Energy cycles

Nature of science:

Making quantitative measurements with replicates to ensure reliability—energy cycles allow for the calculation of values that cannot be determined directly. (3.2)

• Representative equations (eg M⁺(g) ---> M⁺(aq)) can be used for enthalpy/energy of hydration, ionization, atomization, electron affinity, lattice, covalent bond and solution.
• Enthalpy of solution, hydration enthalpy and lattice enthalpy are related in an energy cycle.

Applications and skills:

• Construction of Born-Haber cycles for group 1 and 2 oxides and chlorides.
• Construction of energy cycles from hydration, lattice and solution enthalpy. For example dissolution of solid NaOH or NH₄Cl in water.
• Calculation of enthalpy changes from Born-Haber or dissolution energy cycles.
• Relate size and charge of ions to lattice and hydration enthalpies.
• Perform lab experiments which could include single replacement reactions in aqueous solutions.

Guidance:

• Polarizing effect of some ions producing covalent character in some largely ionic substances will not be assessed.
• The following enthalpy/energy terms should be covered: ionization, atomization, electron affinity, lattice, covalent bond, hydration and solution.
• Value for lattice enthalpies (section 18), enthalpies of aqueous solutions (section 19) and enthalpies of hydration (section 20) are given in the data booklet.

• The importance of being able to obtain measurements of something which cannot be measured directly is significant everywhere. Borehole temperatures, snow cover depth, glacier recession, rates of evaporation and precipitation cycles are among some indirect indicators of global warming. Why is it important for countries to collaborate to combat global problems like global warming?

Utilization:

• Other energy cycles—carbon cycle, the Krebs cycle and electron transfer in biology.

Syllabus and cross-curricular links:
Topics 1.2 and 1.3—stoichiometric relationships
Topic 3.2—ionization energy, atomic and ionic radii
Topic 5.3—bond enthalpy
Aims:

• Aim 4: Discuss the source of accepted values and use this idea to critique experiments.
• Aim 6: A possible experiment is to calculate either the enthalpy of crystallization of water or the heat capacity of water when a cube of ice is added to hot water.
• Aim 7: Use of data loggers to record temperature changes. Use of databases to source accepted values.