why sodium carbonate against hydrochloric acid titration using phenolphthalein indicator does not go to completion but stops at the sodium hydrogen carbonate stage.
this is because sodium hydrogen carbonate will react with H+ or hydrochloric acid,in other word neutralise it
The titration of sodium carbonate with hydrochloric acid using phenolphthalein indicator does not go to completion because sodium carbonate is a weak base, and it undergoes a two-step reaction with hydrochloric acid.
In the first step of the reaction, sodium carbonate reacts with hydrochloric acid to form sodium hydrogen carbonate, water, and carbon dioxide. The balanced chemical equation for this reaction is:
Na2CO3 + 2HCl -> 2NaHCO3 + H2O + CO2
The formation of carbon dioxide gas bubbles during the reaction can be observed as effervescence. This effervescence is an indication that carbon dioxide is being produced and evolved out of the reaction mixture. At this point, the solution turns cloudy due to the formation of small gas bubbles.
Once all the sodium carbonate has reacted to form sodium hydrogen carbonate, the reaction stops, and no further reaction occurs. The appearance of sodium hydrogen carbonate is indicated by the solution becoming clear, indicating the absence of carbon dioxide gas bubbles.
Phenolphthalein indicator is colorless in acidic solutions and turns pink in slightly basic to neutral solutions. During the titration, when all the sodium carbonate has reacted to form the sodium hydrogen carbonate, this solution is still slightly basic. Therefore, phenolphthalein indicator will show a pink color change. This indicates the endpoint of the titration, even though the reaction is not fully complete.
In summary, the reaction between sodium carbonate and hydrochloric acid stops at the formation of sodium hydrogen carbonate because sodium carbonate is a weak base, and phenolphthalein indicator detects the slight basic nature of the solution, which indicates the endpoint of the titration.
In the titration between sodium carbonate (Na2CO3) and hydrochloric acid (HCl) using phenolphthalein as an indicator, the reaction does not go to completion and stops at the sodium hydrogen carbonate (NaHCO3) stage because of the presence of excess carbon dioxide (CO2) in the reaction.
Let's break it down step by step:
1. Sodium carbonate (Na2CO3) is a strong base, while hydrochloric acid (HCl) is a strong acid. When they react, they undergo a neutralization reaction:
Na2CO3 + 2HCl -> 2NaCl + H2O + CO2
This equation shows that one mole of sodium carbonate reacts with two moles of hydrochloric acid to produce two moles of sodium chloride (NaCl), one mole of water (H2O), and one mole of carbon dioxide (CO2).
2. Phenolphthalein is an indicator that changes color at a pH level around 8.2 to 10.0. Initially, when the sodium carbonate reacts with the hydrochloric acid, the pH of the solution increases as sodium carbonate neutralizes the acid. Phenolphthalein remains colorless in this region.
3. As the neutralization reaction progresses, all the sodium carbonate reacts with the hydrochloric acid until a point where all the sodium carbonate is consumed and converted into sodium chloride. At this stage, sodium hydrogen carbonate (NaHCO3) is formed as an intermediate product:
Na2CO3 + HCl -> 2NaCl + H2O + CO2
NaHCO3 is a weak base and is less alkaline than Na2CO3. Its pH is lower, and hence phenolphthalein starts to show a pink color indicating that the solution has reached the endpoint of the titration.
4. However, the reaction does not go to completion, and the endpoint is not the final stage. This is because NaHCO3 reacts with CO2 present in the air to produce sodium carbonate again:
2NaHCO3 + CO2 -> Na2CO3 + H2O + CO2
This reaction shifts the equilibrium back towards Na2CO3, resulting in the formation of sodium carbonate. Therefore, excess CO2 prevents the reaction from proceeding to the completion of NaHCO3 consumption, and the phenolphthalein continues to show a pink color even after the apparent endpoint.
In conclusion, during the titration between sodium carbonate and hydrochloric acid using phenolphthalein as an indicator, the reaction stops at the sodium hydrogen carbonate stage due to the presence of excess CO2, which prevents the conversion of NaHCO3 into sodium chloride and water.