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Sulfuric acid is a colorless oily liquid, and has a vapor pressure of <0.001 mmHg at 25 °C and 1 mmHg at 145.8 °C, [16] and 98% sulfuric acid has a vapor pressure of <1 mmHg at 40 °C. [ 17 ] In the solid state, sulfuric acid is a molecular solid that forms monoclinic crystals with nearly trigonal lattice parameters.
The contact process is a method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum was originally used as the catalyst for this reaction; however, because it is susceptible to reacting with arsenic impurities in the sulfur feedstock, vanadium(V) oxide (V 2 O 5) has since been preferred.
Sulfuric(IV) acid (United Kingdom spelling: sulphuric(IV) acid), also known as sulfurous (UK: sulphurous) acid and thionic acid, [citation needed] is the chemical compound with the formula H 2 SO 3. Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO 2 molecule and the bisulfite ion, HSO − 3 . [ 2 ]
However, SO 3 added to concentrated sulfuric acid readily dissolves, forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid. [4] Typically, above concentrations of 98.3%, sulfuric acid will undergo a spontaneous decomposition into sulfur trioxide and water H 2 SO 4 ⇌ SO 3 + H 2 O
The standard Gibbs free energy of formation (G f °) of a compound is the change of Gibbs free energy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified temperature, usually 298.15 K or 25 °C).
In whatever way SO 3 is formed, it does not behave like SO 2 in that it forms a liquid aerosol known as sulfuric acid (H 2 SO 4) mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to SO 3. Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates.
Treatment of sulfur with hydrogen gives hydrogen sulfide.When dissolved in water, hydrogen sulfide is mildly acidic: [5] H 2 S ⇌ HS − + H +. Hydrogen sulfide gas and the hydrosulfide anion are extremely toxic to mammals, due to their inhibition of the oxygen-carrying capacity of hemoglobin and certain cytochromes in a manner analogous to cyanide and azide.
A typical mixture is 3 parts of concentrated sulfuric acid and 1 part of 30 wt. % hydrogen peroxide solution; [1] other protocols may use a 4:1 or even 7:1 mixture. A closely related mixture, sometimes called "base piranha", is a 5:1:1 mixture of water, ammonia solution ( NH 4 OH , or NH 3 (aq) ), and 30% hydrogen peroxide.