Question

Internal energy change is the sum of heat and work. Explain why the internal energy change of the reaction between hydrochloric acid and sodium hydroxide can be determined with only a small error by determining the heat of the reaction. Explain why the error woul d be larger if sodium carbonate is used as the base in place of the sodium hydroxide.

Answer 1

In case of sodium hydroxide and Hydrochloric acid the reaction is

NaOH + HCl ---> NaCl + H₂O

at equlibrium the final reaction is

H⁺ + OH^- ---> H₂O    H = Heat of reaction.

now according to 2nd law of thermodynamic for a chemical reaction we give

U = H + PV

now for the above reaction only eulibrium of water exits and for this equlibrium there is no large change in pressure and volume of process so we can ignore the value of PV , so we get

U H

means change in internal energy is about equal to heat of reaction and error is less because low value of PV.

but when we use sodium carbonate in place of sodium hydroxide, the reaction is:

Na₂CO₃ + 2 HCl -----> 2 NaCl + H₂CO₃

so at final equlibrium reaction exist

CO₃^-2  + 2 HCl -----> 2 Cl^- + H₂CO₃     H = Heat of reaction.

In this case there exist equlibrium of negative ion CO₃^-2 , which gives H₂CO₃. And due to this reaction there is change in pressure and volume of process so value of   PV will be significant and we can't ignore it so for this process

U = H + PV

and if we write U H for this reaction, there will be large error in value of change in internal energy dur to significant value of PV.