Enthalpy and Entropy RECALL

Enthalpy Change

The change in the heat content of a system during a reaction (kJ mol^-1)

Lattice enthalpy

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

Enthalpy change of formation

enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states

Enthalpy change of atomisation

enthalpy change when 1 mole of gaseous atoms forms 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

second ionisation energy

the enthalpy change when 1 electron is removed from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ 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

Second electron affinity

enthalpy change when 1 electron is added to each ion in 1 mole of gaseous 1- ions to form 1 mole of gaseous 2- ions

Enthalpy change of solution

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

Enthalpy change of hydration

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

Entropy

a measure of disorder within a system

What factors affect lattice enthalpy?

• the charge on the ions

• the radius of the ions

Stronger attractions lead to a more negative lattice enthalpy

Is enthalpy change of solution exothermic or endothermic?

• can be either endothermic or exothermic

• when an ionic compound is dissolved in water, first energy is taken in to break ionic bonds in ionic lattice

• new attractions form between ions and water

• Enthalpy change of solution is endothermic when more energy is taken in when breaking bonds in ionic lattice than released when forming attractions

• Enthalpy change of solution is exothermic when more energy is given out when making new attractions than taken in when breaking ionic bonds

Is enthalpy change of hydration exothermic or endothermic?

Always exothermic because energy is released when attractions towards the molecules are formed

What is meant by a feasible reaction?

if a reaction when no energy, other than activation energy, is required to sustain the reaction

What factors does the feasibility of a reaction depend on?

• the temperature (Kelvin)

• the entropy change (convert JK^-1 mol to kJ K^-1 mol^-1)

• the enthalpy change (kJ mol^-1)

What is the equation (and units) for free energy change?

What is the purpose of the equation?

Finding the value for free energy change predicts the feasibility of a reaction