Thermodynamics

Question 1

What is the enthalpy change of atomisation. Write the general equation and the equation for chlorine?

Answer 1

the enthalpy change when one mole of gaseous atoms is produced from an element in its standard state

1/2Cl₂ (g) →Cl (g)

X(s) → X(g)

Question 2

What is the enthalpy change of hydration enthalpy? Write the equation for magnesium.

Answer 2

the enthalpy change to form one mole of aqueous ions from one mole of gaseous ions

Mg²⁺ (g) → Mg ²⁺ (aq)

Question 3

What is the enthalpy change of solution? Write the equation for magnesium chloride.

Answer 3

The enthalpy change when one mole of a sold ionic compound is dissolved in water to produce aqueous ions

MgCl₂ (s) → Mg²⁺ (aq) + 2Cl ^- (aq)

Question 4

What is the lattice formation enthalpy? Write the equation for magnesium.

Answer 4

the enthalpy change to form one mole of a solid ionic lattice from its ions in the gaseous state

exothermic

Mg²⁺ (g) + 2Cl^- (g) → MgCl₂

Question 5

What is the lattice dissociation enthalpy? Write the general equation

Answer 5

the enthalpy change to break one mole of a solid ionic lattice into its gaseous ions

endothermic

MgCl₂(s) → Mg²⁺ (g) + 2Cl^- (g)

Question 6

What is the enthalpy of vaporisation? Write the equation for sodium.

Answer 6

enthalpy change when one mole of liquid is turned into gas

Na (l) → Na (g)

Question 7

What is the enthalpy of fusion? Write the equation for sodium

Answer 7

The enthalpy change when one mole of a solid is turned into a liquid

Na (s) → Na (l)

Question 8

What is the standard enthalpy change of formation? Write the equation for magnesium chloride .

Answer 8

The enthalpy change to form one mole of a compound from its element, with all substances in their standard states, at 100kPA and the stated temperature

Mg (s) + Cl₂ (g) → MgCl₂ (s)

Question 9

What is the standard enthalpy change of combustion? Write the for ethane

Answer 9

the enthalpy change when ONE mole of a substance reacts completely with oxygen under standard conditions and all of the substances are in their standard state

C₂H₆ (g) + 3 1/2 O₂ (g) → 2 CO₂ (g) + 3H₂O (l)

Question 10

What is the definition for ionisation enthalpy? Write the equation for magensium

Answer 10

The enthalpy change when each atom in one mole of gaseous atoms loses one electron to form one mole of gaseous 1+ ion

Mg⁺ (g) → Mg²⁺ (g) + e-

Question 11

What is the definition for the enthalpy for electron affinity? Write the equation for chlorine

Answer 11

the enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.

Cl (g) + e → Cl^- (g)

Question 12

What is the definition for the mean bond dissociation enthalpy? Write the equation for chlorine

Answer 12

the enthalpy change to break 1 mole of bonds in 1 mole of gaseous molecules (average over a number of compounds).

Cl-Cl (g) → 2Cl (g)

Question 13

What are the two main factors that affect enthalpy change?

Answer 13

size: smaller ions have a larger attraction due to higher density charge

charge on the ion: larger charge have a larger attraction for the water molecules due to larger charge density

Question 14

What does it mean when an ion is hydrated and what happens to this ion?

Answer 14

The ion is surrounded by water molecules.

If the ion is positive an attraction is formed with the delta negative oxygen atoms in H2O

If the ion is negative an attraction is formed with the delta positive hydrogen atoms in H2O

Question 15

Why is hydration exothermic?

Answer 15

energy is released when attractions are formed

Question 16

What is the difference in exothermic hydration enthalpies with smaller and larger ions and why is this?

Answer 16

Smaller ions have more exothermic hydration enthalpies as they have a stronger attraction to water molecules due to their higher charge density (the charge is more concentrated over the volume of the ion)

larger ions have less exothermic hydration for the opposite reasons

Question 17

What must happen for a substance to dissolve?

Answer 17

the attractions formed with water molecules must be the same strength if not greater than those broken

Question 18

Why do small ions generally have stronger attractions?

Answer 18

because they have a higher charge density

Question 19

What is theoretical lattice enthalpy and what is it based on? What does it assume?

Answer 19

lattice enthalpy changes made by calculations based on the pure ionic model of the lattice

based on radius and charges of ions

assumes that all ions are spherical, ions are a known distance apart and that the charge is evenly distributed around the ion

Question 20

What is experimental lattice enthalpy and what is it based on?

