15. Energetics/thermochemistry

15. Energetics/thermochemistry

15.1 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.

Understandings:

  • 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.

15.2 Entropy and spontaneity

Nature of science:

  • Theories can be superseded - the idea of entropy has evolved through the years as a result of developments in statistics and probability.

Understandings:

  • Entropy (S) refers to the distribution of available energy among the particles. The more ways the energy can be distributed the higher the entropy.

  • Gibbs free energy (G) relates the energy that can be obtained from a chemical reaction to the change in enthalpy (△H), change in entropy (△S), and absolute temperature (T).

  • Entropy of gas>liquid>solid under same conditions.

Applications and skills:

  • Prediction of whether a change will result in an increase or decrease in entropy by considering the states of the reactants and products.

  • Calculation of entropy changes (△S) from given standard entropy values (S⁰).

  • Application of △G⁰=△H⁰−T△S⁰ in predicting spontaneity and calculation of various conditions of enthalpy and temperature that will affect this.

  • Relation of △G to position of equilibrium.

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