Question

Hydrogen halides \((\mathrm{HF}, \mathrm{HCl}, \mathrm{HBr}, \mathrm{HI})\) are highly reactive compounds that have many industrial and laboratory uses. (a) In the laboratory, HF and \(\underline{H C l}\) can be generated by combining \(\mathrm{CaF}_{2}\) and \(\mathrm{NaCl}\) with concentrated sulfuric acid. Write appropriate equations for the reactions. (Hint: These are not redox reactions.) (b) Why is it that \(\mathrm{HBr}\) and HI cannot be prepared similarly, that is, by combining NaBr and NaI with concentrated sulfuric acid? (Hint: \(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a stronger oxidizing agent than both \(\mathrm{Br}_{2}\) and \(\mathrm{I}_{2} .\) ) \((\mathrm{c}) \mathrm{HBr}\) can be prepared by reacting phosphorus tribromide \(\left(\mathrm{PBr}_{3}\right)\) with water. Write an equation for this reaction.

Short answer

(a) HF: \(\mathrm{CaF_2 + H_2SO_4 \rightarrow 2\ HF + CaSO_4}\) and HCl: \(\mathrm{NaCl + H_2SO_4 \rightarrow HCl + NaHSO_4}\). (b) \(\mathrm{H_2SO_4}\) is a stronger oxidizing agent. (c) \(\mathrm{PBr_3 + 3H_2O \rightarrow 3HBr + H_3PO_3}\).

Step-by-step solution

Writing the Reaction for HF Generation

When calcium fluoride \((\mathrm{CaF}_2)\) reacts with concentrated sulfuric acid \((\mathrm{H_2SO_4})\), hydrogen fluoride \((\mathrm{HF})\) is generated. The chemical equation for the reaction is:\[\mathrm{CaF_2 + H_2SO_4 \rightarrow 2\ HF + CaSO_4}\] This reaction occurs because \(\mathrm{H_2SO_4}\) is strong enough to displace fluoride ions \((\mathrm{F^-})\), forming \(\mathrm{HF}\) and an insoluble \(\mathrm{CaSO_4}\) precipitate.

Writing the Reaction for HCl Generation

When sodium chloride \((\mathrm{NaCl})\) is treated with concentrated sulfuric acid \((\mathrm{H_2SO_4})\), hydrogen chloride \((\mathrm{HCl})\) is generated. The balanced chemical equation for this reaction is:\[\mathrm{NaCl + H_2SO_4 \rightarrow HCl + NaHSO_4}\]In this process, \(\mathrm{H_2SO_4}\) acts as an acid, capable of donating a proton \((\mathrm{H^+})\) to \(\mathrm{Cl^-}\) ions to form \(\mathrm{HCl}\).

Reason for Not Using NaBr and NaI

The reactions of sodium bromide \((\mathrm{NaBr})\) and sodium iodide \((\mathrm{NaI})\) with concentrated sulfuric acid do not produce hydrogen bromide \((\mathrm{HBr})\) or hydrogen iodide \((\mathrm{HI})\) effectively. This is because \(\mathrm{H_2SO_4}\) acts as a stronger oxidizing agent than both \(\mathrm{Br_2}\) and \(\mathrm{I_2}\), resulting in oxidation of \(\mathrm{Br^-}\) to \(\mathrm{Br_2}\) and \(\mathrm{I^-}\) to \(\mathrm{I_2}\), rather than forming \(\mathrm{HBr}\) and \(\mathrm{HI}\). Thus, the reactions proceed as oxidation rather than simple acid-base reactions.

Writing the Reaction for HBr Generation Using PBr3

The generation of hydrogen bromide \((\mathrm{HBr})\) using phosphorus tribromide \((\mathrm{PBr_3})\) involves a reaction with water. The balanced chemical equation for this reaction is:\[\mathrm{PBr_3 + 3H_2O \rightarrow 3HBr + H_3PO_3}\]This reaction proceeds without oxidizing the bromide ions, and thus results in the formation of \(\mathrm{HBr}\) along with phosphorous acid \((\mathrm{H_3PO_3})\).

