Question

1a- Discuss the vaporization of water in standard condition at 298 K in term of: - exothermicity and endothermicity - change of entropy of the system and the surrounding - spontaneity of the reaction.

1b- Explain how the temperature and the composition of the two phases (liquid and vapour) influence the spontaneity of the vaporization of water.

Answer 1

• Water has a greater entropy than ice and so entropy favours melting.
• But ice has a lower energy than water and so energy favours freezing.
• It is possible to predict what will happen by taking into account the entropy of the surroundings, in addition to the energy of the system.
• Freezing is an exothermic process; energy is lost from the water and dissipated to the surroundings.
• Therefore, as the surroundings get hotter, they are gaining more energy and thus the entropy of the surroundings is increasing.
• The amount by which the entropy of the surroundings has increased can be determined using the following principle: the entropy of the surroundings increases by an amount equal to the heat energy they gain divided by the temperature at which it happens, therefore:

  

• This relationship allows us to make a prediction about the entropy of the surroundings of a chemical process, whatever they are (even the whole universe!), using the measurements we can make on the chemical system.
• The total entropy change can then be used to predict whether a reaction is feasible or not at a given temperature.
• The total entropy is equal to:

  

• S_total is positive for a spontaneous reaction

• This is part of the Second Law of Thermodynamics and it is very important as it allows us to predict whether something can happen or not.
• Even though a reaction can be spontaneous, it is not certainty that the reaction will occur, as the activation energy may be too high for the reaction.
• The second law of thermodynamics can be used to explain the following:

  

1. The reaction will occur, as in an exothermic reaction H is negative, and if the entropy increases, then S is positive, so:

   

   Total entropy change is positive, so reaction is feasible.
2. The reaction can never occur, as H is positive and S is negative:

   

   The total entropy change is negative and so the reaction cannot occur.
3. The reaction can occur depending on the conditions:

   

   As the temperature increases, S_surr increases (becomes less negative), therefore, the higher the temperature, the more likely it is for the reaction to occur. This ties in with La Chatelier’s principle- an increase in the temperature of an endothermic reaction pushes the position of equilibrium to the product side.
4. The reaction can occur depending on the conditions:

   

   As the temperature decreases, S_surr increases, therefore, the lower the temperature, the more likely it is for the reaction to occur.

For most practical purposes (and certainly for this course), the following definitions are good enough:

• gases: 1 atmosphere partial pressure
• pure liquids: the liquid under a total (hydrostatic) pressure of 1 atm.
• solutes: an effective concentration of 1 mol L^–1 (1 mol dm^–3). ("Effective" concentrations approach real concentrations as the latter approach zero; for practical purposes, these can be considered identical at real concentrations smaller than about 10^–4 molar.)
• solids: the pure solid under 1 atm pressure

Note also that

• There is actually no "standard temperature", but because most thermodynamics tables list values for 298.15 K (25° C), this temperature is usually implied.
• These same definitions apply to standard enthalpies and internal energies.
• Don't confuse these thermodynamic standard states with the "standard temperature and pressure" (STP) widely employed in gas law calculations.