Passage VI

When 1-bromebutane (C₄H₉Br) and sodium iodide (NaI) are dissolved together in a solvent—acetone or acetonitrile—they react to from 1-iodobutane (C₄H₉I) and sodium bromide (NaBr):

C₄H₉Br + NaI C₄H₉I + NaBr

NaBr is not very soluble in either solvent, so as NaBr forms, it alone precipitates from the reaction solution. The reaction can be monitored by measuring the conductivity (ability to conduct electrical current) of the reaction solution because NaI is the only component that significantly contributes to the solution's conductivity.

Students did 3 experiments to study the rate of this reaction.

Experiment 1

A probe was used to measure (in millisiemens per centimeter, mS/cm ) the conductivities of solutions with different NaI concentrations (in millimoles per liter, mmol/L) in acetone or in acetonitrile at 40°C (see Figure 1). This trial was repeated twice, once at 30°C and once at 20°C. The results did not significantly differ from those at 40°C.

Experiment 2

A capped test tube containing an 800.0 mmol/L C₄H₉Br-acetone solution was mostly submerged in a water bath maintained at 40°C. Another capped test tube—containing an 800.0 mmol/L NaI-acetone solution, a magnetic stir bar, and the conductivity probe—was mostly submerged in the bath. After 10 min, the tubes were uncapped and the C₄H₉Br solution was added to the NaI solution. The tube was capped and the solution was stirred in the bath. Conductivity readings were recorded at regular intervals. This trial was repeated twice, once at 30°C and once at 20°C ( see Figure 2).

Experiment 3

The trial of Experiment 2 at 40°C and at 20°C were repeated except that the solvent was acetonitrile (see Figure 3).