Calorimetry

Heat of Neutralization

Large concentrations of hydrogen ion and hydroxide ion cannot coexist in solution, because the neutralization reaction (shown below) strongly favors the product (water).

H ⁺ (aq)   +   OH ^- (aq)   →   H₂O (l)

In this experiment, an aqueous hydrochloric acid solution will be added to an aqueous sodium hydroxide solution. The neutralization reaction will occur until either H ⁺ or OH ^- is entirely consumed. The reactant that is consumed first is called the limiting reactant.

The molar enthalpy of neutralization, ΔH_neut , is the heat flow associated with the reaction of one mole of H ⁺ with one mole of OH ^-. ΔH_neut is usually determined by measuring the heat flow, q_neut , for the neutralization reaction and dividing this value by the moles, n , of the limiting reactant (H ⁺ or OH ^-).

Experiment

The goal of this experiment is to determine the molar enthalpy of neutralization using calorimetry.

The experimental apparatus consists of a calorimeter containing 1.00 M aqueous NaOH and a reservoir containing 2.00 M aqueous HCl. The HCl solution is drained into the NaOH solution, resulting in the neutralization reaction. The temperature of the reaction solution will change in response to the heat flow from the neutralization reaction.

There are three heat flows to consider in this system. The heat absorbed or released by the calorimeter is represented by q_cal. The heat absorbed or released by the combined liquid from the HCl and NaOH aqueous solutions is represented by q_aq. Finally, the neutralization reaction releases or absorbs heat. For a perfectly insulating calorimeter, no heat enters or leaves the calorimeter from the surrounding, therefore

0   =   q_cal   +   q_aq   +   q_neut

The values for q_cal and q_aq may be determined from knowledge of the heat capacities of the calorimeter and solution and the temperature change for the system. From this point, q_neut may be calculated.

Finally, determine whether HCl or NaOH is the limiting reactant and calculate the moles of H ⁺ and OH ^- that react. With this information ΔH_neut can be calculated.

Procedure

1. Enter the volume of 1.00 M NaOH to be placed in the calorimeter. Volume must be between 80 and 200 mL.
2. Enter the volume of 2.00 M HCl to be placed in the reservoir. Volume must be between 20 and 100 mL.
3. Click on the Reset button to apply your selections.
4. Record the initial temperature, T_i , measured by the thermometer. The calorimeter, 1.00 M NaOH solution and 2.00 M HCl solution all begin at the same temperature.
5. Click on the Start button to drain the HCl solution from the reservoir into the calorimeter. The neutralization reaction occurs as the HCl and NaOH solutions are mixed.
6. After thermal equilibrium has been reached, record the final temperature, T_f , measured by the thermometer.
7. Calculate the molar heat of neutralization. Express ΔH_neut in units of   kJ mole ^-1 .

Repeat the experiment several times. You should measure the same value for ΔH_neut , within experimental error.