2. Thermodynamics

Question 1

Define enthalpy of solution

Answer 1

1 mole of ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other

Question 2

Hess cycle for enthalpy of solution?

Answer 2

ionic solid—-^Hsol—–>dissolved ions

   LEF                           hydration enthalpys

                    gas ions

Arrows go up from gas ions

Question 3

Define lattice enthalpy of formation

Answer 3

Enthalpy change when 1 mole of a solid ionic compound is formed from its constituent ions in the gas phase

Question 4

What will increase lattice enthalpy?

Answer 4

small ions and/or high charges means stronger attraction means greater lattice enthalpy

Question 5

Define born-haber cycle

Answer 5

A cycle that includes all the enthalpy changes in the formation of an ionic compound

Question 6

born-haber cycle

what does it look like?

Answer 6

gaseous metal ions and gaseous non-metal atoms
u d
ionisation enthalpy d
u electron affinity enthalpy
gaseous atoms d
u gaseous ions
atomisation enthalpy d
u d
elements in normal states d
d lattice enthalpy of F
enthalpy of formation d

                              solid ionic compound

Question 7

What is the experimental lattice enthalpy value?

Answer 7

Born-Haber cycle

Question 8

How is theoretical lattice enthalpy value calculated?

Answer 8

Using an equation that considers size, charge and arrangement of ions

Question 9

Why do the theoretical and experimental lattice enthalpy values differ?

Answer 9

Theoretical assumes perfectly ionic structure whereas in many ionic compounds ions are distorted to some extent

Question 10

What is distortion (lattice enthalpy)?

Answer 10

In ionic compounds, ions are often distorted/ polarised

Question 11

Define covalent character (lattice enthalpy)

Answer 11

When an ionic compound has distortion in its ions- a large amount
larger=more covalent character

Question 12

What does it mean if there is a big difference between the theoretical and experimental lattice enthalpy values?

Answer 12

The ionic compound has great ionic character

Question 13

What does the magnitude of lattice enthalpy signify?

Answer 13

The overall strength of the ionic bonding

Question 14

What is one property of compound with covalent character?

Answer 14

They have low solubility or are insoluble in water

Question 15

Define entropy (S)

Answer 15

The degree of inherent disorder in a system

Question 16

What does a more positive entropy signify?

Answer 16

More disorder

Question 17

What is the second law of entropy?

Answer 17

Over time entropy will naturally increase

Question 18

What is the third law of entropy?

Answer 18

The entropy of a substance is zero at absolute zero and will increase with temperature

Question 19

Why does entropy increase with temperature?

Answer 19

Particles vibrate/move faster so there is more disorder so greater entropy

Question 20

Which of the states has the highest entropy?

Answer 20

Gases then liquids then solids

Question 21

Why is there a bigger increase in entropy in the conversion of a liquid to a gas rather than a solid to a liquid?

Answer 21

Gases have such a greater disorder that liquids or solids that it will significantly increase the entropy

Question 22

What compounds have a low entropy? Think structure

Answer 22

Those with a more ordered structure

Question 23

What is the entropy equation?

Answer 23

entropy = sum products - sum reactants

Question 24

What is gibbs free energy change?

Answer 24

It combines the two thermodynamic factors of enthalpy change and entropy

Question 25

What is the gibbs free energy equation?

Answer 25

Delta G = enthalpy change - (Temp x entropy change)

Question 26

What entropy and enthalpy value is most favourable?

Answer 26

Lower enthalpy

Higher entropy

Question 27

What does the gibbs value need to be for the reaction to be feasible?

Answer 27

It needs to be lower or equal to zero

Question 28

Define feasibility and what does it depend on?

Answer 28

A reaction can take place- depends of temperature

Question 29

What is another name for feasibility?

Answer 29

Spontaneous

Question 30

Why might a feasible reaction might not take place?

Answer 30

It has a too high activation energy

Question 31

If enthalpy is positive and entropy is positive what is the effect on gibbs value? hint: at different temps

Answer 31

Lower temperature = positive value

Higher temperature = negative value

Question 32

If enthalpy is positive and entropy is positive what is the effect on feasibility at different temperatures?

Answer 32

Lower temperature = not feasible

Higher temperature = feasible

Question 33

If enthalpy is positive and entropy is negative what is the effect on gibbs value at different temperatures?

Answer 33

It is always positive

Question 34

If enthalpy is positive and entropy is negative what is the effect on feasibility at different temperatures?

Answer 34

It is never feasible regardless of temperature

Question 35

If enthalpy is negative and entropy is negative what is the effect on gibbs value at different temperatures?

Answer 35

Lower temperature = negative value

Higher temperature = positive value

Question 36

If enthalpy is negative and entropy is negative what is the effect on feasibility at different temperatures?

Answer 36

Lower temperatures = feasible

Higher temperature = not feasible

Question 37

If enthalpy is negative and entropy is positive what is the effect on gibbs value at different temperatures?

Answer 37

Always negative

Question 38

If enthalpy is negative and entropy is positive what is the effect on feasibility at different temperatures?

Answer 38

Feasible at all temperatures

Question 39

When are melting and boiling feasible?

Answer 39

When delta G = zero

Question 40

How do you work out the temperature range for feasibility?

Answer 40

Set G as zero
Rearrange
Divide enthalpy by entropy