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