22 Enthalpy and Entropy

Question 1

Define lattice enthalpy change. (2 marks)

Answer 1

The enthalpy change that accompanies the formation of one mile of an ionic compound from its gaseous ions.

Question 2

Define enthalpy change of atomisation. (2 marks)

Answer 2

The enthalpy change that occurs when the formation of one mole of gaseous atoms from the element in its standard state.

Question 3

Define first electron affinity. (2 marks)

Answer 3

The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions.

Question 4

Why is the second electron affinity endothermic. (2 marks)

Answer 4

In the second one, the electron is being gained by a negative ion, this increases repulsion, so energy must be put in to force the e- onto the ion.

Question 5

Describe the steps required to form a giant ionic lattice. (3 marks)

Answer 5

Formation of gaseous atoms
Formation of gaseous ions
Lattice formation

Question 6

Define enthalpy change of formation. (2 marks)

Answer 6

The enthalpy charge when one mole of a compound is formed from its elements.

Question 7

Define bond dissociation enthalpy. (2 marks)

Answer 7

The bond dissociation enthalpy is the standard enthalpy change when 1 mole of a covalent bond is broken down into two gaseous atoms.

Question 8

Define first ionisation energy. (2 marks)

Answer 8

The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a 1+ charge.

Question 9

Define enthalpy change of hydration. (1 mark)

Answer 9

The enthalpy change when 1 mole of gaseous ions become aqueous ions.

Question 10

Define standard enthalpy change of solution. (2 marks)

Answer 10

The enthalpy change when one mole of a solute dissolved in a solvent.

Question 11

What is the equation involving specific heat capacity and mass of solution? (1 mark)

Answer 11

q=mc x change in T

Question 12

What two enthalpy changes are used to calculate enthalpy change of solution? (2 marks)

Answer 12

Lattice is enthalpy change
Enthalpy of hydration

Question 13

What two factors affect lattice enthalpy? (2 marks)

Answer 13

Ionic size
Ionic charge

Question 14

What two factors affect enthalpy change of hydration? (2 marks)

Answer 14

Ionic size
Ionic charge

Question 15

How does ionic size affect lattice enthalpy? (2 mark)

Answer 15

As ionic radius increases, attraction between ions decreases, so lattice enthalpy becomes less negative.

Question 16

How does ionic size affect hydration enthalpy? (2 mark)

Answer 16

As ionic radius increases, attraction between the ions and water decreases, therefore enthalpy of hydration is less negative.

Question 17

How does ionic charge affect lattice enthalpy? (2 marks)

Answer 17

As ionic charge increases, attraction Bert web ions increases, so lattice enthalpy becomes more negative.

Question 18

How does ionic charge affect hydration enthalpy? (2 marks)

Answer 18

As ionic charge increases, attraction between ions and water molecules increases, so hydration energy becomes more negative.

Question 19

Should a compound with endothermic enthalpy change of solution dissolve? (1 mark)

Answer 19

Yes

Question 20

What is entropy? (2 marks)

Answer 20

The dispersal of energy within the chemicals making up a chemical system.

Question 21

Is the entropy greater in a gas of a liquid, why? (3 marks)

Answer 21

Gas
The system is more random, so energy is spread out more
In a liquid, the system is less random, energy is more concentrated, so entropy is lower.

Question 22

How does Mr affect entropy? (2 marks)

Answer 22

The higher the Mr of a substance, the higher the entropy.

Question 23

Explain the effect of a higher Mr on entropy. (2 marks)

Answer 23

More electrons, the number of ways of arranging particles increases with the number of particles. So entropy is higher.

Question 24

Explain the effect of mixing on entropy. (2 marks)

Answer 24

Mixing increases the entropy of a solution, because there are a greater number of ways of rearranging the particles.

Question 25

Why does sodium chloride dissolve? (3 marks)

Answer 25

Positive changes in entropy favour chemical and physical changes.
Whilst in terms of enthalpy NaCl should not dissolve, there is a positive entropy change as the ions and water molecules mix.

Question 26

What is the unit for entropy? (1 mark)

Answer 26

JK-1mol-1

Question 27

define free energy change. (1 mark)

Answer 27

The overall change in energy during a chemical reaction.

Question 28

What is free energy made up of? (2 marks)

Answer 28

Enthalpy change
Entropy change

Question 29

Define enthalpy change. (2 marks)

Answer 29

The heat transfer between the chemical system and its surroundings.

Question 30

What is the equation for free energy? (2 marks)

Answer 30

Free energy= enthalpy change - (temperature x entropy change)

Question 31

When is the reaction feasible? (1 mark)

Answer 31

When free energy < 0

Question 32

When is a reaction not feasible? (1 mark)

Answer 32

When free energy > 0

Question 33

What is the value of free energy at the minimum temperature for feasibility? (1 mark)

Answer 33

0