S(rhombic)+O2(g)--->SO2(g)
change in H(rxn)= -296.06 kJ/mol
S(monoclinic)+O2(g)--->SO2(g)
change in H(rxn)= -296.36 kJ/mol
1. Calculate the enthalpy change for the transformation. S(rhombic)-->S(monoclinic)
Our goal is to calculate the enthalpy change for the formation of monoclinic sulfur from the rhombic sulfur. To do soo, we must arrange the equations that are given in the problem in such a way that they will sum to the desired overall equation.
So which equation should be reversed: 1st or 2nd?
What is the sign of change in H for the reversed equation: positive or negative?
see above.
To calculate the enthalpy change for the transformation S(rhombic) --> S(monoclinic), we need to rearrange the given equations and make use of Hess's Law, which states that the total enthalpy change of a reaction is the sum of the enthalpy changes of the individual steps of the reaction.
In this case, we are given two reactions:
1. S(rhombic) + O2(g) --> SO2(g); ΔH = -296.06 kJ/mol
2. S(monoclinic) + O2(g) --> SO2(g); ΔH = -296.36 kJ/mol
We need to reverse one of these equations and change the sign of the enthalpy change in order to get the desired overall equation S(rhombic) --> S(monoclinic).
So, we have two choices:
1. Reverse the first equation: SO2(g) --> S(rhombic) + O2(g); ΔH = +296.06 kJ/mol
2. Reverse the second equation: SO2(g) --> S(monoclinic) + O2(g); ΔH = +296.36 kJ/mol
To determine which equation to reverse, we need to look at the sign of the enthalpy change. We want the overall reaction to be S(rhombic) --> S(monoclinic), meaning we want rhombic sulfur to transform into monoclinic sulfur. Since rhombic sulfur is the reactant in the first equation (S(rhombic) + O2(g) --> SO2(g)), we need to use that equation and reverse it.
Now, we have the equation S(monoclinic) + O2(g) --> SO2(g) reversed: SO2(g) --> S(monoclinic) + O2(g); ΔH = +296.36 kJ/mol.
Therefore, the enthalpy change for the transformation S(rhombic) --> S(monoclinic) is +296.36 kJ/mol.