Thermodynamics

Define enthalpy of atomisation

• enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state under standard conditions

  • Mg (s) → Mg (g)

  • ½ O2(g) → O (g)

• POSITIVE VALUE

  
  

Define 2nd ionisation energy

• amount of energy required to remove an electron from 1 mole of singly charged positive ions in gaseous state

  • Mg+ (g) → Mg2+ (g) + e-

• POSITIVE VALUE

Define 1st electron affinity

• enthalpy change when 1 mole of gaseous atoms is converted to 1 mole of singly charged negative ions

  • O (g) + e- → O- (g)

• NEGATIVE VALUE

Define 2nd electron affinity

• enthalpy change when 1 mole of electrons are added to 1 mole of 1- negative ions to form ions with 2- charges

  • O- (g) + e- → O2- (g)

• POSITIVE VALUE

  
  

Define lattice formation enthalpy

• enthalpy change when 1 mole of solid ionic compound is formed from its gaseous ions

  • Mg2+(g) + O2- (g) → MgO (s)

• NEGATIVE VALUE

Define enthalpy of hydration

• enthalpy change when water molecules surround 1 mole of gaseous ions to form 1 mole of hydrated ions

• Mg2+ (g) → Mg2+ (aq)

Define enthalpy of solution

• enthalpy change when 1 mole of solute dissolves completely in sufficient solvent to form a solution where the ions are far apart enough to not interact with each other

  • MgO (s) → Mg2+ (aq) + O2- (aq)

How does ion structure affect lattice enthalpy

• size: smaller ionic radius means the ions pack more closely together in lattice, so more attraction, more exothermic lattice enthalpy

• charge: greater charge density on ions lead to greater attraction in the lattice, more exothermic lattice enthalpy

Why is second electron affinity a positive value

• electron is being added to a negatively charged ion

• leads to electron repulsion, which requires energy to overcome

How to calculate enthalpy of solution

• enthalpy of solution = enthalpy of lattice dissociation + enthalpy of hydration

How to use enthalpy of solution if a compound is more soluble

• the more exothermic the hydration enthalpy is compared to the lattice enthalpy, the more exothermic the enthalpy of solution

How to use enthalpy of solution if a compound is less soluble

• the more exothermic the lattice enthalpy is compared to the hydration enthalpy, the more endothermic the enthalpy of solution

What is the trend down group 7 hydration values

• hydration values become less exothermic

• as the ionic radius increases, water molecules are less attracted to the ion so they pack less closely

  
  

How does charge density affect hydration values

• hydration value becomes more exothermic as charge density increases

• this is because water molecules are more attracted to the ion

What is meant by entropy

• measure of the degree of disorder in a reaction

How does a change in state affect entropy value

• if a liquid turns into a gas, it becomes more disordered

• this makes entropy value more positive

What are the formulas used for Gibbs free energy

• △S = sum of entropy of products - sum of entropy of reactants

• △S÷1000 → kJ K-1 mol-1

• △G = △H - (T△S)

Define feasible and how to calculate if a reaction is feasible

• a reaction is thermodynamically able to occur

• △G is equal to or less than 0

How can you determine feasibility

How can you use temperature to determine at which point the reaction becomes feasible

• △G=0

• T= △H / △S

  (-273 if asked for °C)

How can you predict the sign of △S

• if you produce a gas, S of products increase as more disordered so △S is positive

• f there are gases on both sides, see which has more molecules of gas

How can you link y=mx+c to Gibbs free energy

• gradient (+m) is equal to -△S

• variable (x) is equal to T

• y-intercept (c) is equal to △H

What does the point suggest where the line crosses the x axis on the graph

• the maximum/minimum temperature at which the reaction becomes feasible

What does a negative gradient suggest about feasibility

• △G becomes more negative as temperature increases

• feasibility is ABOVE the x-intercept (higher temperatures)

What does a positive gradient suggest about feasibility

• △G becomes more positive as temperature increases

• feasibility is below the x-intercept (lower temperatures)