Question

8. Suppose you and your lab partner each go to repeat the experiment. You take 4g of NaHCO3 and 4g of CaCl2 and dump them both simultaneously into your calorimeter that is filled with 400mL of water. Then you carefully measure the change in temperature. In a separate experiment, your partner first dissolves 4g of NaHCO3 into 200mL of water and then dissolves 4g of CaCl2 into a different 200mL of water. He then combines both solutions into an empty calorimeter and carefully measures the change in temperature. Will you both measure the same change in temperature? Explain why or why not. I

Answer 1

No, the measured temperature change will be different.

When we dump the solute to solvent, then it gets dissolved in it. For every dissolution there is a thermodynamic parameter called heat of solvation, $\Delta H_{solv}$. Now, for both NaHCO₃ and CaCl₂ there is certain value of $\Delta H_{solv}$, so, if we dump them in calorimeter, then first what we'll see is the combined effect on temperature due to their heat of dissolution and then the effect of the reaction between them, $\Delta H_{rxn}$ :

$CaCl_2 + 2NaHCO_3\rightarrow CaCO_3 + 2NaCl + CO_2 + H_2O$

On the other hand if we first individually dissolve them, then the temperature change due to their dissolution gets gets equilibrated with the surroundings, calorimeter receives no heat due to that. Then, if they are mixed in the calorimeter then we are going to see the effect of only the reaction.

That is why the monitored temperature change in each case is different.