Answer 20

hess cycle/ Born harber cycle

takes into account extra attractions unlike theoretical that holds ions together that makes the lattice have high magnitude

(usually higher than theoretical lattice enthalpy)

Question 21

How do you know when the theoretical value is not using a perfect ionic structure?

Answer 21

if there are much more discrepancies between the theoretical and experimental value

Question 22

What does covalent character mean?

Answer 22

When a positive ion attracts electron density from the negative ion. The positive ion “polarises”/ distorts the electron density on the negative ion towards the positive ion.

This means that the negative ion has become non-spherical resulting in a degree of covalency due to some electron density being shared between the two ions

Question 23

When does covalent character occur?

Answer 23

it only happens if the positive ion has a high charge density to distort the negative ion and if the negative ion is not good at holding on to outer electrons

Question 24

What are the properties of a positive and negative ion with high convalency?

Answer 24

Postive:

• good at attracting electron density
• small and highly charged (as they will have a high charge density

Negative:

• large = outer electrons further away from the nucleus (positive ion can distort easily)
• high negative charge = higher build up of electrons on the outside = positive ion more able to attract them

Question 25

How does high covalency affect theoretical and experimental values

Answer 25

leads to a bigger difference between them, as there are more additional attractions

Question 26

What is entropy

Answer 26

a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared out between the particles the measure of disorder in particles

Question 27

What are the factors that contribute to a high entropy change?

Answer 27

* lots of particles (as there are more ways the particles and their energy can be arranged) aka the complexity of a molecule * the particles containing a high amount of energy *

Question 28

What is a spontaneous change?

Answer 28

one that tends to go naturally (without assistance)

Question 29

What does it mean if a reaction is spontaneous/feasible?

Answer 29

it can go of its own accord thermodynamically

Question 30

Describe the spontaneous change in an exothermic reaction

Answer 30

From a higher energy state to a lower energy state = release energy and the enthalpy change is negative

Question 31

Describe the spontaneous change in an endothermic reaction

Answer 31

from a lower state to a higher state = takes in energy and the enthalpy change is postive

Question 32

Why are reactions feasible/spontaneous?

Answer 32

Because substances always tend towards disorder = they are more energetically stable when there’s more disorder → particles will move to increase their entropy

Question 33

What is the formula for the entropy change?

Answer 33

entropy change of products - entropy change of reactants

Question 34

What is the definition of the standard entropy of a substance?

Answer 34

The entropy of one mole of that substance under standard conditions (at a pressure of 100 kPa and a temperature of 298 K)

Question 35

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

Answer 35

* the enthalpy change * the entropy change * sometimes temperature

Question 36

What is the equation for Gibbs free energy and how do we use this to see whether a reaction is feasible or not?

Answer 36

When a reaction is 0 or negative

Question 37

Explain why the bond enthalpy of a cl-cl bond is greater than that of a Br-Br bond?

Answer 37

* The bonding pair is closer to the nucleus * larger attractions with each other

Question 38

Explain the meaning of the term perfect ionic model (4)

Answer 38

* only ionic bonding/ electrostatic attractions * assumes that all ions are perfectly spherical * no covalent character * ions are point charges

Question 39

By describing the nature of the attractive forces involved, explain why the value of the enthalpy of hydration for the chloride ion is more negative than that of the bromide ion

Answer 39

* chlorine is a smaller ion so it has a higher charge density * So the force of attraction between chloride ions and water is stronger * chlorine ions attract the s+ H ions of water

Question 40

Explain why the free-energy change for the dissolving of potassium chloride in water is negative, even though the enthalpy change is positive

Answer 40

entropy increases Because 1 mol (solid) → 2 mol (aqueous ions) / no of particles increases Therefore T∆S \> ∆H

Question 41

In terms of electrostatic forces, suggest why the electron affinity of fluorine has a negative value.

Answer 41

There is an attraction between the nucleus / protons and (the added) electron(s) Energy is released (when the electron is gained)

Question 42

Suggest why hydration of the chloride ion is an exothermic process.

Answer 42

Water is polar / water has Hδ+ (Chloride ion) attracts (the H in) water molecules

Question 43

Why is the second electron affinity endothermic?

Answer 43

If we want to add one more electron after the addition of one electron then the incoming electron will face repulsion from the electrons in valence shells. So we need to provide some energy if we want to add one more electron to negative − ion