Physical 8: Thermodynamics

Question 1

What is enthalpy change?

Answer 1

Heat change in a reaction at constant pressure

Question 2

What are standard conditions?

Answer 2

100kPA
298K

Question 3

Define standard molar enthalpy of formation.

Answer 3

The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions with all reactants and products in their standard states

Question 4

Define standard molar enthalpy of combustion.

Answer 4

The enthalpy change when one mole of a compound is burned completely in oxygen

Question 5

Define standard enthalpy of atomisation.

Answer 5

The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
e.g. Mg(s) –> Mg(g)

Question 6

Define first ionisation energy.

Answer 6

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of positively charged ions

Question 7

Define second ionisation energy.

Answer 7

The energy required to remove one mole of electrons from one mole of 1+ ions to form one mole of 2+ ions

Question 8

Define first electron affinity.

Answer 8

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions with a single negative charge
E.g. O(g) + e- –> O-(g)

Question 9

Is first electron affinity exothermic or endothermic, and why?

Answer 9

First electron affinity is exothermic because energy is given out when electrons are attracted to the positiely charged nucleus

Question 10

Define second electron affinity.

Answer 10

The standard enthalpy change when one mole of electrons is added to a mole of gaseous 1- ions to form one mole of 2- ions

Question 11

Are second electron affinities exothermic or endothermic?

Answer 11

Endothermic because it is more difficult for a negative ion to gain an electron due to charge repulsion, so energy is needed to overcome this repulsion

Question 12

Define lattice enthalpy of formation.

Answer 12

The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions

Question 12

Define lattice enthalpy of dissociation.

Answer 12

The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions

Question 13

How does lattice enthalpy vary with size of ion?

Answer 13

Larger ions = smaller lattice enthalpy
This is because the opposite charges do not approach each other as closely when the ions are larger

Question 14

Define enthalpy of hydration.

Answer 14

The standard enthalpy change when one mole of gaseous ions become aqueous ions
E.g. Na+(g) + aq –> Na+(aq)
Enthalpy of hydration is exothermic because bonds are made between the ions and water molecules

Question 15

Define enthalpy of solution.

Answer 15

The standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart to not interact
E.g. NaCl(s) –> Na+(aq) + Cl(aq)

Question 16

Define mean bond enthalpy.

Answer 16

The standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms

Question 17

What is a Born-Haber Cycle?

Answer 17

A thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound
They are constructed by starting with the elements in their standard states, the enthalpy of which will be zero

Question 18

Draw a Born-Haber Cycle for sodium chloride.

Answer 18

Question 19

Describe how hydration occurs.

Answer 19

Solvent molecules (i.e. water) cluster around the charged ions:
The positive dipoles face towards negative ions
The negative dipoles face towards positive ions

Question 20

Draw a general thermochemical cycle to calculate enthalpy of solution.

Answer 20

Question 21

Draw a thermochemical cycle to calculate the enthalpy of solution of NaCl.

Answer 21

Question 22

What is entropy and what are its units?

Answer 22

A mathematical expression of the ‘randomness’ of a system
Given the symbol S
And the units JK-1mol-1

Question 23

What happens to entropy as temperature increases?

Answer 23

As temperature increases, entropy also increases, because the particles have more kinetic energy so have more freedom of movement, so randomness increases

Question 24

What are the two factors that govern the feasibility of a chemical reaction?

Answer 24

Enthalpy change
Entropy change

Question 25

What is Gibbs free energy equation?

Answer 25

ΔG = ΔH - TΔS

Question 26

When would a reaction not be feasible?

Answer 26

When ΔG is positive

Question 27

When is a reaction feasible?

Answer 27

When ΔG is less than or equal to zero