Chapter 17 (Thermodynamics) Flashcards
Explain why the feasibility of a reaction changes with temperature
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.
What is the Enthalpy of Formation?
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]
What is the Enthalpy of Atomisation?
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]
What is the Enthalpy of Sublimation?
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]
How is enthalpy of dissociation calculated for diatomic molecules?
2 times the enthalpy of atomisation for that element.
What is the Bond Dissociation Enthalpy?
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
What is first ionisation enthalpy?
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]
What is Second Ionisation Enthalpy?
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]
What is the First Electron Affinity?
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
Is the first electron affinity endo or exo thermic and why?
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
What is the Second Electron Affinity?
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
Is the second electron affinity endo or exo thermic and why?
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
What is the Enthalpy of Lattice Formation?
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]
What is Enthalpy of Lattice Dissociation?
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]
What is Enthalpy of Hydration?
Enthalpy change when 1 mole of gaseous ions becomes aqueous ions. (∆hydH)
X+(g) + aq > X+(aq)
X-(g) + aq > X-(aq)
Is Enthalpy of Hydration endo or ex thermic and why?
Exothermic because bonds are made between the ions and the water molecules.
What is the Enthalpy of Solution?
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)
How does the size of the ion impact the strength of the enthalpy of lattice formation?
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.
How does the charge of the ion impact the strength of the enthalpy of lattice formation?
The bigger the charge of the ion, the greater the attraction between the ions so the stronger the lattice enthalpy (more negative values).
What is the perfect ionic model?
The ions are 100% ionic and spherical and the attractions are purely electrostatic.
What is the difference between theoretical and Born Haber lattice enthalpies?
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.
What impacts the tendency of covalent character in ionic substances?
- The positive ion is small
- The positive ion has multiple charges
- The negative ion is large.
- The negative ion had multiple negative charges.
What is meant by polarised?
The negative ions becoming distorted and more covalent.
What is a spontaneous process?
A process which will proceed on its own without any external influence.
