A sulfuric acid plant produces an enormous amount of heat. To keep costs as low as possible, much of this heat is used to make steam to generate electricity. Some of the electricity is used to run the plant, and the excess is sold to the local electrical utility. Three reactions are important in sulfuric acid production: (1) burning S to SO₂; (2) oxidation of SO₂ to SO₃; and (3) reaction of SO₃ with H₂O:

SO 3 ( g ) +  H 2 O (in 98% H 2 SO 4 )  →  H 2 SO 4 ( l )

The enthalpy change of the third reaction is −130 kJ/mol. Estimate the enthalpy change when 1.00 mol of S is used to produce 1.00 mol of H₂SO₄. How much energy is produced per metric ton (1.00 × 10³ kg) of H₂SO₄?

Interpretation:

To calculate enthalpy change when one mole of sulfur produced one mole of sulfuric acid and energy produced per metric ton of sulfuric acid is to be stated.

Concept introduction:

The amount of heat released or absorbed in a chemical reaction under constant pressure is known as enthalpy change. It is denoted by ΔH.

The enthalpy change of any reaction is written as,

ΔH=∑ΔfHproducts−∑ΔfHreactants                                 (1)

Enthalpy of formation is ithe enthalpy change when one mole of a compound is formed from its elements.

Number  of  moles  =  MassMolecular mass

The enthalpy change of one mole of sulfur produced one mole of sulfuric acid and energy produced per matric ton of sulfuric acid is calculated below.

Given:

The mass of sulphuric acid per matric ton is 106 g.

The molar mass of sulphuric acid is 98 g⋅mol−1.

Refer to the appendix L for enthalpy values.

The enthalpy of formation (ΔfH) of water is −285.83 kJ⋅mol−1.

For the production of sulfuric acid from sulfur atom, three reactions are involved. The first reaction is production of sulfur dioxide from sulfur atom is written as,

    S(s)+O2(g)→SO2(g) (2)

The heat of formation (ΔfH) of SO2 is −296.84 kJ⋅mol−1. Since, enthalpy of formation of substance which present in its elemental form in the reaction is zero. Hence, enthalpy change of sulfur dioxide is equal to the heat of formation of sulfur dioxide.

The second reaction is production of SO3 from SO2 is written as,

    SO2(g)+12O2(g)→SO3(g)                                                                       (3)

The enthalpy change of above reaction is −98 kJ⋅mol−1.

The final reaction is production sulphuric acid is written as follows,

    SO3(g)+H2O(l)→H2SO4(l)                                                                      (4)

The enthalpy change (ΔH) of this reaction is −130 kJ⋅mol−1