Key concepts

Sulfuric Acid Reactions

Sulfuric acid is a highly reactive substance and a strong acid with a multitude of applications. It is well-known for its ability to displace weaker acids in chemical reactions. For instance, when \[\mathrm{CaF_2}\] (calcium fluoride) is combined with concentrated \[\mathrm{H_2SO_4}\], the result is the production of hydrogen fluoride (HF) and calcium sulfate \[\mathrm{CaSO_4}\]. This reaction showcases sulfuric acid's power to provide protons and displace fluoride ions from calcium fluoride.
In another reaction with sodium chloride \[\mathrm{NaCl}\], sulfuric acid generates hydrogen chloride (HCl) and \[\mathrm{NaHSO_4}\]. These reactions exemplify sulfuric acid's versatility and its capacity to form different halides.

Acid-Base Chemistry

Acid-base chemistry plays a pivotal role in these reactions involving hydrogen halides and sulfuric acid. At its core, acid-base chemistry involves the transfer of protons from acids to bases. Sulfuric acid \[\mathrm{H_2SO_4}\] behaves as a proton donor in its reactions, which fits its definition as a strong acid.
An example of this behavior is seen in the reaction with sodium chloride, where hydrogen chloride (HCl) is formed. Here, the chloride ion \[\mathrm{Cl^-}\] acts as a base and accepts a proton from sulfuric acid, converting it into the neutral hydrogen chloride molecule. Similarly, in the reaction with calcium fluoride, fluoride ions \[\mathrm{F^-}\] receive protons to become hydrogen fluoride. This essential aspect of acid-base reactions is what facilitates the formation of these important industrial compounds.

Oxidation-Reduction Reactions

Oxidation-reduction reactions involve the transfer of electrons between molecules, leading to an alteration in their oxidation states. In the case of sulfuric acid reacting with \[\mathrm{NaBr}\] and \[\mathrm{NaI}\], sulfuric acid does not merely act as an acid, but also as an oxidizing agent. It initiates the oxidation of bromide \[\mathrm{Br^-}\] and iodide \[\mathrm{I^-}\] ions.
Sulfuric acid's strong oxidizing properties lead to \[\mathrm{Br^-}\] and \[\mathrm{I^-}\] losing electrons and turning into \[\mathrm{Br_2}\] and \[\mathrm{I_2}\], rather than producing hydrogen halide gases like \[\mathrm{HBr}\] and \[\mathrm{HI}\]. This process prevents a simple acid-base reaction from occurring, instead favoring an intricate redox chemistry, highlighting the complexities and the powerful nature of sulfuric acid.

Chemical Equations

Chemical equations are a standardized method to represent chemical reactions, illustrating the reactants consumed and products formed. A balanced chemical equation ensures that the law of conservation of mass is observed, indicating the same number of each type of atom on both sides of the equation.
For instance, in the generation of hydrogen fluoride, the equation is:\[\mathrm{CaF_2 + H_2SO_4 \rightarrow 2\ HF + CaSO_4}\] Here, this portrays the complete transformation of reactants to products with all elements accounted for. This method of visual representation is crucial for understanding the stoichiometry and proportions of chemical reactions in both laboratory and industry settings.

Industrial Chemistry

Industrial chemistry utilizes these chemical reactions to produce a wide range of substances essential for manufacturing and industrial applications. Hydrogen halides such as \[\mathrm{HF}\] and \[\mathrm{HCl}\] are crucial in many industries, including pharmaceuticals, manufacturing, and cleaning processes.

• Hydrogen fluoride is essential in etching glass and producing industrial chemicals like fluorocarbons.
• Hydrogen chloride is commonly used in producing plastics, refining metals, and preparing other chemicals.

Understanding the reactivity, particularly that of powerful acids like sulfuric acid, is vital in developing efficient and safe industrial procedures. This expertise ensures that the production of these chemicals meets the demands of various sectors while adhering to safety and environmental regulations.