Chapter 17 (Thermodynamics)

Question 1

Explain why the feasibility of a reaction changes with temperature

Answer 1

A reaction is considered feasible when ΔG ≤ 0, where ΔG = ΔH - TΔS.

as ΔH is positive and ΔS is positive, at low temperatures ΔG is also positive as ΔG is greater than TΔS.

at higher temperatures, ΔG becomes negative as TΔS is greater than ΔH.

Question 2

What is the Enthalpy of Formation?

Answer 2

Energy transferred when 1 mole of a compound is formed from its constituent elements under standard conditions (298K and 100kpa) with all reactants and products in their standard state.

∆Na (s) + 1⁄2Cl (g )>NaCl (s) [∆f H = - 411.2 kJ mol-1]

Question 3

What is the Enthalpy of Atomisation?

Answer 3

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

Na (s) > Na(g) [∆atH = +148 kJ mol-1]
1⁄2O (g)> O(g) [∆at H=+249kJmol-1]

Question 4

What is the Enthalpy of Sublimation?

Answer 4

The enthalpy change for a solid metal turning to gaseous atoms and will numerically be the same as the enthalpy of atomisation.

Na (s) > Na(g) [∆subH = +148 kJ mol-1]

Question 5

How is enthalpy of dissociation calculated for diatomic molecules?

Answer 5

2 times the enthalpy of atomisation for that element.

Question 6

What is the Bond Dissociation Enthalpy?

Answer 6

The standard molar enthalpy change when one mole of a covalent bond is broken into two gaseous atoms (or free radicals).

Cl2(g) > 2Cl(g) ∆dissH = +242 kjmol-1

CH4(g) > CH3(g) + H(g) ∆dissH = +435 kjmol-1

Question 7

What is first ionisation enthalpy?

Answer 7

Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous ions with a 1+ charge.

Mg(g) > Mg+(g) + e- [∆ie1H]

Question 8

What is Second Ionisation Enthalpy?

Answer 8

The enthalpy change to remove one mole of electrons from one mole of gaseous 1+ions to produce one mole of gaseous 2+ ions.

Mg+(g) > Mg2+(g) + e- [∆ie2H]

Question 9

What is the First Electron Affinity?

Answer 9

The enthalpy change that occurs when 1 mole of gaseous atoms gains 1 mole of electrons to form 1 mole of gaseous ions with a 1- charge.

O(g) + e- > o-(g) [∆ea1H] = -141.1 kjmol-1

Question 10

Is the first electron affinity endo or exo thermic and why?

Answer 10

Exothermic for atoms that normally form negative ions.

This is because the ion is more stable than the atoms and there is an attraction between the nucleus and the electron.

O(g) + e- > O-(g) [∆ea1H] = -141.1 kjmol-1

Question 11

What is the Second Electron Affinity?

Answer 11

The enthalpy change when one mole of gaseous 1- ions gains one electron per ions to produce gaseous 2- ions.

O-(g) + e- > O2-(g) [∆ea2H] = +788 kjmol-1

Question 12

Is the second electron affinity endo or exo thermic and why?

Answer 12

Endothermic because it takes energy to overcome the repulsive force between the negative ion and the electron.

O-(g) + e- > O2-(g) [∆ea2H] = +788 kjmol-1

Question 13

What is the Enthalpy of Lattice Formation?

Answer 13

The standard enthalpy change when 1 mole of ionic crystal lattice is formed form its constituent ions in gaseous form.

Na+(g) + Cl- > NaCl(s) [∆lattH = -787 kjmol-1]

Question 14

What is Enthalpy of Lattice Dissociation?

Answer 14

The standard enthalpy change when 1 mole of an ionic crystal lattice is separated into its constituent ions in gaseous form.

NaCl(s) > Na+(g) + Cl-(g) [∆lattH = +787 kjmol-1]

Question 15

What is Enthalpy of Hydration?

Answer 15

Enthalpy change when 1 mole of gaseous ions becomes aqueous ions. (∆hydH)

X+(g) + aq > X+(aq)

X-(g) + aq > X-(aq)

Question 16

Is Enthalpy of Hydration endo or ex thermic and why?

Answer 16

Exothermic because bonds are made between the ions and the water molecules.

Question 17

What is the Enthalpy of Solution?

