greater charge
• magnesium ion attracts water more strongly

Question 22

A 5.00 g sample of potassium chloride was added to 50.0 g of water initially at 20.0 °C. The
mixture was stirred and as the potassium chloride dissolved, the temperature of the
solution decreased.
Describe the steps you would take to determine an accurate minimum temperature
that is not influenced by heat from the surroundings.

Answer 22

• start a clock when KCl is added to water
• Record the temperature every subsequent minute for about 5 minutes
• Plot a graph of temperature vs time
• Extrapolate back to time of mixing = 0 and determine the temperature

Question 23

A 5.00 g sample of potassium chloride was added to 50.0 g of water initially at 20.0 °C.
The temperature of the water decreased to 14.6 °C
Calculate a value, in kJ mol−1, for the enthalpy of solution of potassium chloride.
You should assume that only the 50.0 g of water changes in temperature and that
the specific heat capacity of water is 4.18 J K−1 g−1.

Answer 23

Q = mc x delta t
change in temperature = 5.4 degrees
Q = 50 x 4.18 x 5.4 = 1128.6 J
moles = 5 / 74.6 = 0.0670
energy change per mole = 1128.6 / 0.0670 = 16839 J mol^-1
= + 16.8 kJ mol^-1

Question 24

Write an equation for the process that has an enthalpy change equal to the electron affinity
of chlorine

Answer 24

Cl(g) + e- → Cl-(g)

Question 25

In terms of electrostatic forces, suggest why the electron affinity of fluorine has a
negative value.

Answer 25

There is an attraction between the nucleus and the electron
Energy is released when the electron is gained

Question 26

define atomisation enthalpy

Answer 26

energy required for formation of a mole of gaseous atoms under standard conditions

Question 27

define electron affinity

Answer 27

the enthalpy change when one mole of electrons is added to a mole of gaseous atoms under standard conditions

Question 28

define enthalpy of solution

Answer 28

enthalpy change when one mole of ionic solid is dissolved in water to infinite dilution so that the ions no longer interact under standard conditions.

Question 29

define enthalpy of hydration

Answer 29

enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions under standard conditions

Question 30

Suggest why hydration of the chloride ion is an exothermic process.

Answer 30

Water is polar
chlorine attract the hydrogen in water which has a delta plus

Question 31

what is the boiling point of water in kelvin

Answer 31

373.2 kelvin

Question 32

Lattice enthalpy can be calculated theoretically using a perfect ionic model.
Explain the meaning of the term perfect ionic model.

Answer 32

• Ions are perfect spheres
• Ions are point charges
• Only electrostatic attraction
• no covalent character
• only ionic bonding

Question 33

Suggest two properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model.

Answer 33

Ionic radius
Charge

Question 34

enthalpy of atomisation of chloirine

Answer 34

1/2 Cl2 (g) -> Cl (g)
- when using born Haber cycle don’t multiply by 2

Question 35

Predict whether the magnitude of the lattice enthalpy of dissociation that you have calculated for AgCl will be less than, equal to or greater than the value that is obtained from a perfect ionic model. Explain your answer.

Answer 35

Equal
AgCl is perfectly ionic / no covalent character

Question 36

The enthalpy of solution for potassium chloride is +17.2 kJ mol−1
Explain why the free-energy change for the dissolving of potassium chloride in water is negative, even though the enthalpy change is positive.

Answer 36

entropy change is positive as entropy increases
because 1 mole of solid turns into 2 moles of aqueous ions
aqueous ions are more disordered than the solid

Question 37

Explain why the second electron affinity of oxygen has a positive value

Answer 37

O^- repels electron being added
as all negative ions repel electrons

Question 38

Explain why the enthalpy of lattice dissociation for sodium oxide is greater than the
enthalpy of lattice dissociation for sodium chloride.

Answer 38

O2- has a higher negative charge
smaller size
has stronger attraction to Na+

Question 39

Give a reason why data books do not contain a value for the enthalpy of solution of
sodium oxide.

Answer 39

it reacts with water
or
it reacts to form a solution of Na(OH)

Question 40

In your answer you should
* compare the three values based on a perfect ionic model
* compare the values based on a perfect ionic model to the values from a
Born–Haber cycle for each compound.

Answer 40

1a Value for CaCl2 is larger
OR
Values for KCl and AgCl are similar
OR
Values for CaCl2 > AgCl > KCl
1b Ca2+ has a larger charge/ is a smaller ion
OR
Ag+ and K+ have smaller charge or larger ions
1c CaCl2 has stronger ionic bonds or stronger
attraction between + and – ions (Ca2+and Cl
OR
AgCl and KCl have weaker ionic bonds or weaker
attraction between + and – ions (Ag+/ K+ and Cl
Stage 2 similarities in the perfect ionic model and
Born-Haber cycle values
2a CaCl2 has similar values (between the perfect ionic
model and Born-Haber cycle)
2b KCl has similar values (between the perfect ionic
model and Born-Haber cycle)
2c CaCl2 and KCl have (almost) perfect ionic bonding
or + ions are point charges/(perfectly) spherical
Stage 3 difference in the perfect ionic model and
Born-Haber cycle values
3a AgCl has larger difference in values (between the
perfect ionic model and Born-Haber cycle)
3b AgCl contains (some) covalent character
3c Ag+ more polarising/distorts electron cloud more