[jennaw23]

Hello, I am trying to get an endothermic reaction of less than 0℃ when mixing HCl and baking soda. It was my belief that a higher endothermic change of enthalpy would yield a cooler reaction (I calculated a higher ΔH when I had less baking soda), but it ended up barely changing the overall temperature. What amounts of baking soda and HCl (1, 2, 3, and 6 molarity are all options) would combine to the lowest possible temperature?

I have already tried using;

• approx. 1g of baking soda in 25mL of 6M HCl (got to 20℃),
  
• approx. 5g of baking soda in 25mL of 6M HCl (got to 10℃)
  
• approx. 3g of baking soda in 25mL of 3M HCl (got to 14℃).
  

[Enthalpy]

Hi jennaw23,

ΔH relates with the amount of the products of your reaction. Additional water, excess HCl, excess NaHCO₃ would only increase the heat capacity hence dilute the temperature change, as long as all products remain in the same state. So what about writing the reaction equation and trying to balance it?

You could double-check your computation of ΔH versus NaHCO₃ amount. And HCl isn't the usual endothermic choice among similar reactions.

If computing ΔH, pay attention to whether the compounds are dissolved or not. For HCl the difference is big. The concentration matters a bit too. And is all the NaCl amount soluble?

If you wanted to estimate the temperature change, you'd need to take some mean temperature for the evolved CO₂, as it begins to leave at the start temperature and ends at the stop temperature. No dramatic change here, since water is the main contributor to heat capacity in your reaction.