Problem 71 - Chemistry

Question

Sulfuric acid is a dehydrating agent. Write balanced equations for the reactions between sulfuric acid and the following substances: (a) \(\mathrm{HCOOH}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (c) \(\mathrm{HNO}_{3}\), (d) \(\mathrm{HClO}_{3}\). (Hint: Sulfuric acid is not decomposed by the dehydrating action.)

Step-by-Step Solution

Answer

The balanced chemical reactions are: For (a) \[\mathrm{HCOOH} + \mathrm{H_2SO_4} \rightarrow \mathrm{CO}_2 + \mathrm{2H_2O} + \mathrm{HSO_4^{-}}\]; for (b) \[2\mathrm{H}_{3} \mathrm{PO}_{4} + \mathrm{H_2SO_4} \rightarrow \mathrm{P}_2\mathrm{O}_5 + 5\mathrm{H_2O} + \mathrm{HSO_4^{-}}\]; for (c) \[\mathrm{HNO}_{3} + \mathrm{H_2SO_4} \rightarrow \mathrm{NO}_2 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\]; for (d) \[\mathrm{HClO}_{3} + \mathrm{H_2SO_4} \rightarrow \mathrm{ClO}_2 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\].

Step 1: Reaction with \(\mathrm{HCOOH}\)

The equation for this reaction is \(\mathrm{HCOOH} + \mathrm{H_2SO_4} \rightarrow \mathrm{CO}_2 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\). The balanced equation becomes \(\mathrm{HCOOH} + \mathrm{H_2SO_4} \rightarrow \mathrm{CO}_2 + \mathrm{2H_2O} + \mathrm{HSO_4^{-}}\). In this reaction one molecule of formic acid reacts with one molecule of sulfuric acid to form one molecule of carbon dioxide, two molecules of water and one molecule of hydrogen sulfate.

Step 2: Reaction with \(\mathrm{H}_{3} \mathrm{PO}_{4}\)

The equation is \(\mathrm{H}_{3} \mathrm{PO}_{4} + \mathrm{H_2SO_4} \rightarrow \mathrm{P}_2\mathrm{O}_5 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\). All atoms are balanced except for Phosphorus (P) and Oxygen (O), the balanced equation is \(2\mathrm{H}_{3} \mathrm{PO}_{4} + \mathrm{H_2SO_4} \rightarrow \mathrm{P}_2\mathrm{O}_5 + 5\mathrm{H_2O} + \mathrm{HSO_4^{-}}\). Two molecules of phosphoric acid react with sulfuric acid, producing diphosphorus pentoxide, five molecules of water and hydrogen sulfate.

Step 3: Reaction with \(\mathrm{HNO}_{3}\)

The equation for this reaction is \(\mathrm{HNO}_{3} + \mathrm{H_2SO_4} \rightarrow \mathrm{NO}_2 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\). This reaction is already balanced. One molecule of nitric acid reacts with sulfuric acid to form nitrogen dioxide, water and hydrogen sulfate.

Step 4: Reaction with \(\mathrm{HClO}_{3}\)

The equation for this reaction is \(\mathrm{HClO}_{3} + \mathrm{H_2SO_4} \rightarrow \mathrm{ClO}_2 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\). This reaction is already balanced. One molecule of chloric acid reacts with sulfuric acid to form chlorine dioxide, water and hydrogen sulfate.

Key Concepts

Balanced Chemical Equations

Balanced chemical equations are crucial for understanding how substances interact in a chemical reaction. They ensure that the same number of each type of atom appears on both sides of the equation. This reflects the conservation of mass, one of the fundamental principles of chemistry.
Consider the reaction of sulfuric acid with formic acid:

• Original: \(\mathrm{HCOOH} + \mathrm{H_2SO_4} \rightarrow \mathrm{CO}_2 + \mathrm{H_2O} + \mathrm{HSO_4^{-}}\)
• Balanced: \(\mathrm{HCOOH} + \mathrm{H_2SO_4} \rightarrow \mathrm{CO}_2 + \mathrm{2H_2O} + \mathrm{HSO_4^{-}}\)

Notice how we adjust the number of water molecules to ensure that each atom type is equal on both sides. Balancing equations requires careful counting of each element and adjusting coefficients where necessary. This ensures no atoms are lost or gained, as is required by the law of conservation of matter.

Reaction Mechanism

A reaction mechanism is a detailed step-by-step description of how reactants become products. It shows the pathway and the sequence of elementary steps in the transformation.
For instance, in the dehydration of formic acid by sulfuric acid:

• Formic acid \((\mathrm{HCOOH})\) loses water under the action of sulfuric acid \((\mathrm{H_2SO_4})\).
• This results in the formation of carbon dioxide \((\mathrm{CO}_2})\) and water \((\mathrm{H_2O})\).
• The reactive intermediate, hydrogen sulfate \((\mathrm{HSO_4^{-}})\), is produced.

The mechanism involves breaking bonds in the reactants and forming new bonds in the products. Understanding the mechanism provides insight into why certain reactions occur and can help predict the behavior of the substances involved.

Acid Dehydration Reactions

Acid dehydration reactions are processes where an acid, like sulfuric acid, removes water molecules from other substances. This is why sulfuric acid is called a 'dehydrating agent.'
In these reactions, water is generally removed as steam and this leaves behind dehydrated products. For example:

• With \(\mathrm{HCOOH}\), water is removed to yield \(\mathrm{CO}_2\).
• With \(\mathrm{H_3PO_4}\), water is removed to form \(\mathrm{P_2O_5}\).

Sulfuric acid's high affinity for water allows it to act as an effective dehydrator. This nature is often exploited in industrial and laboratory settings where specific dehydration processes are required. Sulfuric acid's dehydrating power comes from its ability to bind strongly with water, making it a very strong and efficient dehydrating agent.