Neutralization Reactions: NaOH, H3PO4 & H2SO4, Na2S

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Hey guys! Let's dive into the fascinating world of neutralization reactions, where acids and bases react in a molar ratio to neutralize each other. We're going to explore two specific scenarios: the reaction between sodium hydroxide (NaOH) and phosphoric acid (H3PO4), and the reaction between sulfuric acid (H2SO4) and sodium sulfide (Na2S). We'll break down each reaction, perform the necessary calculations, and explain the underlying chemistry. Buckle up, it's chemistry time!

a) NaOH and H3PO4: A Balancing Act

When sodium hydroxide (NaOH), a strong base, reacts with phosphoric acid (H3PO4), a triprotic acid, the neutralization process involves multiple steps, as phosphoric acid can donate three protons (H+). The reaction isn't as straightforward as a simple 1:1 acid-base reaction. To fully understand the neutralization, we need to consider the stepwise dissociation of phosphoric acid and its reactions with NaOH. Phosphoric acid (H3PO4) is a triprotic acid, meaning it has three acidic protons that can be neutralized by a base. Sodium hydroxide (NaOH) is a strong base that completely dissociates in water to provide hydroxide ions (OH-). The reaction between NaOH and H3PO4 can occur in three steps, leading to the formation of different salts:

  1. Formation of Sodium Dihydrogen Phosphate (NaH2PO4):

NaOH + H3PO4 → NaH2PO4 + H2O

In this first step, one mole of NaOH reacts with one mole of H3PO4 to form one mole of sodium dihydrogen phosphate (NaH2PO4) and one mole of water. The H3PO4 donates one proton, which is accepted by the hydroxide ion (OH-) from NaOH to form water.

  1. Formation of Disodium Hydrogen Phosphate (Na2HPO4):

NaH2PO4 + NaOH → Na2HPO4 + H2O

In the second step, the sodium dihydrogen phosphate (NaH2PO4) formed in the first step reacts with another mole of NaOH to form one mole of disodium hydrogen phosphate (Na2HPO4) and one mole of water. Here, NaH2PO4 donates its remaining acidic proton to the hydroxide ion from NaOH.

  1. Formation of Trisodium Phosphate (Na3PO4):

Na2HPO4 + NaOH → Na3PO4 + H2O

Finally, in the third step, the disodium hydrogen phosphate (Na2HPO4) reacts with another mole of NaOH to form one mole of trisodium phosphate (Na3PO4) and one mole of water. This step completes the neutralization of all three acidic protons in the phosphoric acid.

For complete neutralization, all three protons from H3PO4 must react with NaOH. This requires a 1:3 molar ratio of H3PO4 to NaOH. Let's illustrate this with an example. Suppose we have 1 mole of H3PO4. To completely neutralize it, we would need 3 moles of NaOH. The overall reaction would be:

H3PO4 + 3NaOH → Na3PO4 + 3H2O

This balanced equation shows that one mole of phosphoric acid reacts with three moles of sodium hydroxide to produce one mole of trisodium phosphate and three moles of water. It’s crucial to consider all three steps to determine the complete neutralization point. If you only add one mole of NaOH, you'll end up with NaH2PO4 and water. If you add two moles of NaOH, you'll get Na2HPO4 and water. Only by adding three moles of NaOH will you achieve complete neutralization, resulting in Na3PO4 and water. Therefore, complete neutralization necessitates a 1:3 molar ratio of H3PO4 to NaOH, highlighting the importance of considering all dissociation steps in polyprotic acids. Keep in mind that the pH at each step will vary, reflecting the different degrees of neutralization. This step-by-step neutralization is essential in various applications, such as in buffer solutions and chemical titrations, where precise pH control is needed.

b) H2SO4 and Na2S: A Gas-Releasing Reaction

Now, let's examine the reaction between sulfuric acid (H2SO4), a strong diprotic acid, and sodium sulfide (Na2S), a salt of a weak acid. This reaction is not a simple neutralization in the traditional sense because it produces hydrogen sulfide (H2S) gas. Sulfuric acid (H2SO4) is a strong diprotic acid, which means it can donate two protons (H+). Sodium sulfide (Na2S) is a salt of a weak acid (H2S) and a strong base (NaOH). When H2SO4 reacts with Na2S, it forms hydrogen sulfide gas (H2S) and sodium sulfate (Na2SO4). The reaction between sulfuric acid (H2SO4) and sodium sulfide (Na2S) proceeds in two main steps:

  1. Formation of Sodium Bisulfide (NaHS):

H2SO4 + Na2S → NaHS + NaHSO4

In the first step, one mole of sulfuric acid (H2SO4) reacts with one mole of sodium sulfide (Na2S) to form one mole of sodium bisulfide (NaHS) and one mole of sodium bisulfate (NaHSO4). This step involves the transfer of one proton from H2SO4 to S2- from Na2S, forming HS-.

  1. Formation of Hydrogen Sulfide (H2S) Gas:

NaHS + NaHSO4 → H2S(g) + Na2SO4

In the second step, the sodium bisulfide (NaHS) and sodium bisulfate (NaHSO4) react to form one mole of hydrogen sulfide gas (H2S) and one mole of sodium sulfate (Na2SO4). This step involves the transfer of another proton, leading to the evolution of H2S gas, which has a characteristic rotten egg smell.

The overall balanced equation for the reaction is:

H2SO4 + Na2S → H2S(g) + Na2SO4

In this reaction, one mole of sulfuric acid reacts with one mole of sodium sulfide to produce one mole of hydrogen sulfide gas and one mole of sodium sulfate. The key aspect here is the formation of H2S gas, which drives the reaction to completion. Unlike the NaOH and H3PO4 reaction, where we had stepwise neutralization, this reaction primarily results in gas formation. Now, let's consider the molar ratios. According to the balanced equation, a 1:1 molar ratio of H2SO4 to Na2S is required for the reaction to occur completely. If you have 1 mole of H2SO4, you need 1 mole of Na2S to produce 1 mole of H2S and 1 mole of Na2SO4. Any excess of either reactant will remain unreacted. Understanding this molar ratio is crucial for controlling the amount of H2S gas produced, especially in industrial or laboratory settings where H2S can be hazardous. Remember, hydrogen sulfide gas is toxic and has a distinct odor of rotten eggs, so this reaction should be performed with caution, typically under a fume hood to prevent exposure to the gas. In summary, the reaction between H2SO4 and Na2S is not just a simple neutralization but a gas-evolving reaction that follows a 1:1 molar ratio, producing H2S gas and sodium sulfate. Always ensure proper safety measures when dealing with this reaction due to the toxicity of H2S.

Conclusion: Mastering Neutralization

So, there you have it! We've explored two different types of reactions: a stepwise neutralization with NaOH and H3PO4 and a gas-evolving reaction with H2SO4 and Na2S. In the case of NaOH and H3PO4, remember the importance of considering the stepwise neutralization to achieve complete reaction, requiring a 1:3 molar ratio. For H2SO4 and Na2S, the reaction leads to the formation of H2S gas and requires a 1:1 molar ratio. Understanding these reactions and their molar ratios is fundamental in chemistry. Whether you're in the lab or just curious, grasping these concepts will boost your chemistry knowledge. Keep experimenting and stay curious!