In an ice calorimeter, a chemical reaction is allowed to occur in thermal contact with an ice–water — Thermodynamics and Thermochemistry Chemistry Question
Question
In an ice calorimeter, a chemical reaction is allowed to occur in thermal contact with an ice–water mixture at $0^\circ\text{C}$. Any heat liberated by the reaction is used to melt some ice; the volume change of the ice–water mixture indicates the amount of melting. When solutions containing 1.0 millimole each of $\text{AgNO}_3$ and NaCl are mixed in such a calorimeter, both solutions having been pre-cooled to $0^\circ\text{C}$, 0.20 g of ice melted. Assuming complete reaction in this experiment, what is $\Delta H$ for the reaction: $\text{Ag}^+\text{ (aq)} + \text{Cl}^-\text{ (aq)} \to \text{AgCl(s)}$? Latent heat of fusion of ice at $0^\circ\text{C}$ is 80 cal/g.
💡 Solution & Explanation
Heat liberated melts the ice: $q = 0.20\text{ g} \times 80\text{ cal/g} = 16\text{ cal}$. Since heat is released, $\Delta H$ is negative. This heat is produced by 1.0 millimole (0.001 mol) of reactants. For 1 mole, $\Delta H = -16\text{ cal} / 0.001\text{ mol} = -16000\text{ cal/mol} = -16\text{ kcal/mol}$.