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Lattice enthalpy
- enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
- exothermic change, negative sign
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Key enthalpy changes
- Formation: one mole of a compound is formed from its constituent elements in there standard states under standard conditions
- Atomisation: one mole of gaseous atoms is formed from its element in its standard state
- First ionisation energy: one mole of gaseous 1+ ions is formed from one mole of gaseous atoms
- Second ionisation energy: one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions
- First electron affinity: one mole of gaseous 1- ions is made from one mole of gaseous atoms
- Second electron affinity: one mole of gaseous 2- ions is made from one mole of gaseous 1- ions
- Neutralisation: one mole of water is formed by the reaction between an acid and a base under standard conditions
- Hydration: one mole of aqueous ions is formed from gaseous ions
- Solution: one mole of solute is dissolved in sufficient solvent that no further enthalpy changes occurs on further dilution
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Enthalpy change of solution
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Ionic charge and radius
- Higher charge: higher attraction, more exothermic, more negative
- Smaller radius: more attraction, more exothermic
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Entropy
- a measure of the disorder of a system
- system becomes energetically more stable when it becomes more disordered, increase in entropy
- Physical state: solids have lowest entropy, gases have highest
- Dissolving: increases entropy
- Change in number of gaseous molecules: more particles/moles, more entropy
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Free-energy change
- the tendency of a process to take place depends on temperature T, the entropy change in the system S, and enthalpy change H, with the surroundings
- free-energy change, G is the balance between enthalpy, entropy and temperature for a process
- Spontaneous reaction: free-energy has to be 0 or negative (for endothermic S must be positive, TS>H)
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Redox
- redox: a reaction in which both reduction and oxidation take place
- oxidation number: a measure of the number of electrons that an atom uses to bond with atoms of another element
- half-reaction:
- oxidising agent: a reagent that oxidises (takes electrons from) another species
- reducing agent: a reagent that reduces (adds electrons to) another species
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Electrode potentials
- Standard electrode potential: the emf of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1 mol dm-3 and a gas pressure of 100 kPa (1 atm)
- How to measure SEP using a hydrogen electrode: hydrogen electrode is always on the left, has a value of 0V, whole EP=RHS-LHS
- most negative side is where oxidation happens
- Metal/metal ion half cells:
- Non-metal/non-metal ion half cells: plantinum electrode is placed in the solution
- Metal ion/metal ion half cells: inert platinum electrode allows electrons to pass into or out of the half cell
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Feasibility of reactions
- the cell potential is feasible when over +0.40
- changing the concentration shifts the equilibrium to oppose the change
- rate of reaction may be slow, so may appear not to happen
- may have a high activation energy, stop it happening
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Storage and fuel cells: hydrogen fuel cells
- energy storage cells are like electrochemical cells
- a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
- 1. hydrogen is oxidised to H+ ions/protons at the anode
- 2. the polymer electrolyte only allows the H+ ions across forcing e- to travel round the circuit
- 3. electric circuit creates a current
- 4. oxygen is reduced at the cathode
- 5. water is produced
- Catalyst: platinum
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Storage and fuel cells: FCVs
- Fuel cell vehicles (FCV)
- fuel cells develloped to be fuelled by hydrogen gas (emits only water) or by hydrogen-rich fuels (methanol, natural gas or petrol) (relese small amounts of pollutants)
- Advantages of FCV's: less pollution, less CO2 and more efficient
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Storage and fuel cells: storage of hydrogen
- as a liquid under pressure (very low temperature required)
- adsorbed onto the surface of a solid material
- absorbed within some solid materials
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Limitations of hydrogen fuel cells
- platinum catalysts are expensive
- toxic chemicals used
- fuel cells have a limited life span
- disposing of a fuel cell is expensive
- high production costs
- if stored as a gas it's explosive
- if a liquid need expensive storage equipment
- limited life of adsorbers and absorbers
- manufacturing hydrogen takes energy
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Hydrogen economy
- may contribute largely to future energy
- But....
- many people have concerns about the safety and reliability of hydrogen
- hydrogen fuel supplies are expensive to set up, so it needs to be cheaper
- clean energy sources need to be used to produce hydrogen
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