Question

I'm being asked to compute the literature "true" value of the heat of solution of KCll at infinite solution. What do I need/how do I do this?

Answer 1

The heat of solution at infinite dilution may be regarded as, ' the heat liberated or adsorbed when one mole ofsolute dissolved in a large amount of pure solvent.'

the experimentally it can be calculated with a calorimeters are designed to be well-insulated, so no heat is gained from or lost to the surroundings. If no heating element is used to introduce heat in the system, the total heat (q) for the entire calorimeter system must equal zero. The total heat can be split into heats for each component in the system.

perform a reaction in which solid potassium chloride is dissolved in water to produce an aqueous KCl solution.

KCl (s)   → K⁺(aq) + Cl^-(aq)

The heat flow (q_rxn) for this reaction is called the heat of solution for KCl. When the reaction is finished, the system contains two substances, the calorimeter itself and the aqueous solution, and there is a heat associated with each component. The heat balance for this experiment is thus

0 = q = q_rxn + q_cal + q_soln

The basic strategy in calorimetry is to use a temperature change and a heat capacity to determine a heat flow. In this experiment all substances have the same initial and final temperatures.

q_cal = C_calΔT = C_cal( T_f - T_i)

q_soln = C_solnΔT = m_soln s_soln( T_f - T_i)

One typically determines the heat capacity of the aqueous solution (C_soln) from the mass of the solution (m_soln) and the specific heat capacity of the solution (s_soln). The mass of the solution is the sum of the masses of the water and KCl originally placed in the calorimeter. The specific heat capacity of the aqueous solution is usually close to that of pure water.

The objective of this experiment is to determine the heat of reaction (in this case a heat of solution). The above equations can be combined and rearranged to give a equation:

q_rxn = - q_cal - q_soln = - ( C_cal + m_soln s_soln) ( T_f - T_i)

As the heat capacity of a substance is an extensive property, so the heat of solution is an extensive property. It is generally more convenient to report intensive properties, thus the heat capacity of a substance is usually reported as a specific heat capacity, that is, the heat capacity per gram of substance. Similarly one can report a specific heat of solution, which is the heat a solution per gram of solute. though the molar heat of solution (ΔH_soln) is the heat of solution (q_rxn) per mole of solute (n).

H_solu = q_rxn / n.