Answer 17

The standard enthalpy change when 1 mole of an ionic crystal lattice form is separated unto its constituent ions in gaseous form.

NaCl(s) + aq > Na+(aq) + Cl-(aq)

Question 18

How does the size of the ion impact the strength of the enthalpy of lattice formation?

Answer 18

The larger the ion the less negative the enthalpies of lattice formation e.g. a weaker lattice. As the ions are larger the charges become further apart and so have weaker attractive forces between them.

Question 19

How does the charge of the ion impact the strength of the enthalpy of lattice formation?

Answer 19

The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).

Question 20

What is the perfect ionic model?

Answer 20

The ions are 100% ionic and spherical and the attractions are purely electrostatic.

Question 21

What is the difference between theoretical and Born Haber lattice enthalpies?

Answer 21

Born Haber is the real experimental value.

When a compound shows covalent character the Born Haber and theoretical values differ. The more covalent character, the bigger the difference between the two values.

Question 22

What impacts the tendency of covalent character in ionic substances?

Answer 22

• The positive ion is small
• The positive ion has multiple charges
• The negative ion is large.
• The negative ion had multiple negative charges.

Question 23

What is meant by polarised?

Answer 23

The negative ions becoming distorted and more covalent.

Question 24

What is a spontaneous process?

Answer 24

A process which will proceed on its own without any external influence.

Question 25

What causes spontaneous reactions?

Answer 25

When a reaction is exothermic and results in products which are more thermodynamically stable than the reactants.

Question 26

What is entropy?

Answer 26

The number of ways atoms can share quanta of energy.

Question 27

What is entropy measured in?

Answer 27

Jmol-1K-1

Question 28

How does entropy vary in different states of matter?

Answer 28

• Solids have lower entropies than liquid which are lower than gas.
• When solids increase in temperature its entropy increases as the particles vibrate more.
• There is a bigger jump in entropy with boiling point than melting.
• Gas have large entropies as they are much more distorted.

Question 29

What causes a significant increase in entropy?

Answer 29

• There is a change in state from solid to liquid to gas

* There is a significant increase in number of molecules between products and reactants.

Question 30

When do substances have zero entropy and why?

Answer 30

At 0K as the particles are stationary.

Question 31

How is entropy calculated?

Answer 31

∆Sº = ∑Sºproducts - ∑Sºreactants

Question 32

What is Gibbs free energy?

Answer 32

A term that combines the effect of enthalpy and entropy into one number.

Question 33

What is the equation for Gibbs free energy?

Answer 33

∆G = ∆H - T∆S

Question 34

What is required for the reaction to be feasible with Gibbs free energy?

Answer 34

∆G to be negative, an increasing entropy (=ve ∆S) and is exothermic (-ve ∆H)

Question 35

Why might a reaction not occur even if Gibbs free energy is negative?

Answer 35

The reaction might have a high activation energy.

Question 36

Why is dissolving a lattice substance endothermic?

Answer 36

The lattice must be broken up . The enthalpy of the lattice dissociation is equal to the energy needed to break up the lattice.

Question 37

What is the formula for finding enthalpy of solution?

Answer 37

∆Hsolution = ∆Hldissociation + ∑∆hydH

or

∆Hsolution = -∆Hlformation + ∑∆hydH

Question 38

Why are hydration enthalpies exothermic?

Answer 38

Energy is given out when water molecules bond to metal ions.

Question 39

What impacts the hydration enthalpy and why?

Answer 39

The charge density as the ions attracted water molecules more strongly.

Question 40

What causes charge density?

Answer 40

Caused by negative ions attracted to δ+ hydrogens on polar water molecules and the positive ions are attracted to δ- oxygen on the polar water molecules

Question 41

What can be found from a ∆Hsolution?

Answer 41

An exothermic ∆Hsolution will more likely be soluble.

An endothermic ∆Hsolution is more likely to not dissolve because the lattice enthalpy is much larger than the hydration enthalpy and it is not energetically favourable to break up the lattice.

Question 42

Why is entropy sometimes needed to find if a substance is soluble?

Answer 42

When a solid dissolves into a liquid the entropy increases as there is more disorder as the number of particles increases. The positive ∆S can make ∆G negative even if ∆H is endothermic.