Thermodynamics

Question 1

What is lattice enthalpy?

Answer 1

The enthalpy change when 1 mole of a solid ionic lattice forms from its gaseous ions under standard conditions

More energy released, the stronger the bonds are

Question 2

Define lattice enthalpy of formation

Answer 2

The enthalpy change when 1 mole of solid ionic lattice is formed from its gaseous ions under standard conditions

Question 3

Define standard enthalpy of formation

Answer 3

The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.

Question 4

Define standard enthalpy of atomisation

Answer 4

The enthalpy change which accompanies the formation of 1 mole of gaseous atoms from the element in its standard state under standard conditions

Question 5

Define the fist ionisation energy

Answer 5

The enthalpy change when 1 mole of gaseous atoms is converted into a mole of gaseous iona each with a single positive charge

Question 6

Define second ionisation energy

Answer 6

The enthalpy change when a mole of electrons I lost from 1 mole of a singly positively charged ion to form 1 mole of 2+ ions

Question 7

Define the first electron affinity

Answer 7

The enthalpy change when 1 mole of gaseous atoms is converted to 1 mole of gaseous ions each with a single negative charge

Question 8

Define the second electron affinity

Answer 8

The enthalpy change when 1 mole of electrons is added to 1 mole of gaseous ions each with a single negative change to form ions each with 2 negative charges

Question 9

Define lattice enthalpy of dissociation

Answer 9

The enthalpy change when 1 mole of solid ionic compound is dissociated into its gaseous ions

Question 10

Define enthalpy of hydration

Answer 10

The standard enthalpy change when 1 mole of gaseous ions is converted into 1 mole of aqueous ions

Question 11

Define enthalpy of solution

Answer 11

The enthalpy change when one mole of dilute dissolved completely in sufficient solvent to form a solution where the ions are far enough apart to not interact with one another.

Question 12

Define the mean bond enthalpy

Answer 12

The enthalpy change when 1 mole of gaseous molecules each breaks a covalent bond to form 2 free radicals averaged over a range of compounds

Question 13

What are he stages in a born haber cycle?

Answer 13

1. Atomisation of elements
2. Ionisation if elements
3. Electron affinity of elements
4. Lattice enthalpy

Question 14

What is the formula for charge density?

Answer 14

Charge density = charge/size

Question 15

What happens when we dissolve a solid?

Answer 15

First the ionic solid is broken down.
Then free ions become part of the solution (hydration)
This has an enthalpy change (enthalpy change of solution)
This can be an exo or endothermic reaction.

Question 16

How is the ionic solid broken down when dissolving a solid compound?

Answer 16

Break the solid into gaseous ions

NaCl ——> Na+ + Cl-

The reverse of this is lattice enthalpy of formation
Therefore break down = - lattice enthalpy of formation

Breaking down a lattice is always endothermic

Question 17

What factors influence the size of lattice enthalpy?

Answer 17

Small ions - smaller ions can get close to the molecules and therefore form stronger bonds

Greater charge density- increased attractive force so the bonds are stronger

Therefore lattices made from smaller more highly charged ions will have the strongest bonds and give out he most energy on formation (polarisation)

Question 18

How are ions hydrated when dissolving a solid compound?

Answer 18

Ionic solids must be dissolved in polar solvents (water)

This separates the ions and stops the solid reforming

Standard enthalpy of hydration

The magnitude of this is affected by the same factors as ionic bond strength

Question 19

What is entropy?

Answer 19

A measure of disorder

More disordered something is, the higher the entropy

Question 20

Explain why a gas has a higher entropy than a solid?

Answer 20

gas has a higher entropy than a solid as there are many more arrangements the particles can be configured/distributed compared to a solid so the entropy is higher

Question 21

What is a spontaneous reaction?

Answer 21

A reaction without external help

Entropy must increase in a spontaneous reaction

Question 22

What are the units for enthalpy?

Answer 22

Kjmol^-1

Question 23

What are the units for entropy?

Answer 23

JK^1mol^-1

J/K/mol

Question 24

How do you work out entropy change for the system?

Answer 24

Entropy change of system = sum of products - sum of reactants

Question 25

How do you calculate entropy of the surroundings?

Answer 25

Entropy of the surroundings = - enthalpy change / temperature

Question 26

How do you calculate the total entropy change?

Answer 26

Entropy change = entropy of system + entropy of surroundings
Only reactions with a positive entropy change are feasible

Question 27

How can we predict entropy changes?

Answer 27

Can predict if the entropy of system will be positive or negative without calculating it.

Change of state:
Solid/liquid/gas
Entropy———> increase

Change in the number of moles:
More moles= more configurations = more disorder = more entropy

Dissolving ionic solids in water:
Breakdown of lattice = more ways to arrange free ions (increase in entropy)
Hydration of ions = ordered around water (decrease in entropy)

Question 28

What is the equation for Gibbs free energy?

Answer 28

Gibbs energy = change in enthalpy - T(change in entropy)

Gibbs energy must be negative for a reaction to be feasible

Question 29

How is entropy change and Gibbs energy related?

Answer 29

Change in Gibbs free energy has the opposite charge to the total entropy

Change in entropy = - Gibbs energy / temperature

Question 30

How do you work out at what temperature a reaction becomes feasible?

Answer 30

When Gibbs energy is 0

T = enthalpy change / entropy change

Question 31

Explain why the first electron affinity of Halogens has a large negative value?

Answer 31

Halogens have a net attractive force, between the positive chlorine nucleus and the extra negative electron.

Question 32

Explain why the second electron affinities of elements are usually large positive values

Answer 32

Elements already have a negative charge after the gain of the first electron.
electrons repel each other so their electron affinity is more positive.

Question 33

What is the perfect ionic model?

Answer 33

Ions are perfect spherical shapes
100% ionic bonding
No distortion in the electron clouds

Question 34

why may experimental values of lattice enthalpy be higher than the theoretical values?

Answer 34

Due to the compounds not being purely ionic and therefore not having spherical shapes. This means that the electron clouds may overlap meaning there is some covalent character so the bonds/forces holding the lattice together are stronger and hence there is a higher lattice enthalpy.