Enthalpy and Entropy A2 Flashcards
What is the definition of Enthalpy of Formation? (ΔfH) Exo or endothermic?
Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states.
Exothermic for most substances.
What is the definition of Enthalpy of Combustion? (ΔcH) Exo or endothermic?
Enthalpy change when one mole of substance undergoes complete combustion in oxygen with all substances in standard states.
Exothermic.
What is the definition of Enthalpy of Neutralisation?(ΔneutH) Exo or endothermic?
Enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions.
Exothermic.
What are the definitions of Ionisation enthalpy? (ΔIEH) Exo or endothermic?
First ionisation energy = enthalpy change when each atom from one mole of atoms loses an electron to form one mole of gaseous 1+ ions.
Second ionisation energy = enthalpy change when each ion from one mole of 1+ ions loses an electron to form one mole of gaseous 2+ ions.
Endothermic
What are the definitions of Electron Affinity (ΔEAH) Exo or endothermic?
First electron affinity = enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.
Exothermic for many non-metals.
Second electron affinity = enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions.
Endothermic as adding -ve electron to -ve ion = +ve
What is the definition of Enthalpy of Atomisation? (ΔatH) Exo or endothermic?
Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.
e.g. I2 (s) –> I (g)
Endothermic
What is the definition of Hydration enthalpy? (ΔhydH) Exo or endothermic?
Enthalpy change when one more of gaseous ions becomes hydrated.
e.g. Mg2+(g) + aq –> Mg2+(aq)
Exothermic
What is the definition of Enthalpy of Solution? (ΔsolH) Exo or endothermic?
Enthalpy change when one mole of ionic solid dissolves in a solvent.
e.g. MgCl2(s) + aq –> Mg2+(aq) + 2 Cl-(aq)
Varies
What is the definition of Bond Dissociation enthalpy? (ΔdisH) Exo or endothermic?
Enthalpy change when one mole of covalent bonds are broken in gaseous state.
Endothermic.
What is the definition of lattice enthalpy? (ΔLEH) Exo or endothermic?
Enthalpy change when one moles of a solid ionic compound is formed from its constituent gaseous ions.
Exothermic
What is the definition of Enthalpy of vaporisation? (ΔvapH) Exo or endothermic?
Enthalpy change when one mole of a liquid is turned into a gas.
Endothermic
What is the definition of Enthalpy of Fusion? (ΔfusH) Exo or endothermic?
Enthalpy when one mole of a solid is turned into a liquid.
Endothermic
What type of enthalpy cycle relates lattice enthalpy and enthalpy of formation?
Born-Haber cycle
What two processes make up dissolving a substance in water?
Breaking ionic lattice (reverse lattice enthalpy) and water molecules surrounding (hydrating) the ions.
Enthalpy of solution = reverse lattice enthalpy* + hydration enthalpy
How does ionic size/radius and ionic charge affect lattice enthalpy?
IONIC RADIUS:
- The larger the ionic radius, the further apart and the ions are and so the forces of attraction are weaker. (So less exothermic)
- The smaller the ionic radius, the more closely packed together the ions are so greater forces of attraction. (So lattice enthalpy more exothermic).
IONIC CHARGE:
As ionic charge increases, attraction between ions increases and latice enthalpy becomes more negative/exothermic.
How does ionic radius and ionic charge affect hydration enthalpy?
IONIC RADIUS:
As ionic radius increases, attraction between water molecules and ions decreases and hydration enthalpy becomes less negative/exothermic.
(smaller molecules have a greater attraction between water molecules).
IONIC CHARGE:
As ionic charge increases, attraction between water molecules and ions increases and hydration enthalpy becomes more negative/exothermic.
What is the definition of Entropy? What is the symbol to represent entropy? Units?
A measure of THE DISPERSAL OF ENERGY IN A SYSTEM, which increases as the system gets more disordered.
ΔS - J / K / mol
(higher the entropy, greater the chaoticness!)
Does lowering the entropy make the value more or less negative?
Decreasing entropy = more negative
Why are the values for enthalpy negative?
Energy is lost (to the surroundings).
Put the 3 states of matter in order of increasing entropy.
Solid –> liquid –> gas
When…
- a liquid boils to become a gas
- a lquid freezes to become a solid
is ΔS postive or negative?
Evaporation = Positive
Freezing = negative
If there are more moles of products than reactants, does entropy increase or decrease?
Increase
What is the equation to work out standard entropy? What is standard entropy?
ΔS = ΣS (products) - ΣS (reactants)
The entropy of one mole of a substance under standard conditions (always positive values).
If there are the same number of moles of products and reactants and they are in the same state, what happens to entropy?
Entropy change is very small so you can’t tell a difference (increase/decrease) without calculations.
In an equation, when gas is a product but not a reactant, is ΔS positive or negative?
Positive
e.g. CaCO3 + HCl –> CaCl2 + H2O + CO2(g)
What is free energy change? What is the symbol?
ΔG
The overall energy change caused by the:
- heat transfer between the system and surroundings (ΔH)
- dispersal of energy within the system at the temperature of the reaction (TΔS)
Internal and external energy change
What is the Gibbs equation?
ΔG = ΔH - (T x ΔS)
Free energy change = enthalpy change - (Temperature (K) x entropy change)
When is a reaction feasible, in relation to free energy?
When ΔG < 0 (negative value)
If ΔH is positive and ΔS is negative, can the reaction ever be feasible?
No - two negatives make a positive
How do you arrange the Gibbs equation to calculate the minimum temperature that the reaction is feasible?
ΔG = ΔH - (T x ΔS)
Set ΔG to 0.
Then rearrange to T = ΔH / ΔS
How can endothermic reactions be made feasible?
If ΔS is positive enough and T really high, then then -TΔS may be negative enough to outweigh ΔH.
Why might a feasible reaction not take place?
May have a very high activation energy, so is very slow at normal conditions.
What is the thing I keep forgetting to do?
Use a negative sign for exothermic reaction!!!! ΔH
Ask yourself for everything ΔH reaction - is this endo or exothermic?
