A student performs an experiment to determine the enthalpy of neutralization of hydrochloric acid and sodium hydroxide. The student combines 25 mL of equimolar solutions of acid and base, both having an initial temperature of 22.5 C, in an open polystyrene calorimeter. Each solution has a density of 1 g/mL and a specific heat of 4.18 J/g C. The student collects data until a maximum temperature of 26.4 C is reached. What assumption is the student allowed to make and explain how the student knows that data collection is complete.

In addition to the usual assumptions about accurately reading the thermometers, accurately measuring out the 25 mL of acid and base, etc., the student is assuming that the calorimeter has no loss of heat to the surroundings and no heat input to the calorimeter from the surroundings. The data is complete when the temperature has reached a maximum.

The assumption that the student is allowed to make is that the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) is complete.

The student knows that data collection is complete when a maximum temperature is reached and there is no further increase in temperature. In this case, the maximum temperature reached is 26.4 C. This indicates that the reaction has consumed all the heat produced during the neutralization process and that there is no further exothermic reaction occurring.

Once the maximum temperature is reached and no further increase occurs, it means that the reactants have completely reacted and reached their equilibrium state. At this point, the energy released from the reaction is completely transferred to the surroundings, leading to a constant temperature. Therefore, the student can assume that the neutralization reaction is complete and that data collection can be stopped.

The assumption that the student is allowed to make is that the entire energy released or absorbed during the reaction is transferred only to the solution inside the calorimeter, and there are no heat losses to the surroundings. This assumption is based on the fact that the calorimeter is an open polystyrene calorimeter, which is well-insulated and minimizes heat transfer to the surroundings.

To determine if the data collection is complete, the student should look for a consistent plateau in the temperature readings over a sufficient amount of time. This indicates that the reaction has reached its equilibrium state and that the temperature has stabilized. In this case, the student observes a maximum temperature of 26.4 °C, so they can assume that the data collection is complete once the temperature remains constant or changes insignificantly over several measurements. This indicates that the energy transfer has reached its maximum and the reaction has come to completion.