Consider the strengths of the following changes:
separate NaI into its ions: 702 kJ/mol
separate water into molecules 60 kJ/mol
form solvent shells around sodium ions and iodide ions 770 kJ/mol
When dissolving NaI in water, the change is likely to be (endo/exo thermic) and the reaction will feel (hot/cold) to the touch
Question 2 options:
endothermic, hot
endothermic; cold
exothermic, hot
exothermic, cold
separate NaI into its ions: 702 kJ/mol
separate water into molecules 60 kJ/mol
form solvent shells around sodium ions and iodide ions 770 kJ/mol
So it takes 702 kJ/mol energy for this step + 60 kJ/mol for step 2. Then you get back 770 kJ/mol for step 3.
dH = 702 + 60 - 770 = -8 kJ/mol
That tells you that NaI dissolves in water, it is exothermic, so the vessel will feel warm.
To determine whether the dissolving of NaI in water is endothermic or exothermic, we need to calculate the overall change in enthalpy (ΔH) of the process. The overall change in enthalpy is the sum of the individual energy changes required for each step involved in the process.
Step 1: Separating NaI into its ions (702 kJ/mol)
This step requires energy input to overcome the electrostatic forces holding the sodium and iodide ions together. Therefore, this step is endothermic, meaning it absorbs energy.
Step 2: Separating water into molecules (60 kJ/mol)
Similarly, this step also requires energy to break the intermolecular forces between water molecules. Therefore, this step is endothermic.
Step 3: Forming solvent shells around sodium ions and iodide ions (770 kJ/mol)
When sodium ions and iodide ions dissolve in water, the water molecules surround and interact with these ions through ion-dipole interactions, forming solvent shells. This process releases energy as it's energetically favorable for the ions to be surrounded by solvent molecules. Therefore, this step is exothermic.
To determine the overall change in enthalpy, we sum up the energy changes for each step:
ΔH = Step 1 + Step 2 + Step 3
= 702 kJ/mol + 60 kJ/mol + 770 kJ/mol
Calculating this sum gives us ΔH = 1532 kJ/mol.
The positive value of ΔH (1532 kJ/mol) indicates that the dissolving of NaI in water is endothermic. Furthermore, since energy is absorbed from the surroundings during the process, it will feel cold to the touch, meaning the reaction is endothermic and cold.
Therefore, the correct answer is: endothermic, cold.