[Energetics] Enthalpy and Entropy

standard enthalpy change of combustion

standard enthalpy change of combustion

energy evolved when one mole of substance is completely burnt in oxygen under standard conditions

standard enthalpy change of neutralisation

energy evolved when one mole of water is evolved from the neutralisation reaction between acid and base under standard conditions

standard enthalpy change of formation

energy change when one mole of substance is formed from its constituent elements under standard conditions

bond energy

average energy absorbed to break one mole of X-Y covalent bonds in gaseous state to form gaseous atoms under standard conditions

lattice energy

energy evolved when one mole of solid ionic compound is formed from its constituent gaseous ions under standard conditions

standard enthalpy change of atomisation

energy absorbed when one mole of gaseous atoms is formed from them element under standard conditions

first ionisation energy

energy absorbed when one mole of gaseous atoms loses one mole of electrons to form one mole of singly positively charged gaseous ions

second ionisation energy

energy absorbed when one mole of singly positively charged gaseous ions loses one mole of electrons to form one mole of doubly positively charged ions

first electron affinity

energy evolved when one mole of gaseous atoms acquires one mole of electrons to give one mole of singly negatively charged gaseous ions

standard enthalpy change of hydration

energy evolved when one mole of gaseous ions is hydrated under standard conditions

standard enthalpy change of solution

energy change when one mole of substance is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions

why is 1st EA exothermic while 2nd EA is endothermic?

1. In 1st EA, the energy released when the nucleus attracts the electron is larger than the energy absorbed to overcome inter-electron repulsion.

2. In the 2nd EA, electrons are being added to a negatively charged ion, hence energy has to be absorbed in order to overcome repulsive forces between the two negatively charged species.

Lattice energy is proportional to:

product of charges over sum of ionic radii

Enthalpy change of hydration is proportional to:

charge over ionic radius

∆Hsoln =

∆Hhyd - ∆Hlatt

Soup = HaiDiLao

What does the process of an ionic substance dissolving (∆Hsoln) entail?

1. lattice structure destroyed to release ions (-∆Hlatt)

2. ions undergo hydration (∆Hhyd)

a solid with a more exothermic ∆Hsoln is more/less likely to be soluble in water

more

energy released during hydration is enough to compensate for the energy required to break down the crystal lattice for the compound to dissolve in water

Q =

mc∆T

∆H =

Q/n

∆Hr =

1. ∆Hf (products) - ∆HF (reactants)

2. ∆Hc (reactants) - ∆Hc (products)

3. BE (reactants) - BE products

∆Hf =

∑∆Hatom + ∑IE + ∑EA + ∆Hlatt

in what order do you construct a Born-Haber Cycle?

FAEIL

1. ∆Hf

2. ∆Hatom

3. IE

4. EA

5. LE

what is entropy?

the degree of disorder or randomness in a system

higher entropy means?

• more ways to distribute particles in space

• more ways to distribute available energy among particles

how does an increase in temperature affect entropy?

Entropy increases as there is more energy to be distributed among particles, creating greater disorder.

how does a change in phase from solid to liquid to gas affect entropy?

Entropy increases as particles can move around freely/at high speeds, and there is greater disorder. There are now more ways to distribute energy and particles.

how does an increase in gaseous particles affect entropy?

Entropy increases. The product contains more gaseous particles moving randomly, meaning there are more ways to distribute energy and particles.

how does an increase in volume for a gaseous system affect entropy?

Entropy increases. There are more ways to distribute the energy and particles in a larger volume.

how does the mixing of gaseous particles affect entropy?

Entropy increases. Each gas expands to occupy the whole container, meaning there are more ways to distribute the energy and particles in a larger volume.

∆G =

∆H - T∆S

Go Home To Sleep

-RTlnk

limitations of using ∆G to determine spontaneity of a reaction

1. only valid under standard conditions

2. only gives indication of thermodynamic feasibility, not kinetic feasibility

enthalpy change of formation of element is

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