Enthalpy And Entropy Flashcards
Enthalpy change of reaction
The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states.
Enthalpy of combustion
The enthalpy change when one mole of a compound completely reacts with oxygen under standard conditions, with reactants and products in their standard states. - Exo
Enthalpy of neutralisation
The enthalpy change when one mole of water is after is formed from an acid by base reaction, under standard conditions.
Enthalpy change of formation
Enthalpy change due to 1 mole of a compound forming from its constituent elements under standard conditions. -Exo
Lattice enthalpy
Enthalpy change that takes place due to the formation of 1 mole of an ionic lattice from its gaseous ions under standard conditions. (Used to determine strength of ionic bond in ionic compound)
-Exo
Na+ (g) + Cl-(g) —> NaCl (s)
First ionisation energy
Energy required to remove one mole of electron from 1 mole of gaseous atom to form one mole of +1 gaseous ions.
- Endo
Na(g) —> Na+(g) + e-
Enthalpy of hydration
The enthalpy change that takes place when one mole gaseous ions are dissolved in water to form aqueous ions.
- Exo
Na+ (g) + aq —> Na+ (aq)
Enthalpy of solution
Standard enthalpy change when one mole of solute completely dissolved in a solvent and forms aqueous ions.
(goes below LE on LHS side when negative) or
(below hydration when positive on RHS)
NaCl(s) + aq —> Na+ (aq) + Cl- (aq)
Enthalpy change of atomisation
- Endothermic
- 1/2Cl2 (g) —> Cl (g)
One mole of gaseous atom from elements
Lattice dissociation enthalpy (reverse of lattice enthalpy)
- Endo
NaCl (s) —> Na+ (g) + Cl- (g) - 1 mole of solid ionic compound separates into its gaseous ions
First electron affinity (opposite of ionisation)
- Exo
Cl (g) + e- —> Cl- (g)
Mean bond enthalpy
- Endo
- enthalpy change required to break one mole of a specific type of bond within a gaseous molecule across a range of compounds
- CH4 (g) —> C(g) + 4H (g)
Properties of ionic compounds
- high melting point
- soluble in polar solvents
- conduct electricity when molten or in aqueous solution
What is the effect of increasing ionic size on lattice enthalpy?
the ionic radius increases, weaker attraction between ions, lattice energy gets less exothermic and melting point decreases.
What is the effect of increasing ionic charge on lattice enthalpy?
Ionic charge increases, stronger attraction between ions, lattice energy becomes more exothermic and melting point increases.
What is the effect of increasing ionic size on the enthalpy of hydration?
Ionic radius increases, attraction between ion and water molecules decrease and hydration energy gets less exothermic.
What is the effect of increasing ionic charge on hydration enthalpy?
Attraction between ions and water molecules increases and hydration energy becomes more exothermic.
What is entropy?
A measure of the dispersal of energy in a system, which increases as the system becomes more disordered.
Why does entropy always have a positive value?
At below 0K there is no energy within a system and every system has energy so entropy has to have a value of 0 JK-1mol-1, must be above 0 for it to have an entropy value.
What effect does changing temperature have on entropy?
As temp increases, kinetic energy of particles increase, system becomes more disordered so entropy increases. (More positive)
What effect does dissolving ionic solids have on entropy?
As ionic solids dissolve, ions spread out, disorder increases and entropy increases.
What effect does the number of gas molecules have on entropy?
As the number of gas molecules increase, entropy increases (vice versa)
Entropy Equation
^S = Products - Reactants
Units for entropy
JK-1mol-1
Feasibility
Describes whether a reaction takes place.
Gibbs’s Equation
^G = ^H - T^S
^G = kJmol-1
^H = kJmol-1
^S = JK-1mol-1 /1000 = kJK-1mol-1
When is a process feasible?
When the Gibbs’s equation is less than or equal to 0, so it is negative
What is a limitation of predictions made for feasibility?
Does not take kinetics or rate of reaction into account. For example if a reaction has a large activation energy it would take a long time to occur even at high temps (requires a catalyst).
Lattice enthalpy equation
Route 2=Route 1
What happens to entropy when particles are dissolved
Aqeous is more disordered than solid
What is feasibility dependent on?
It is dependent on temperature, enthalpy change and entropy change.
Why is second ionisation energy more endothermic than first ionisation energy
Ion is smaller than the atom because there is stronger attraction to the nucleus
Why is second electron affinity positive?
Ion and electron are both negative so energy is required to overcome the repulsion
