Calculate the standard entropy change for the following reaction
at 25 degree C.
https://sites.google.com/site/chempendix/thermo. the S values
can be found on this website
MgCl2(s)+H2O(l) equals MgO(s)+2HCl(g).
Delta S rxn= ?
Answer
General guidance
can be calculated by using the standard values of 
for the individual reactants and products and then applying the extension of Hess’s law.
Hess’s law: According to the Hess’s law, also known as Hess’s law of constant heat summation, the total enthalpy change for a particular reaction during the complete course of time is same irrespective of the path it follows.
The standard enthalpy change for a given chemical reaction can be calculated by using the following expression:
Here, 
represents the standard enthalpy change of formation.
Extension of Hess’s law: The Hess’s law can be extended to calculate the standard entropy change, 
, in the same manner as above.
Step-by-step
Step 1 of 2
The reaction is given as:
Calculate standard entropy change for the reactants as follows:
Similarly, calculate standard entropy change for the products as follows:
The extension of Hess’s law is used to calculate the standard entropy change for the given reaction. The number of moles of products and reactants are multiplied with their respective .
By considering the literature, the values of is used for the reactants and the products of the reaction.
Step 2 of 2
Calculate the standard entropy change as follows:
The standard entropy change for the reaction at 
is 
.
The total sum of 
is subtracted from the total sum of 
to find the standard entropy change for the reaction at .
Answer
The standard entropy change for the reaction at is .
Answer only
The standard entropy change for the reaction at is .