EXPLAIN WHY NA2CO3 AGAINST HCL TITRATION USING PHENOLPHTHALEIN INDICATOR DOES NOT UNDERGO COMPLETION BUT STOPS AT NAHCO3 STAGE

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In a titration, two substances are reacted together until their chemical reaction is complete. The goal is to determine the amount of a substance in a solution by reacting it with a solution of a known concentration. However, in the case of sodium carbonate (Na2CO3) and hydrochloric acid (HCl) titration using phenolphthalein indicator, the completion of the reaction is not observed, and it stops at the formation of sodium bicarbonate (NaHCO3).

To understand why this happens, let's break down the reaction between Na2CO3 and HCl.

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

As you can see, the reaction produces sodium chloride (NaCl), water (H2O), and carbon dioxide (CO2). The formation of CO2 gas makes this reaction quite different from a normal acid-base titration.

During the titration, when HCl is added to the Na2CO3 solution, it reacts with the sodium carbonate to produce sodium bicarbonate, NaHCO3.

Na2CO3 + HCl → NaHCO3 + NaCl

At the initial stages of the titration, you will see the alkaline solution turning pink due to the presence of phenolphthalein, an indicator that changes color in the presence of a basic solution.

However, as more HCl is added, the pink color starts to fade, indicating that the solution is becoming acidic. This color change happens because sodium bicarbonate is a weak base. As the NaHCO3 is formed, it reacts with the excess HCl to produce carbonic acid (H2CO3), which is a weak acid.

NaHCO3 + HCl → NaCl + H2CO3

The carbonic acid is unstable and decomposes into water and carbon dioxide:

H2CO3 → H2O + CO2

Since the carbon dioxide (CO2) is a gas, it escapes from the solution, leaving behind a clear, colorless solution.

At this point, the phenolphthalein indicator becomes colorless, indicating that the reaction endpoint has been reached. However, it's important to note that this reaction is not fully complete, as NaHCO3 is still present in the solution.

To obtain a complete reaction, the titration would need to be performed using a different indicator, like methyl orange, which changes color in the presence of acidic conditions. With methyl orange, the endpoint would be reached when the solution turns slightly acidic, indicating that all the sodium carbonate has reacted with HCl to form sodium chloride and water.