Equilibrium Na2CO3 and NaHCO3

1.
Explain why would we consider sodium carbonatereacting with the HCl only but not the both reactions?

Sodium carbonate reacts with HCl in two stages:
Firstly, HCl reacts with sodium carbonate to give sodium chloride and sodium hydrogencarboante and water.
HCl(aq) + Na2CO3(aq) → NaCl(aq)+ NaHCO3(aq)
Secondly, when the above stage completes, the HCl added reacts with NaHCO3 solution to give sodium chloride, water and carbon dioxide.
NaHCO3(aq) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(aq)
HCl reacts with NaHCO3 when all Na2CO3 iscompletely reacted. This is because Na2CO3 has a much higher basic strength than NaHCO3.

Therefore, when HCl is added to a mixture of Na2CO3 and NaHCO3, HCl firstly reacts with Na2CO3.


= = = = =
2.
Explain why phenolphthalein indicates the reaction between sodium carbonate and HCl , and methyl orange indicates the reaction between sodium hydrogencarbonate and HCl ?

It is known that phenolphthalein indicator changes its colour in slightlyalkaline solution (pH 8.3-10), while methyl orange indicator changes its colourin slightly acidic solution (pH 3.1-4.5).

Consider the titration between sodium carbonate and hydrochloric acid.
HCl(aq) + Na2CO3(aq) → NaCl(aq)+ NaHCO3(aq)
At the equivalence point, HCl and Na2CO3 completely react. Sodium ions and chloride ions are not hydrolysed. The hydrolysis of hydrogencarbonate ions gives a slightly alkaline solution at the equivalence point, and thus phenolphthalein is suitable to be used as an indicator in the titration.

Consider the titration between sodium hydrogencarbonate and hydrochloric acid.
NaHCO3(aq) + HCl(aq) → NaCl(aq) + H2O(l) + CO2(aq)
At the equivalence point, HCl and NaHCO3 completely react. A small amount of carbon dioxide dissolves in water to give a slightly acidic solutionat the equivalence point, and thus methyl orange is suitable to be used as indicator in the titration.

"phenolphthalein change from colourless to pale pink when more acid is added" ?!
Jason, you can go back and study now.

1. Explain why would we consider sodium carbonate reacting with the HCl only but not the both reactions??
No, we always consider the reaction of both the NCl and NaCo3 as long as we are considering the concentrations of both the solution. It is not a right concept of only considering the HCl.

2. Explain why phenolphthalein indicates the reaction between sodium carbonate and HCl , and methyl orange indicates the reaction between sodium hydrogencarbonate and HCl ??
You should have a clear concept that phenolphthalein change from colourless to pale pink when more acid is added. Methyl orange change from Red to orange to yellow when more alkline is added. Both of the indicators can use to indicate the reaction between HCl and NaCo3. But the only thing is when more NaCo3 is added to HCl, it means that the solution will become more acidic, then it is better for us to observe the colour change from colourless to pale pink by phenolphthalein. For a better colour observation only.