Thermodynamics

Question 1

Define enthalpy change.

Answer 1

The heat energy transferred in a reaction at constant pressure.

Question 2

What are standard conditions?

Answer 2

100kPa

298K

Question 3

What is the enthalpy change for an exothermic reaction?

Answer 3

Negative.

Question 4

What is the enthalpy change for an endothermic reaction?

Answer 4

Positive.

Question 5

Lattice enthalpy is a measure of what?

Answer 5

Ionic bond strength.

Question 6

Define lattice formation enthalpy,

Answer 6

The total enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions.

Question 7

Define lattice dissociation enthalpy.

Answer 7

The total enthalpy chaneg when 1 mole of a solid ionic substance is completely dissociated into its gaseous ions under standard conditions.

Question 8

Define enthalpy change of formation.

Answer 8

The total enthalpy change when 1 mole of a substance is formed from its constiuent elements which are in their standard states under standard conditions.

Question 9

Define enthalpy change of atomisation of an element.

Answer 9

The total enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.

Question 10

Define first ionisation enthalpy.

Answer 10

The total enthalpy change when 1 mole of gaseous 1+ ions is formed from 1 mole of gaseous atoms.

Question 11

Define first electron affinity.

Answer 11

The total enthalpy change when 1 mole of gasesou 1- ions is formed from 1 mole of gaseous atoms and 1 mole of electrons.

Question 12

Define enthalpy of hydration.

Answer 12

The total enthalpy change when 1 mole of aqueous ions is forme from 1 mole of gaseous ions.

Question 13

Define bond dissociation enthalpy.

Answer 13

The total enthalpy change when all the bonds of the same type in 1 mole of molecules are broken.

Question 14

Define the enthalpy of atomisation of a compound.

Answer 14

The total enthalpy change when 1 mole of a compound in its standard state is converted to gaseous atoms.

Question 15

Define second ionisation enthalpy.

Answer 15

The enthalpy change when 1 mole of gaseous 2+ ions and 1 mole of electrons is formed from 1 mole of gaseous 1+ ions.

Question 16

Define second electron affinity.

Answer 16

The total enthalpy change when 1 mole of gaseous 2- ions is formed from 1 mole of gaseous 1- ions and 1 mole of electrons.

Question 17

Define enthalpy change of solution.

Answer 17

The total enthalpy change when 1 mole of solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution.

Question 18

What is Hess’s law?

Answer 18

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

Question 19

To find the lattice formation enthalpy for NaCl, what is the Born-Haber cycle that must be constructed.

Answer 19

First the -(enthalpy of formation for NaCl)

Then the atomisation enthalpies for both Na_(s) and 1/2Cl_2(g)

Then the first ionisation enthalpy for Na_(g)

Then the first electron affinity of Cl_(g)

Question 20

Why is it that theoretical lattice enthalpies differ from experimental values?

Answer 20

Because the theoretical values assumet that the lattice is purely ionic - experimental values suggest that ionic compounds have a degree of covalent character.

Question 21

Why is it that ionic lattices have a degree of covalency?

Answer 21

Because the positive ions polarise their negative neighbourds, withdrawing electron density from them - the more polarisation that occurs, the greater the withdrawn electron density, the greater the covalency.

Question 22

How can the covalency of an ionic bond be established?

Answer 22

By comparing the experimental and theoretical values for the bond - the greater the difference, the more covalent the bond is.

Question 23

Which are stronger, covalent or ionic bonds?

Answer 23

Covalent bonds - their formation is more exothermic than ionic bonds’.

Question 24

What are the process that occur when an ionic lattice is dissolved?

Answer 24

The bonds between the ions break - an endothermic process (lattice enthalpy of dissociation)

New bonds between the ions and water form - an exothermic process (enthalpy of hydration)

Question 25

Enthalpy change of solution = ?

Answer 25

Lattice enthalpy of dissociation + Enthalpy of hydration

Question 26

Why is it that the data book value for a bond enthalpy can differ drom the actual enthalpy of a particular bond?

Answer 26

Because data book values are mean values across a range of compounds - actual bond strength varies between compounds and even within a compound. For an accurate bond enthalpy, acquire experimental data for the relevant compound.

Question 27

Define entropy.

Answer 27

A measure of disorder in a system that defines how much work that system is able to do.

Question 28

What variables affect the entropy of a substance?

Answer 28

The physical state of the substance (Solid = ordered, liquid = less so, gas = chaotic)

Whether the substance has been dissolved.

The number of particles.

Question 29

What is a spontaneous reaction?

Answer 29

A reaction which occurs by itself.

Question 30

What is the total entropy change of a reaction?

Answer 30

The entropy of the products minus the entropy of the reactants.

ΔS_system= S_products - S_reactants

Question 31

What is the entropy change of surroundings defined as?

Answer 31

ΔS_surroundings = -ΔH / T

Where: ΔH = Enthalpy change

T = temperature of surroundings

Question 32

What is Gibbs’ free energy used for?

Answer 32

Predicting the feasibility of a reaction.

Question 33

When is a reaction spontaneous?

Answer 33

When the Gibbs’ free energy is less than or equal to zero.

Question 34

What is the equation for Gibbs free energy?

Answer 34

ΔG = ΔH - TΔS

G = Gibbs free energy

ΔH = Enthalpy change

ΔS = Entropy change

T = Temperature

Question 35

How is it possible to calculate the temperature at which a reaction becomes feasible?

Answer 35

By setting the Gibbs free energy equal to zero - this occurs just as the reaction becomes feasible.

This gives:

ΔH/ΔS_system = T