Problem 86 - Chemistry

Question

Hydrogen fluoride can be prepared by the action of sulfuric acid on sodium fluoride. Explain why hydrogen bromide cannot be prepared by the action of the same acid on sodium bromide.

Step-by-Step Solution

Answer

Hydrogen bromide cannot be produced by the action of sulfuric acid on sodium bromide because HBr is a stronger acid than H2SO4. A stronger acid cannot be prepared from a weaker acid.

Step 1: Analyzing Reactions

Begin by analyzing the reaction between sulfuric acid and sodium fluoride: \n\n\( NaF + H_2SO_4 \rightarrow HF + NaHSO_4 \) \n\nHydrogen fluoride (HF) is produced which is a weaker acid than the sulfuric acid used.

Step 2: Predicting Reactions

Now try to predict the reaction between sulfuric acid and sodium bromide:\n\n\( NaBr + H_2SO_4 ?\rightarrow \)\n\nOne might think that Hydrogen bromide (HBr) would be produced, because it matches the pattern of the first reaction.

Step 3: Understanding Chemical Properties

However, Hydrogen bromide (HBr) cannot be produced. This is because HBr is a stronger acid than sulfuric acid (H2SO4). In general, a stronger acid cannot be prepared from a weaker acid. So the reaction does not happen.

Key Concepts

Hydrogen Fluoride

Hydrogen fluoride (HF) is an interesting compound primarily due to its unique chemical properties. It is produced by the reaction of a fluoride salt, such as sodium fluoride (NaF), with sulfuric acid (H₂SO₄). This reaction can be represented by the equation: \( NaF + H_2SO_4 \rightarrow HF + NaHSO_4 \).In this process, hydrogen fluoride is liberated, and sodium hydrogen sulfate (NaHSO₄) forms as a by-product. HF is notable for being a polar covalent bond with strong hydrogen bonding abilities which play a significant role in its properties.

• It is a colorless gas that readily dissolves in water to form hydrofluoric acid, a highly corrosive liquid used in various industrial processes.
• Hydrogen fluoride is unique among hydrogen halides due to its weak acidic behavior in dilute solutions, despite being very reactive and toxic.

Hydrogen Bromide

Hydrogen bromide (HBr) is another hydrogen halide that is understood through its interactions with different reagents. It forms as a colorless gas which turns into hydrobromic acid when dissolved in water. However, unlike HF, HBr cannot be prepared from the reaction of sodium bromide (NaBr) with sulfuric acid (H₂SO₄).The predicted reaction that one might expect is: \( NaBr + H_2SO_4 ?\rightarrow \)This reaction does not proceed primarily due to the strength of HBr as an acid compared to H₂SO₄. Since HBr is a stronger acid than sulfuric acid, the reaction does not happen, preventing the formation of HBr through this method. In preparative chemistry, such considerations of acid strength are crucial to assure successful synthesis.

Acid Strength

In chemistry, acid strength is a measure of an acid's ability to donate a proton (H⁺). It is key when predicting reactions involving acids, as seen with hydrogen halides like HF and HBr.

• Hydrogen fluoride (HF) is a weak acid in aqueous solution but is more reactive compared to some common acids due to its formation of strong hydrogen bonds and complex ion interactions when dissolved.
• Hydrogen bromide (HBr) is considered a strong acid, capable of fully dissociating in water, which means it gives up protons easily compared to weaker acids like HF.

The general rule is that a stronger acid cannot be produced directly from a weaker acid because of its higher tendency to hold onto its protons. This principle is why sulfuric acid cannot be used to produce HBr, as HBr's strength surpasses that of H₂SO₄ in releasing protons.

Sulfuric Acid Reactions

Sulfuric acid (H₂SO₄) is a highly utilized industrial acid known for its potent reactive nature and ability to produce numerous chemical transformations. Its application in reactions with sodium compounds can yield different hydrogen halides, as seen with its reaction with sodium fluoride to form hydrogen fluoride.

• Sulfuric acid can donate protons to substances that take up these protons more readily, like fluoride ions, making it suitable for HF preparation.
• However, when the reaction involves sodium bromide (NaBr), sulfuric acid is ineffective as the source of HBr, because HBr, as a stronger acid, will not be displaced by an acid of lesser strength like H₂SO₄.

Understanding the reactivity patterns between sulfuric acid and halogen salts highlights the importance of acid-base reactivity dynamics in chemical syntheses.