A-level Only Physical Chemistry Flashcards
What is Enthalpy change?
Heat energy change at constant pressure under standard conditions (100KPa and 298K) in standard states.
Concentration = 1mol dm-3
What is Enthaply of Formation?
1 mole of the substance is made under standard conditions with elements in standard states.
What is Enthalpy of Combustion?
1 mole is completely combusted in excess oxygen in standard conditions, with elements in their standard states.
What is Enthalpy of first ionisation energy?
Energy required to remove an electron from 1 mole of GASEOUS atoms to form 1 mole of GASEOUS 1+ ions
ENDOTHERMIC
What is Enthalpy of second ionisation energy?
Energy required to removed an electron from 1 mole of GASEOUS 1+ ions to form 1 mole of GASEOUS 2+ ions
ENDOTHERMIC
What is Enthalpy of first electron affinity?
Energy change required when 1 mole of gaseous atoms gain an electron to from 1 mole of gaseous 1- ions
EXOTHERMIC
E.g. N (g) + e- —> N- (g)
As (g) + e- —> As- (g)
Always a gas
What is Enthalpy of second electron affinity?
Energy change when 1 mole of gaseous 1- ions gain an electron to form 1 mole of gaseous 2- ions
ENDOTHERMIC
E.g. U- (g) + e- —> U2- (g)
What is Enthalpy of Atomisation?
Enthalpy change when 1 mole of gaseous atoms are forms from an element in its standard states
ENDOTHERMIC
E.g. Na (s) —> Na (g)
1/4 P4 (s) —> P (g)
1/2 F2 (g) —> F (g)
What is lattice Enthalpy of formation?
Energy change when 1 mole of an ionic compound is formed from its constituent (ions in the compound) gaseous ions
EXOTHERMIC
E.g. Na+(g) + Cl- (g) —> NaCl (s)
Zn2+ (g) + 2Br- (g) —> ZnBr (s)
What is Lattice Enthalpy of dissociation?
Enthalpy change when 1 mole of an ionic compound is broken into its constituent gaseous ions
ENDOTHERMIC
E.g. NaCl(s) —> Na+ (g) + Cl- (g)
What is Bond Dissociation Enthalpy?
Enthalpy change when 1 mole of covalent bonds is broken in gaseous state.
ENDOTHERMIC
E.g. Cl2 (g) —> 2Cl (g)
What are the rules of a Born-Haber cycle?
- Arrows represent a reaction
- Upwards arrow = endothermic
- Downwards arrow = exothermic
- Cycle counters the direction of the arrow = sign of the Enthalpy change is reversed.
When finding one of the Enthalpies: - Start = beginning of arrow
- Finish = head of arrow
Enthalpies of Solution and Hydration = another way of measuring lattice Enthalpies indirectly.
What is Enthalpy of Solution?
Enthalpy change when one mole of ionic solid is dissolved in water so that the ions no longer interact under standard conditions
ENDOTHERMIC or EXOTHERMIC
Enthalpies of Solution and Hydration = another way of measuring lattice Enthalpies indirectly.
What is Enthalpy of Hydration?
Enthalpy change when one mole of gaseous ions are dissolved in water to from one mole of aqueous ions under standard conditions
Always EXOTHERMIC
What is the Perfect Ionic Model theory?
- Hydration Enthalpy is heavily influenced by the size and charge of the molecules being dissolved.
- Ions are therefore assumed to be ‘perfectly ionic’
What does perfect ionic model assume?
- All ions are perfectly spherical
- Bonding is 100% ionic (no covalent character)
When does Covalent character occur?
When two joined ions have varying sizes or charges meaning the distribution of charge is not even.
E.g. Al 3+ and O 2- so Al2O3 has a ‘degree of covalent character’
What determines something to have a large degree of covalent character?
- If the value from the Born-Haber cycle and the value using the perfect ionic model differ significantly there is a large degree of covalent character.
Why does Na+cl- have lower lattice enthalpy than mg2+cl2-?
- The higher the charge of the +ve and -ve the stronger the electrostatic attraction.
- The smaller the ionic radii, the stronger the electrostatic attraction.
What is Entropy?
- Entropy, S, is a measure of disorder within a system or substance.
What is Entropy?
- Entropy, S, is a measure of disorder within a system or substance.
What is entropy measured in?
JK-1mold-1
Why would Na+ more exothermic than K+?
It is a smaller ion.
F- will be more -ve than I-
Why will Mg2+ have a greater 🔺solH?
It has a larger charge density, smaller ion = higher charge = more exothermic
What is the trend for entropy?
- Solids < Liquids < Gas
- The larger the molecule/Mr, the higher the entropy
- The larger the temperature = higher the entropy
- Liquid to gas causes a significant increase in energy
How to calculate entropy?
🔺S = S(products) - S(reactants)
What is Gibbs Free energy?
A way of linking enthalpy and entropy
Gives information about the feasibility of a reaction
What is the Gibbs Energy equation?
🔺G = 🔺H - T🔺S
When is a reaction able to be feasible?
When 🔺G is equal or less than zero
What are the rules of feasibility?
🔺H 🔺S 🔺G
- Ve +Ve Always negative! = always feasible
+Ve -Ve Always positive! = never feasible
-Ve -Ve Only feasible at low temperatures
+Ve +Ve Only feasible at high temperatures
How to find temperature at which a reaction is feasible?
When 🔺G = O, a reaction is feasible
T=🔺H divided by 🔺S
Graphical use of 🔺G:
🔺G = 🔺H - T🔺S:
🔺G = -🔺S(T) - 🔺H
Y = mx + c
What is a spontaneous reaction?
- Has a sufficiently low activation energy to take place immediately
What is a feasible reaction in comparison to a spontaneous one?
- Has a higher activation energy, this must be supplied for the reaction to start
