Enthalpies of Solution - Activity

Introduction

When ionic compounds are added to water, there are often energy changes as the compound dissolves. These changes can be observed as temperature changes. They are the result of two main processes. The ionic lattice breaks down, and energy is required to do this. Then as the ions are set free they become 'hydrated' - that is, water molecules form bonds with the ions of the compound. This releases energy. The overall temperature change depends on the relative amounts of energy taken in to break down the lattice and released when the ions become hydrated. In this model, we calculate the enthalpy of solution, per mole, for each of three ionic compounds.

Task 1: Enthalpy of Solution

1. Open Yenka file Model 1.
2. The model shows three containers of ionic compounds. There are 5 g in each container. There is also a beaker of water, a 5 ml pipette, and an empty beaker. The temperature of the beaker can be read in its reaction details.
   • Use the pipette to transfer 50 ml of water to the empty beaker. Note the temperature of the water in this beaker.
   • Add the sodium hydroxide to the water. Note the maximum temperature reached.
     Reload the simulation (press F5) and repeat the two points above using the ammonium chloride, and note the minimum temperature reached. Reload the simulation again and repeat using the sodium chloride, and note the minimum temperature reached.
     Calculate the change in temperature for each chemical, and make a table showing your results.
     Answer

     Your results may not be quite the same as these but they should be similar.

     Compound	Temp. at start / °C	Temp. at end / °C	Temp. change / °C
     NaOH	24.9	79	+54.1
     NH4Cl	24.9	23	−1.9
     NaCl	24.9	24	−0.9

3. The amount of heat involved in each reaction can be calculated using the expression:
   
   Heat = cmΔT
   
   where c is the specific heat capacity of water = 4.18 kJ / kg, m = mass of water in kg, and Δ_T_ is the temperature change. The units are kJ. For each chemical, calculate the amount of heat involved, and enter your results as a table. Note that 1 ml of water has a mass of 1 g.
   Answer

   Again, your results should be similar to these.

   Compound	Heat. change / °C
   NaOH	22.6
   NH4Cl	.79
   NaCl	0.37

4. Now, calculate the mass of one mole of each compound. Then calculate the number of moles present in 5 g of the compound. Enter your results as a table.

   Compound	Mass of 1 mole / g	Moles in 5 g
   NaOH	Answer 40	Answer 0.125
   NH4Cl	Answer 53.5	Answer 0.093
   NaCl	Answer 58.5	Answer 0.085

5. Now calculate the enthalpy of solution per mole, by dividing the heat changes in Q3 by the number of moles in Q4. Enter your results as a table.

   Compound	Heat change per mole / kJmol​​-
   NaOH	Answer 180.8
   NH4Cl	Answer 8.5
   NaCl	Answer 4.35

6. In an exothermic reaction, heat is given out, and the mixture gets hotter. This is shown by placing a negative sign in front of the heat change per mole. Which of the solution processes was exothermic?
   Answer

   The solution of sodium hydroxide.
7. Complete the following by adding the correct sign and units to the following. For the solution of sodium hydroxide in water, ΔH = 39.76
   Answer

   ΔH = −39.76 kJ mol​^​-1

Summary

The energy changes that accompany the solution of an ionic compound in water can be measured using temperature changes. In an exothermic process, heat is given out and the temperature rises. In an endothermic process, heat is taken in and the temperature falls. The enthalpy change per mole can be calculated using the expression

where c is the specific heat capacity of water, m is the mass of water used and n is the number of moles of compound dissolved.