Question

Mg(OH)2(s) = MgO(s) + H2O(g) 100.000 200.000 300.000 400.000 500.000 600.000 700.000 800.000 900.000 1000.000 ΔΗ kcal 19.237 18.867 18.418 17.924 17.401 16.859 16.302 15.735 15.159 14.576 AS cal/K 35.954 35.080 34.221 33.427 32.703 32.044 31.441 30.886 30.373 29.896 AG kcal 5.821 2.269 -1.196 -4.577 -7.883 -11.120 -14.294 -17.410 -20.473 -23.486 Use the above table of quantities to discuss the thermodynamic potential for this reaction, effect of temperature and whether the chemical reaction gives off or requires energy. The Gibbs Energy Minimization for heating one kmolo magnesium hydroxide) with one kmole of air is shown below. Discuss what might happen as the material is heated up to 1000 C What temperature would you select and why?

Answer 1

Answer: By using the above table of quantities we discuss the thermodynamic potential for this reaction as follows:

The thermodynamic potential of reaction is characterized by its Entropy ($\Delta$S)/internal energy (Q), enthalpy ($\Delta$H), Gibbs free energy ($\Delta$G).

Entropy is the measure of degree of randomness in the reaction system and in this reaction system on increasing the temperature entropy ($\Delta$S) decreases which suggest that the reaction is endothermic. Since in endothermic reaction entropy ($\Delta$S) decreases while in exothermic reaction entropy ($\Delta$S) increases.

Internal energy is the energy of the reaction system. It depends upon temperature and entropy ($\Delta$S). Since in the our reaction system on increasing the temperature entropy ($\Delta$S) decreases but temperature increases. So the internal energy of the reaction system will increase on increasing the temperature which suggest that the reaction is endothermic. Since in endothermic reaction the products are higher in energy than reactant as observed in our case.

Enthalpy ($\Delta$H) is the another thermodynamic potential property that decreases when the reaction is favoured in forward direction. As observed in the above  reaction system enthalpy ($\Delta$H) decreases on increasing the temperature which further suggest that the above reaction system is favoured in the forward direction.

Gibbs free energy ($\Delta$G) is a thermodynamic potential property that  can be used to calculate the maximum of reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. So if the Gibbs free energy ($\Delta$G) decreases the reaction is favoured in the forward direction and in the backward direction if it decreases in the reaction system. As observed in the above  reaction system Gibbs free energy ($\Delta$G) decreases on increasing the temperature which further suggest that the above reaction system is favoured in the forward direction.

So from all the above finding we conclude that the above reaction system is endothermic and on increasing the temperature the reaction is favoured in the forward direction.

Since the above reaction system is endothermic so it requires energy which is provided in the reaction system by increasing the temperature (as heat) for reaction to proceed in the forward direction.

The Gibbs energy Minimization for heating one kmole magnesium hydroxide with one kmole of air is shown above. Since the magnesium hydroxide is heated to 1000 0C i.e. calcining temperature (i.e. heating to high temperature in air or oxygen) so it produces light burned magnesia a reactive form also known as caustic calcined magnesia. When calcining temperature is further increased it produces hard burned magnesia with limited reactivity.

We select 1000 0C because at this temperature the reaction system is at most favourable thermodynamic potential reaction conditions i.e. lowest free energy ($\Delta$G); highest internal energy, minimum entropy ($\Delta$S) and enthalpy ($\Delta$H).