3 replies to this topic

[InquisitiveMind]

I want to calculate the change in temperature when diluting 25% NaOH to 20% NaOH. I know how to calculate molarity and volume but I think I need to incorporate enthalpy and capacity of heat in order to calculate the change in temperature. Can you guide me the right direction?

[kkala]

I want to calculate the change in temperature when diluting 25% NaOH to 20% NaOH. I know how to calculate molarity and volume but I think I need to incorporate enthalpy and capacity of heat in order to calculate the change in temperature. Can you guide me the right direction?

You need precise enthalpy (from table or diagram) and heat capacity data. Perry 7th ed (Physical and chemical data) does not show values precise enough for the calculation. Following in only indicative, subject to others' criticism:
Assuming liquids (25% NaOH and H2O) of 77 0F initial temperature, enthalpy of 25% NaOH solution is (say) 32 Btu/lb and enthalpy of 20% NaOH is (say) 31 Btu/lb (scale does not permit to distinguish these figures well), according to Perry's Fig 2-29 (enthalpy concentration diagram for aqueous NaOH solution).
We also note that 100 lb of 25% NaOH solution + 25 lb H2O = 125 lb of 20% solution (mass balance).
Heat content of 100 lb 25% NaOH solution + 25 lb H2O = 32x100 Btu + 25x1x(77-32) Btu = 4325 Btu (reference for water enthalpy is 32 0F, as stated in Perry).
Heat content of 125 lb 20% NaOH solution = 31x125 Btu =3875 Btu
Heat removed during isothermal dilution (77 0F) 4325 Btu - 3875 Btu = 450 Btu
Specific heats of NaOH solutions (at 20 0C) can be found in Perry's Table 2-216, it is about 0.82 Btu/lb/0 F for 22.5% NaOH solution.
If the dilution was adiabatic, heat of 450 Btu would not be removed, but raised solution temperature by 450/(125x0.82)=4.4 0C. So final temperature would be about 81.4 0F.
Perry also states that partial molal enthalpy of infinitely dilute NaOH solution at 64 0F is considered zero (another reference state, in addition to this of water), which is confusing to me.

Edited by kkala, 10 February 2010 - 02:50 PM.

[breizh]

Hi ,
Attached a good link to support your query .

http://www.dow.com/P...fromPage=GetDoc

Regards

Breizh

[InquisitiveMind]

Thank you for your help kkala and breizh!