Entropy Change, ΔS (CIE A Level Chemistry)

Potassium nitrate is a soluble salt. When it dissolves in water the value of the enthalpy change ΔH^θ = +34.9 kJ mol^–1 and the value of the entropy change ΔS^θ = +117 J K^–1 mol^–1 .

Write an equation, including state symbols, for the process that occurs when potassium nitrate dissolves in water. 

Explain why ΔS^θ is positive for this process.

Calculate the minimum temperature at which the reaction will become feasible. Show your working.

Ammonia, NH₃, is produced by the Haber process and is an important chemical in the manufacture of fertilisers and clearing products. 

Ammonia gas can react with oxygen to produce nitrogen monoxide and steam, and is the first step in the Ostwald process which produces nitric acid. 

   Calculate ΔS^θ for change for this reaction at 298 K.

ΔS^θ = ................................... J K^-1 mol^-1

[2]

Explain why the standard entropy change for the reaction in (a) is positive.

The second step in the Ostwald process produces nitrogen dioxide as shown in the equation 

2NO (g) + O₂ (g) → 2NO₂ (g)             ΔH^θ = -112 kJ mol^-1   

The standard entropy for NO₂ (g) is 240.0 J K^-1 mol^-1

Using Table 2.1 calculate ΔS^θ or this reaction at 298 K

ΔS^θ = ....................... J K^-1 mol^-1

Using Table 2.1 calculate ΔG^θ or this reaction at 298 K

ΔG^θ = ....................... kJ mol^–1

Fig. 3.1 shows how entropy changes with temperature.

Fig. 3.1

Standard entropies can be used to calculate the entropy change of a reaction, ΔS.
For example, for the formation of nitrogen monoxide from nitrogen and oxygen. 

NO (g) + O₃ (g) → NO₂ (g) + O₂ (g)

Standard entropy values are shown in Table 3.1

Table 3.1

Using Table 3.1 calculate ΔS^θ of the reaction between nitric oxide and ozone at 298K.

ΔS^θ  = .......................... J K^-1 mol^-1

The contact process is a method used industrially to form sulfur trioxide, by reacting sulfur dioxide and oxygen together over a vanadium(V) oxide catalyst.

The equation for this reaction is shown below:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

Enthalpy of formation values are shown in Table 3.2

Table 3.2