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2

# Solid Naoh Dissolving In Water

chemistry

3 replies to this topic
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### #1 lak1r

lak1r

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Posted 10 April 2023 - 05:56 AM

Hello,

I would like to do  4 % NaOH solution that is 40 degrees. I have got demineralised water that is 30 degrees and if  I add 7,2 kg of solid NaOH to 180 L of 30 degree water,

should the exothermic reaction give approximately 40 °C  4% NaOH solution? (So the temperature should rise from 30 to 40 degree celcius?)

Could you give me some basic calculations?

What i have tried myself:

moles of solute = 7200g/(40g/mol) = 180 mol NaOH

molar heat of solution: ΔHsolution/(moles of solute) = -44.2kJ/mol

heat of solution: ΔHsolution = -44.2kJ/mol • 180 mol = -7956kJ

masssolution = 7,2 kg + 180 = 187,2 kg

ΔTsolution = -ΔHsolution/(masssolution • specific heatsolution) ≈

≈ -ΔHsolution/(masssolution • specific heatwater) =

= 7956kJ/(187,2 kg • 4.184kJ/kg°C) = 10,15 °C

So the final temperature would be: 30 °C+ 10,15°C= 40,15°C.

If i am wrong somewhere please guide me with this calculation.

Also the same problem is with Hydrochloric acid solution. I am adding 32,5 %  (m.v) HCL  to 180 L of 30 degree water to get 3% HCL solution. So I add 16,6L of 32,5 % HCL to 30 °C 180L demineralised water.

How much will the diluted hydrocloric acid solution temperature increases after adding 16,6 L the concentrated HCL to 180 L of demineralised water that temperature is 30 degrees and concentrated HCL (32,5 %) temperature is 23 degrees?

Could you give me some guidance with this calculation?

Edited by lak1r, 10 April 2023 - 05:59 AM.

### #2 breizh

breizh

Gold Member

• 6,075 posts

Posted 10 April 2023 - 06:12 AM

Hi,

Be extremely careful with those chemicals and the exothermic reactions, I got an operator badly injured during the process of dissolution of NaOH pearls in hot water. A lack of control of introduction of caustic leaded to a big exotherm and severe burns.

Consider the documents attached to support. A good set of PPE is required.

The problem is how do you manage the introduction of caustic soda? How fast ?

Let you study these 2 handbooks and follow the instructions related to the preparation of diluted solutions.

Good luck and stay safe.

Breizh

### #3 lak1r

lak1r

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Posted 10 April 2023 - 08:26 AM

Hi,

Be extremely careful with those chemicals and the exothermic reactions, I got an operator badly injured during the process of dissolution of NaOH pearls in hot water. A lack of control of introduction of caustic leaded to a big exotherm and severe burns.

Consider the documents attached to support. A good set of PPE is required.

The problem is how do you manage the introduction of caustic soda? How fast ?

Let you study these 2 handbooks and follow the instructions related to the preparation of diluted solutions.

Good luck and stay safe.

Breizh

I Have the correct PPE. I will dissolve first 3,6 kg of NaOH in 20 L bucket that is with water and add it to the tank where is  the 140 liters of demineralised water. I very slowly add the bucket NaOH solution to 140 distilled water.

Later i will add again 3,6 kg of NaOH to 20 L of water in bucket and that bucket goes to 160 L of water that has already inside the 3,6 kg of NaOH. But i add the NaOH very slowly to the bigger container from the bucket of water.

Just previousluy i have done it with 20 degrees of water but now need to do 40 degree NaOH solution. I have boiler heater from i take the preheated water and mix it with cold water to 30 degrees. So after that i start adding NaOH powder to get 4 % solution. I do it very slowly. I put from the bucket the water to the big containter which i use our membrane washing. So practically i add 7,2 kg of caustic very very slowly from bucket of water over time to 180 L of water that is in 30 degrees. There shouldnt be anything dangerous if i do it slowly? What is your opinion?

My big question is that if I look your NaOH handbook then the solution temperature should increase from 30 degrees to 38 degrees?  Because on the chart 4% NaOH increases from 21-29 degrees and then should  from 30 degrees to 38 degrees if i do 4 % solution.

I dont know if my logic is correct but the temperature should increase around 7-9 degrees if I add 7,2 kg of caustic to 30 degrees 180 L deminarlised water.

If you have some more tips to give then i am happy to hear;)

### #4 breizh

breizh

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Posted 10 April 2023 - 06:35 PM

Hi,

Seems OK to me.

Breizh