Explain why Na2CO3 against HCL titration using phenolphthalein indicator does not go to completion but stops at the NaHCO3 stage.
Na2CO3 act as a weak base and HCl is a strong acid. Phenolphthalein is not a suitable indicator for titrating weak base against strong acid because the equivalence point of the titration will result in an acid salt(NaHCO3) which cannot go to completion. That is; the greater acidity of the acid overcomes the basicity of the base. Thus it will occur at a pH less than 7.
The chemical equation is given by;
Na2CO3 + HCl = NaHCO3 + NaCl.
By: *Ogidi Power*
(Gideon Nyarkoh)
I need help
The reaction between sodium carbonate (Na2CO3) and hydrochloric acid (HCl) can be represented by the following equation:
Na2CO3 + 2HCl -> 2NaCl + CO2 + H2O
In this reaction, sodium carbonate reacts with hydrochloric acid to form sodium chloride (NaCl), carbon dioxide (CO2), and water (H2O). The reaction proceeds via a series of steps, and it is important to understand why it stops at the stage where sodium bicarbonate (NaHCO3) is formed.
Initially, as the sodium carbonate is added to the hydrochloric acid, a rapid reaction occurs where the first sodium carbonate molecule reacts with two hydrochloric acid molecules to produce sodium chloride, carbon dioxide, and water. However, this reaction is exothermic, meaning it releases heat.
The release of heat from the reaction causes an increase in temperature, which in turn increases the solubility of the carbon dioxide in the solution. As a result, some of the carbon dioxide will stay in solution rather than escape as a gas. The presence of undissolved carbon dioxide gas creates a state of equilibrium between dissolved carbon dioxide and its gaseous form.
Next, the reaction between sodium carbonate and hydrochloric acid continues, and sodium bicarbonate (NaHCO3) is formed as an intermediate compound. This is because carbonic acid (H2CO3), which is formed when carbon dioxide dissolves in water, reacts with excess hydrochloric acid to form sodium bicarbonate.
H2CO3 + 2HCl -> NaHCO3 + NaCl + H2O
At this stage, the reaction reaches a new equilibrium, where the formation of sodium bicarbonate and its subsequent dissociation into sodium ions (Na+) and bicarbonate ions (HCO3-) occur simultaneously.
The presence of phenolphthalein as an indicator gives an additional insight into this reaction. Phenolphthalein is colorless in acidic solution (pH less than 8), and it turns pink when the solution becomes basic (pH greater than 8). During the titration, as hydrochloric acid is gradually added to sodium carbonate, the phenolphthalein remains colorless until the sodium bicarbonate stage is reached.
The reason why the titration stops at the NaHCO3 stage is that the formation of sodium bicarbonate is a reversible reaction. The carbonic acid formed reacts with excess hydrochloric acid to form sodium bicarbonate, but sodium bicarbonate can also react with hydrochloric acid to form carbonic acid.
NaHCO3 + HCl -> H2CO3 + NaCl
As a result, the equilibrium is maintained between sodium bicarbonate and carbonic acid, preventing the reaction from going to completion by converting all sodium carbonate into sodium chloride. The addition of more hydrochloric acid would cause the conversion of some sodium bicarbonate back into sodium carbonate until a new equilibrium is reached.
In conclusion, the titration of sodium carbonate against hydrochloric acid using phenolphthalein indicator does not go to completion and stops at the sodium bicarbonate (NaHCO3) stage due to the reversible nature of the reaction and the establishment of a new equilibrium.
In order to understand why the titration of Na2CO3 against HCl using phenolphthalein indicator does not go to completion but stops at the NaHCO3 stage, we need to examine the reactions that take place during the titration.
Na2CO3 is a strong base, while HCl is a strong acid. When Na2CO3 is titrated with HCl, a neutralization reaction occurs between the two. The balanced chemical equation for the reaction can be written as:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
In this reaction, two moles of HCl react with one mole of Na2CO3 to form two moles of NaCl (sodium chloride), water (H2O), and carbon dioxide (CO2).
Phenolphthalein is used as an indicator during this titration. It undergoes a color change from pink to colorless at the equivalence point. However, the titration of Na2CO3 against HCl using phenolphthalein as an indicator does not go to completion because of the presence of NaHCO3 (sodium bicarbonate).
During the reaction between Na2CO3 and HCl, the first equivalent of HCl reacts with Na2CO3 to form NaHCO3 (sodium bicarbonate) as an intermediate product:
Na2CO3 + HCl → NaHCO3 + NaCl
This reaction occurs because the first protonation of Na2CO3 yields NaHCO3, which is a weak acid compared to Na2CO3. Therefore, the reaction stops after the formation of NaHCO3.
The reason why phenolphthalein indicator does not go through a color change at this stage is because NaHCO3 is still a slightly basic solution. Phenolphthalein only undergoes a color change in the presence of strong bases, not weak bases. Therefore, the color change indicating the completion of the titration is not observed.
To completely titrate Na2CO3 against HCl and reach the endpoint, a different indicator, such as methyl orange or bromothymol blue, which undergo color changes in the presence of weak bases like NaHCO3, can be used instead of phenolphthalein. These indicators will allow for the detection of the completion of the titration at the NaHCO3 stage.