Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Win,odd Dhamnekar on July 03, 2021, 08:37:05 AM

How much heat is produced when 100 mL of 0.250 M HCl (density, 1.00 g/mL) and 200 mL of 0.150 M NaOH (density, 1.00 g/mL) are mixed?
HCl(aq) + NaOH (aq) :rarrow: NaCl(aq) + H_{2}O(l) ΔT = 58 kJ
If both solutions are at the same temperature and the heat capacity of the products is 4.19 J/g°C, how much will the temperature increase? What assumption did you make in your calculation?
My answer:
Reaction given produces 58 kJ of heat with 1 mol of NaOH. We have 0.150 M of NaOH (limiting reagent) so it will produce 0.150 M × 58 kJ/1.0 M= 8.7 kJ of heat.
We know that q=cm∆T, substituting the values we have , in the equation 8.7 kJ = 4.19 J/g℃ × 300 g × ∆T, ⇒ ∆T= 6.9°C.
Is this answer correct?

No, it is wrong.

Would you tell me where i am wrong?

HCl is limitting agent (not NaOH) + emitted heat is calculated in wrong way.

OP, Do you understand what the symbol M means?

Please read the following, interpret it and draw out the appropriate inference.

The appropriate inference is  no, you don't understand it.
(First of all, to dispose of what may be just a careless error, the unit of moles is not g/mol. It is moles. g/mol is the unit of molar mass.)
The symbol M DOES NOT mean moles. It means moles per litre. It is a unit of concentration, not amount. You do NOT have 0.250 moles of HCl. You have 100 mL of a 0.250 mol/L solution of HCl. How many moles is that?

Thanks for finding out my mistake in my chemistry working.
Now, my rectified answer: We have 0.025 M of HCl (limiting reagent), so it will produce 0.025 M × 58 kJ/1 mol = 1.45 kJ of heat.
Substituting the known values in the heat equation q=cmΔT =4.19 J/g℃ × 300 g × ∆T=1.45 kJ,
we get ΔT= 1.15℃
In my opinion, now this answer is correct.

The numerical calculation is correct. However:
1. You do not have 0.025 M of HCl. You have 0.025 moles. Reread my previous post. M DOES NOT MEAN MOLES!!!
2. A negative ΔH means heat is given out, and (unless there is efficient heat transfer to the surroundings) the temperature rises. A ΔH of 1.45 kJ means q, the heat entering the system, is +1.45 kJ. The original question asked "how much does the temperature increase?" It should be obvious that a negative ΔT is wrong.

Yes, you are correct. ΔT =1.15°C is positive. Thanks again for pointing out where i was wrong in my chemistry workings.