3.1.8 Thermodynamics

enthalpy of formation

The standards molar enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states

Enthalpy of combustion

the enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all the substances in standard states

enthalpy of atomisation

enthalpy change when one mole of gaseous atoms is produced from elements in its standard state.

lattice enthalpy of formation

Enthalpy change when one mole of a solid ionic compound is formed from its constituent ions in the gaseous state

lattice enthalpy of dissociation

enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase

enthalpy of neutralisation

enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions

why do combustion reactions occur and why do they give out energy?

why is thermodynamics important?

it is important to understand the stability of compounds and why chemical reactions occur

first ionisation enthalpy

enthalpy change when each atom in one mole of gaseous atoms lose one electron to form one mole of gaseous 1+ ions.

first electron affinity

enthalpy change when each atom in one mole of gaseous atoms gains one election to form one mole of gaseous 1- ions

enthalpy of atomisation

enthalpy change when one mole of gaseous atoms is produced from an element in its standard state

hydration enthalpy

enthalpy change when one mole of gaseous ions become hydrated (Dissolved in water)

enthalpy of solution

enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other.

bond dissociation enthalpy

enthalpy change when one mole of covalent bonds is broken in the gaseous state

Enthalpy of vaporisation

enthalpy change when one mole of a liquid is turned into a gas

enthalpy of fusion

enthalpy change when one mole of a solid is turned into a liquid

What is a born harbour cycle?

it is a special kind type of Hess Cycle that represent the formation of an ionic compound from its elements in a series of steps.

what are the five steps in the born haber cycles

step 1 - atomisation of the metal reactant

step 2 - atomisation of the non metal reactant

step 3 - ionisation of metal

step 4 - electron affinity of the non metal

step 5 - formation of the ionic lattice

exothermic enthalpies

enthalpy of formation

enthalpy of electron affinity

lattice formation enthalpy

endothermic enthalpies

enthalpy of atomisation

enthalpy of electron affinity

ionisation enthalpy

What two factors increase the attraction and therefore the lattice formation enthalpy

higher charge of anion, smaller sizer of anion

which has a higher lattice enthalpy and why, NaBr or NaCl? (3 marks)

Cl- is smaller that Br-

Higher charge density

More strongly attracted to Na

what are the assumptions of the perfect ionic model?

Ions are point charges, their charge is distributed evenly

perfectly spherical

100% ionic bonding

no electron cloud distortion

what happens when i introduce a large anion to a small, highly charged cation?

attraction to valence electrons, valence electrons move further away from the nucleus

cloud distortion

called polarisation

what is covalent character?

when valence electrons from anion mover further away from nucleus and are drawn to the nucleus of the cation

overlap between oppositely charged ions, this is an ionic bond with covalent character

theoretical values

formation enthalpy of ionic compounds, calculated using born harbor cycle based on the perfect ionic model

experimental values

normally higher that theoretical values

this suggests that the compounds are not completely ionic.

what does the term polarisable mean

it is when the anion is large so that the outer electrons have a weaker electrostatic attraction with the nucleus and therefore the nucleus in the cation is able to attract those outer electrons, this means the anion is polarisable

what is entropy?

A measure of disorder/randomness, it is given the symbol S: J K^-1mol^-1

describe a substance with low entropy

Ordered, and regular arrangement of particles

Describe a substance with high entropy

Disorder, random arrangement of particles

states of matter in order of low to high entropy

Solid < Liquid < Gas

How calculate Entropy change

Total entropy of products - total entropy of reactants

Entropy change in surroundings

S = entropy change in the system + entropy change in the surrounding

How does an exothermic reaction increase the entropy in surroundings

Heat is released from the system into the surroundings, this increases the random motion of the particles in the surroundings, which increases the entropy

How does an endothermic reaction decrease the entropy in the surroundings

Heat is absorbed by the system from the surrounding, this decreases the motion of particles in the surroundings, decreasing the entropy

what does the entropy change in the surroundings depend on

enthalpy change of the system and temperature of surroundings

= -enthalpy change of the system/ temperature of the surroundings

what is Gibbs Free energy?

The way of predicting whether or not a chemical reaction will take place when looking at the thermodynamics of a reaction and whether it is feasible, it combines the effect of enthalpy and entropy

How to calculate Gibbs free energy?

G=H-TS

G - KJ mol-1

H - KJ mol-1

T - K

S - JK-1 mol-1 (needs to be divided by 1000)

what does feasibility

If a reaction is feasible it will occur spontaneous which no external energy input

in the gibbs free energy equation how do you know that it is feasible

when G<0 or G=0

What is the effect of temperature on feasibility?

it will change the -TS part of the equation