CHEM5 - Thermodynamics

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Thermo - Definitions

Enthalpy change - The heat energy transferred in a reaction at constant pressure.

Lattice Formation Enthalpy - The enthalpy change when one mole of an ionic solid is formed from its gaseous ions under standard conditions.

Lattice Dissociation Enthalpy - The enthalpy change when one mole of an ionic solid is completely dissociated into its gaseous ions under standard conditions.

Enthalpy of Formation - The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Enthalpy change of combustion - The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions.

Enthalpy change of atomisation - The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state

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Thermo - Definitions

First ionisation enthalpy - The enthalpy change when one mole of gaseous unipositive ions are formed from one mole of gaseous atoms.

Second ionisation enthalpy - The enthalpy change when one mole of gaseous dipositive ions are formed from one mole of gaseous unipositive ions.

First electron affinity - The enthalpy change when one mole of gaseous uninegative ions are formed from one mole of gaseous atoms.

Second electron affinity - The enthalpy change when one mole of gaseous dinegative ions are formed from one mole of gaseous uninegative ions.

Enthalpy change of hydration - The enthalpy change when one mole of aqueous ions is formed from gaseous ions.

Enthalpy change of solution - The enthalpy change when one mole of a compound completely dissolves in water. 

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Thermo - Enthalpy Change

Hess' Law - The energy change of a reaction is independent of the route.

Calculating Enthalpy of Formation using Enthalpy of Combustion values:
- Combustion (elements/reactants) - (compounds/products)
Calculating Enthalpy of Reaction using Enthalpy of Formation values:
- Formation (products) - (reactants
Calculating Enthalpy of Reaction using Mean Bond Enthalpies:
- (Bonds broken) - (Bonds formed

Born-Haber Cycles
- Are used to calculate Lattice Formation Enthalpies: Na+(g) + Cl-(g) > NaCl(s)
1) Enthalpy Change of Atomisation of the metal
 [Na(s) > Na(g)]
2) First Ionisation Enthalpy of metal [Na(g) > Na+(g)]
3) Enthalpy Change of Atomisation of the non-metal [1/2Cl2(g) > Cl(g)]
4) First Electron Affinity of non-metal [Cl(g) > Cl-(g)]
Lattice Formation Enthalpy = Enthalpy of Formation - Enthalpy Change 1

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Thermo - Theoretical and Experimental

Purely/Perfect ionic model - Assumes all ions are spherical and their charge is evenly distributed.

- However, experimental lattice enthalpies differ from theoretical, suggesting ionic compounds have some covalent character.

- The more polarisation there is, the more covalent bonding there will be.

- The closer the experimental and theoretical values are, the closer the lattice is to being purely ionic.

- Therefore, the more different the experimental and theoretical values are, the more polarisation there is, and the further the lattice is to being purely ionic.

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Thermo - Enthalpy of Solution

Enthalpy of Solution [NaCl(s) + aq > Na+(aq) + Cl-(aq)]

1) Lattice Dissociation Enthalpy of solid [NaCl(s) > Na+(g) + Cl-(g)]
2) Enthalpy of Hydration of metal [Na+(g) > Na+(aq)]
3) Enthalpy of Hydration of non-metal [Cl-(g) > Cl-(aq)]

Enthalpy of Solution = Lattice Dissociaton Enthalpy + Enthalpy of Hydrations 

- Lattice Dissociation Enthalpies are ENDOTHERMIC
- Enthalpy of Hydrations are EXOTHERMIC

- As temperature increases, the equilibrium shifts to the endothermic direction
(i.e. the right) to oppose the change. Therefore the solubility of the solid increases

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Thermo - Entropy

Spontaneous Reactions - Occur without having any additional energy added.
Feasible Reactions - Are thermodynamically possible but will only occur if energy is supplied due to the large activation energy.
Entropy - A measure of the number of ways that particles can be arranged and the number of ways the energy can be shared out between the particles.

Factors that affect Entropy:

Physical State: - Solid particles vibrate about a fixed point, so have the least disorder, therefore have the lowest entropy;
- Gas particles move freely, so have most disorder and therefore highest entropy

Dissolving:  - Dissolved particles move freely so theres lots of disorder and therefore a high entropy.

More particles: - More particles mean more ways they and their energy can be arranged, therefore more disorder and a higher entropy. 

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Thermo - Entropy

- Exothermic reactions tend to be spontaneous, but so do some endothermic.
- A change in state from liquid to gas increases entropy.
- A reaction wont occur unless the total entropy change is positive.

Total Entropy Change = Entropy of System + Entropy of Surroundings.

Entropy of System = Entropy (Products - Reactants)
Entropy of Surroundings = -Enthalpy Change / Temperature 

- The Second Law of Thermodynamics states:
In a spontaneous process, the entropy of the Universe increases.

Entropy change is calculated in JK-1mol-1 

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Thermo - Free Energy Change

For a reaction to be feasible: Free Energy Change must be less than or equal to 0.

Free Energy Change = Enthalpy Change - (Temperature x Entropy Change) 
[FE = kJmol-1 ; EC = kJmol-1 ; T = K ; ES = JK-1mol-1]

  •  Exothermic reaction with an increase in entropy
    - Free Energy is always negative at any temperature (always feasible)
  • Exothermic reaction with a decrease in entropy:
    - Free Energy is negative at low temperatures
  • Endothermic reaction with an increase in entropy:
    - Free Energy is negative at high temperatures
  • Endothermic reaction with decrease in entropy:
    - Free Energy is always positive at any temperature (never feasible)

- To calculate the temperature at which a reaction becomes feasible;
Free Energy = 0. So, T = Enthalpy Change / Entropy Change of System 

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