Enthalpy and entropy

Simple recall

lattice enthalpy

the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions

Standard enthalpy change of solution

when one mole of a compound dissolves in water under standard conditions

Standard enthalpy change of hydration

when one mole of aqueous ions are formed from their gaseous ions under standard conditions

What is entropy?

a measure of disorder within a system

• describes how randomly things area arranged, if particles are moving, they have some degree of disorder

• So entropy (S) always a positive number

Is enthalpy change of solution exothermic or endothermic

can be either because:

• energy is taken in to break bonds in ionic lattice when dissolving

• Then new attractions form between ions and water (metal ions with negative dipole on H₂O and non-metal ion with positive dipole on H₂O)

is enthalpy change of hydration exothermic or endothermic

always exothermic because

Energy is released when the attractions towards other molecules are formed

Enthalpy change of formation

the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states

Enthalpy change of atomisation

the enthalpy change when 1 mole gaseous atoms is formed from the element in its standard state

First ionisation energy

Enthalpy change when 1 electron is removed from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

First electron affinity

enthalpy change when 1 electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions

Is enthalpy change of atomisation endo or exothermic

endothermic because energy needs to be supplied to break the molecules into atoms and turn it into a gas

is ionisation energy endothermic or exothermic

endothermic because energy must be supplied to remove an electron from each atom due to the electron’s attraction to the nucleus

Is first electron affinity endo or exothermic

exothermic because energy is released when the electron is accepted - it is readily attracted to the atom

Is second electron affinity endo or exothermic

endothermic because energy must be supplied to overcome the small repulsion of adding another electron to a negative ion

what is standard entropy and its units?

the entropy content of one mole of the substance under standard conditions

J K^-1 mol^-1

how to calculate change in entropy

ΔS = ∑S^θ_{(products) -} ∑S^θ_(reactants)

what does it mean if a reaction is spontaneous/feasible?

no energy other than activation energy is required to sustain the reaction

what is the equation for free energy change and how do you know if the reaction is feasible?

ΔG = ΔH - TΔS

ΔG - free energy change kJ mol^-1

ΔH - enthalpy change kJ mol^-1

T - temperature Kelvin

ΔS - entropy change kJ K^-1 mol^-1

negative free energy change value means reaction is feasible at given temperature