Enthalpy change, of solution

The standard enthalpy change of solution is the enthalpy change that occurs when one mole of a solute dissolves in a large excess of water, so that no further heat change occurs when more water is added to the solution. 

For example, the enthalpy change of solution when dissolving sodium chloride in water is the enthalpy change of the following reaction ... 

This enthalpy change of solution is positive, that is, the reaction is endothermic. This reaction proceeds because a large positive entropy change occurs during the dissolving process (Chapter 15). Many ionic compounds have enthalpies of solution that are negative, that is, the reactions are exothermic. 

The enthalpy change of solution, AH [, is the enthalpy change when one mole of solute dissolves in an infinite volume of water. The value cannot be determined direcdy by experiment and must be found by a process of extrapolation. In practice, there comes a point when further dilution has no measurable effect on tbe value of the enthalpy change of solution, and this may be taken as infinite dilution. 

The enthalpy change of mixing is often expressed in terms of the heat of solution or enthalpy change of solution. For a two-component solution, the molar integral heat of solution of component I in a solution with component 2 is defined by... 

The standard enthalpy change of solution (AH ) is the enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions. 

The enthalpy change of solution of sodium hydroxide can be found using a polystyrene cup as a calorimeter. We use known amounts of solute and solvent with the solvent in excess to make sure that all the solute dissolves. 

Astudent added 10g(0.25mol) of sodium hydroxide to 40 cm of water to make a concentrated solution. All the sodium hydroxide dissolved. He measured the maximum temperature rise. He suggested that these results would give an accurate value for the standard enthalpy change of solution. Give two reasons why he is incorrect. 

We can, however, use an enthalpy cycle to calculate this. We use the standard enthalpy changes of solution to complete the enthalpy cycle (Figure 6.12). 

The enthalpy change of solution, is the energy absorbed or released when 1 mole of an ionic solid dissolves in sufficient water to form a very dilute solution. 

The enthalpy changes of solution for magnesium chloride and sodium chloride are described by the equations below ... 

We can calculate the enthalpy change of solution or the enthalpy change of hydration by constructing an enthalpy cycle and using Hess s law (Figure 19.10). 

Figure 19.10 An enthalpy cycle involving lattice energy, enthalpy change of hydration and enthalpy change of solution.

Determine the enthalpy change of solution of sodium fluoride using the following data lattice energy=-902 kJ mob enthalpy change of hydration of sodium ions =-406 kJmob ... 

Difference in enthalpy change of solution of Group 2 sulfates... 

Figure 19.14 Enthalpy cycles comparing the enthalpy change of solution of calcium sulfate and strontium sulfate.

Hess s law can be applied to construct energy cycles to determine enthalpy changes of solution and enthalpy changes of hydration. 

In Chapter 4 (page 66) we learnt that most ionic compounds dissolve in water and in Chapter 19 (page 265) we related solubility to enthalpy change of solution. Some ionic compounds, however, are insoluble or only slightly soluble in water. But even insoluble ionic compounds may dissolve to a very small extent in water. Solubility is generally quoted as the number of grams or number of moles of compound needed to saturate 100 g or 1 kg of water at a given temperature. AVe say that a solution is saturated when no more solute dissolves in it. 

B is enthalpy change of hydration / AWfyj [1] C is enthalpy change of solution / AH, [1]... 

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