Unit 5 Thermodynamics

Question 0

Why are first electron affinities always exothermic?

Answer 0

There is an electrostatic attraction between the negative external electron and positive nucleus of the atom.

Question 1

What must the overall enthalpy change be for each reaction in the Born-Haber cycle?

Answer 1

0 kJ/mol

Question 2

What is the Gibbs free energy equation?

Answer 2

ΔG = ΔH - TΔS

Question 3

Why is the melting of water endothermic?

Answer 3

Energy must be supplied to break hydrogen bonds between water molecules.

Question 4

In the Born-Haber cycle, what is the direction of the arrow for the 1st electron affinity?

Answer 4

Down - exothermic

Question 5

In the Born-Haber cycle, what is the direction of the arrow for the 2nd electron affinity?

Answer 5

Up - exothermic

Question 6

In the Born-Haber cycle, what is the length of each arrow proportional to?

Answer 6

The size of the enthalpy change

Question 7

What is meant by the Hydration enthalpy?

Answer 7

The enthalpy change when one mole of gaseous ions become hydrated (dissolved in water).

Question 8

What is meant by the Enthalpy of solution?

Answer 8

The enthalpy change when one mole of an 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 9

Gibbs free energy is ……….. for a feasible reaction

Answer 9

Negative

Question 10

What are the units of entropy

Answer 10

JK-1mol-1

Joules per kelvin per mole

Question 11

What happens when Gibbs free energy is zero?

Answer 11

The temperature at which a reaction becomes feasible

Question 12

What is the formula to calculate enthalpy of solution using lattice dissociation?

Answer 12

Enthalpy of solution = lattice enthalpy of dissociation + sum of hydration enthalpies

Question 13

What is meant by lattice formation enthalpy?

Answer 13

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

Question 14

What is meant by first electron affinity?

Answer 14

The standard enthalpy change when a mole of gaseous atoms is converted to a mole of gaseous ions, each with a single negative charge

Question 15

What is meant by first ionisation energy?

Answer 15

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions each with a single positive charge

Question 16

What is meant by mean bond energy?

Answer 16

The enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form two free radicals, averaged over a range of compounds

Question 17

How do you calculate enthalpy change using mean bond enthalpies?

Answer 17

Sigma bonds broken - sigma bonds formed

Question 19

If the equation for Gibbs free energy is applied to the equation of a straight line y = mx + c, what is the gradient of the graph equal to?

Answer 19

-delta S

Question 20

What are the units of entropy?

Answer 20

J K^-1 mol^-1

Joules per Kelvin per mole

Question 20

What does covalent character do to lattice enthalpy values?

Answer 20

Increases it.

Question 21

What does covalent character do to lattice enthalpy values?

Answer 21

Increases them.

Question 22

What has 0 entropy?

Answer 22

Perfect ionic crystal, at 0K

Question 23

What is the state of a reaction, if ΔG=0?

Answer 23

At equilibrium.

Question 24

What is meant by enthalpy of atomisation?

Answer 24

The enthalpy change when one mole of gaseous atoms, are formed from their elements in their standard states, under standard conditions.

Question 25

What is meant by enthalpy change?

Answer 25

The change in heat, at constant pressure (100kPa)

Question 26

Why is chelation “thermodynamically favourable”?

Answer 26

Increase in entropy, due to more molecule on RHS of equation.

Question 27

State Hess’s Law.

Answer 27

The enthalpy change of a reaction is independent of the route taken.

Question 28

In terms of Gibbs free energy, when is a reaction feasible?

Answer 28

ΔG < 0