Passage IV

Chemical reaction that release heat areexothermic reactions. The amount of heat released depends on the number of moles of reactants consumed in the reaction. A mole of any substance is 6 × 10²³ molecules or formula units of the substance.

When sodium hypochlorite (NaCIO) and sodium iodide (NaI) are dissolved in acidic H₂O, an exothermic reaction occurs

NaCIO + NaI → products + heat

Students did an experiment to study this reaction.

Experiment

In each of 8 trails, the students performed Steps 1-5:

1. A known volume of a 0.2 mole/L aqueous NaCIO solu­tion was poured into a foam coffee cup. A lid was placed on the cup.

2. A thermometer was placed into the solution through a hole in the lid. The solution's initial temperature, T_i, of 22.0° C was recorded.

3. The lid was lifted, and a known volume of a 0.2 mole/L aqueous NaI solution, also at a T_i of 22.0°C, was poured into the cup. The lid was put back on the cup, and the solution was swirled.

4. The solution's final (maximum) temperature, T_f was measured.

5. The change in temperature, △T, was calculated :

△T= T_f - T_i

The data for each trail are shown in Table 1.

The students plotted △T versus the volume of NaCIO solution for each trail (see Figure 1).

Then they identified the trial for which △T had its greatest value. The ratio of volume of the NaCIO solution to the volume of the NaI solution for this trial is the mole ratio for the reaction.