Question

CaO can be used as a drying agent. One such application occurs when water is added to dry concrete or cement. The reaction that occurs is

CaO(s)+H2O(l)⇌Ca(OH)2(s)
The product is commonly called slaked lime.
Assuming the commonly used standard-state temperature of 25∘C, calculate ΔSuniv for this reaction using table from the table below.
Substance S∘
[J/(K⋅mol)] ΔH∘f
(kJ/mol)
CaO(s) 39.9 −635.1
H2O(l) 69.9 −285.8
Ca(OH)2(s) 83.4 −986.1

Answer 1

To calculate ΔSuniv for the reaction CaO(s) + H2O(l) ⇌ Ca(OH)2(s), we need to use the standard state entropy values of the substances involved. The change in entropy for the reaction can be calculated using the formula ΔSuniv = ΣS°products - ΣS°reactants.

Step-by-step explanation:

To calculate ΔSuniv for the reaction CaO(s) + H2O(l) ⇌ Ca(OH)2(s), we need to use the standard state entropy values of the substances involved. The change in entropy for the reaction can be calculated using the formula ΔSuniv = ΣS°products - ΣS°reactants. Let's calculate it:

1. CaO(s): S° = 39.9 J/(K⋅mol)
2. H2O(l): S° = 69.9 J/(K⋅mol)
3. Ca(OH)2(s): S° = 83.4 J/(K⋅mol)

Now we can substitute the values into the formula: ΔSuniv = (83.4 J/(K⋅mol)) - (39.9 J/(K⋅mol)) - (69.9 J/(K⋅mol)).

Simplifying the equation gives us: ΔSuniv = 83.4 J/(K⋅mol) - 109.8 J/(K⋅mol).

Therefore, ΔSuniv = -26.4 J/(K⋅mol).

Answer 2

To find ΔSuniv for the formation of slaked lime from CaO and H2O, we calculate ΔSsystem using standard molar entropies and ΔSsurroundings using standard enthalpies of formation. The resulting ΔSuniv is -28.6 J/(K·mol).

Step-by-step explanation:

The student is asking to calculate the ΔSuniv for the reaction CaO(s) + H2O(l) ⇌ Ca(OH)2(s), commonly known as the formation of slaked lime, at a standard-state temperature of 25°C using given thermodynamic data.

To calculate the change in the entropy of the universe (ΔSuniv) for this reaction, we will use the formula:

ΔSuniv = ΔSsystem + ΔSsurroundings

To find ΔSsystem, we use the standard molar entropies (S°) of the products minus the reactants:

(83.4 J/(K·mol)) - (39.9 J/(K·mol) + 69.9 J/(K·mol)) = -26.4 J/(K·mol)

To find ΔSsurroundings, we use the standard enthalpy of formation (ΔH°f) of the products minus the reactants and divide by the temperature (T):

(ΔH°f Ca(OH)2 - (ΔH°f CaO + ΔH°f H2O)) / T

(-986.1 kJ/mol - (-635.1 kJ/mol - 285.8 kJ/mol)) / 298.15 K = -2.2 J/(K·mol)

Finally, combining these two changes gives us ΔSuniv:

ΔSuniv = -26.4 J/(K·mol) - 2.2 J/(K·mol) = -28.6 J/(K·mol)