Thermodynamics Flashcards
Enthalpy of formation
- Enthalpy change 1 mole of a compound is formed from its constituent elements
- Under standard conditions, all reactants and products in their standard states
- Na (s) + ½Cl2 (g) NaCl (s) [Hf = - 411.2 kJ mol-1]
Lattice enthalpy of dissociation
- Enthalpy change when 1 mol of an ionic substance is dissociated into its component gaseous ions
- Under standard conditions
- NaCl (s) ⇒ Na+ (g) + Cl- (g)
Enthalpy of atomistation
- Enthalpy change when 1 mole of gaseous atoms are made from its element in its standard state
- Na (s) ⇒ Na(g)
- ½ O2 (g) ⇒ O (g)
First ionisation energy
- Enthalpy change when 1 mole of gaseous atoms is converted into 1 mole of gaseous ions with a single positive charge
- Mg (g) ⇒ Mg+ (g) + e-
First electron affinity
- Enthalpy change when 1 mole of a gaseous atoms is converted to 1 mole of gaseous ions, with a single negative charge
- By the addition of 1 electron
- O (g) + e- ⇒ O- (g)
Second electron affinity
- Enthalpy change when 1 mole of electrons is added to 1 mole of gaseous ions with 1- charge to form gasous ions with 2-
- O- (g) + e- ⇒O2- (g)
Lattice formation energy
- Enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions
- Na+ (g) + Cl- (g) ⇒ NaCl (s)
Enthalpy of hydration
- Enthalpy change when 1 mole of a gaseous ion is converted to an aqueous ion
- X+(g) + aq ⇒ X+
- X- (g) + aq ⇒ X- (aq)
Enthalpy of solution
- Enthalpy change when 1 mole of a solute dissolves completely** in **enough solvent to form a solution which the molecules or ions are far enough apart to not interact with each other
- NaCl (s) + aq ⇒ Na+ (aq) + Cl-(aq)
Mean bond dissociation enthalpy
- Mean enthalpy change when 1 mole of covalent bond is broken homolytically, forming 2 gaseous atoms
- This is averaged over a range of compounds
Even though G would be more positive, why do we increase the temperature
- To speed up the reaction as Reaction would proceed too slowly
- Make sure the energy of the molecule exceeds that of the Ea
Hess’s law
The enthalpy change of a chemical reaction is the same irrespective of the route taken
Why is the second electron affinity of ‘S’ endothermic
- The negative S– ion repels the electron being added
- Energy must be supplied to overcome the repulsion
Why is the second ionisation energy of Ca bigger than the first?
- Electron being removed from a +ve ion
- The electron is closer** and more strongly **attracted to the nucleus
Why is the entropy of anything 0 at 0k
- Molecules are stationary
- No disorder
- entropy is zero at 0 k by definition
Why is the entropy change bigger when a liquid turns to gas than ice to water?
The increase in disorder is greater when a gas is formed from a liquid
Why is the ionisation energy of Na (top) greater than Cs (bottom) of group 1?
- Na is a smaller atom, so it has less shells so less shielding
- The atomic radius is also smaller
- Stronger attraction between the nucleus and outer electrons so more energy required
Feasible
G is less than or equal to 0
Why would there be a decrease in entropy?
- Less number of moles
- Fewer gas molecules
- Solid instead of solution
- All of this means there is more order in a system
Enthalpy change
- Heat energy change
- Measured at constant pressure
Why does the electron affinity of fluorine has a negative value.
- There is an attraction between the nucleus and the added electron
- Energy is released when the electron is gained so exothermic
- F- more stable than F
Explain why the theoretical enthalpy of lattice dissociation is different from the experimental value
- Experimental lattice enthalpy value includes covalent interaction
- Theoretical lattice enthalpy value assumes there’s only ionic interaction, evenly distrubuted charges and perfect spheres
- Forces in the actual lattice are stronger** than pure ionic attractions so **more energy is needed to break them
The theoretical enthalpy of lattice dissociation for AgCl is +770 but AgF is +995. Explain the difference
- Cl- is bigger than F- so F- has a bigger charge density
- Attraction between Ag+ and F– is stronger
Why is the hydration of the chloride ion an exothermic process
- Water is polar, as it has Hδ+
- Cl- attracts the H in water molecules
- energy is released
2 properties of ions that influence the value of a lattice enthalpy calculated using a perfect ionic model.
- Ionic radius as this affects the distance between ions
- Charge density of ions
Explain why the ΔG for the dissolving of KCl in water can negative, even though the ΔH is positive.
- The entropy change must be positive meaning there’s an increase in disorder
- Because no of particles increases as 1 mol (solid) → 2 mol (aqueous ions)
- Therefore T∆S > ∆H
Why is there a positive correlation between temperature and entropy?
- As T increases, there is an increased kinetic energy
- particles start vibrate more
- Disorder and randomness increases
Why is the bond enthalpy for Cl bigger than Br?
- In Cl2 the bonding pair closer to nucleus and there’s less shielding due to less electron shells
- So attraction between nucleus and bond pair is stronger
Why is the bond enthalpy of ClF different to that of ClF3
Cl–F bond in ClF is different from that in ClF3
Explain why the lattice dissociation enthalpy of MgCl2 is greater than that of CaCl2.
- The Mg2+ is smaller so it has a greater charge density
- The attraction between the Cl- is stronger
- Stronger ionic bonds are formed
Explain why the lattice dissociation enthalpy of MgO is greater than that of MgCl2
- The O2- has a greater charge so the charge density is greater than the Cl-
- So it attracts the Mg2+ more strongly
why can’t we obtain the enthalpy of solution of magnesium oxide by experiment
Allow MgO does not dissolve in water as it is insoluble
Why does N2 have a higher entropy than C?
- N2 is a gas whereas C is a solid
- N2 has a greater kinetic energy so there’s more vibration
- There’s more disorder
Why is turning ice into water vapour endothermic
- Hydrogen bonds between water molecules
- Energy needed to break them
Why does pure ice look pale blue when illuminated by white light.
- Complementary colour to blue light in visible spectrum absorbed by ice
- Blue light transmitted and observed
Why is the hydration enthalpy of the fluoride ion more negative than that of the chloride ion.
- Fluoride ions smaller than chloride so greater charge density
- Water is polar, it has Hδ+
- This means negative charge attracts Hδ+ in water more strongly
- More energy is released (exothermic)
Why does Mg2+ attract water during hydration
- H2O is polar as O is delta -ve
- Mg2+ attracts the lone pair on O
