Thermal Analysis of Acid-Base Reactions: Heat of Ionization in Strong and Weak Acids

To investigate the heat of ionization of a weak acid, a calorimeter was employed to measure the heat produced during neutralization reactions involving two different acids: the strong acid hydrochloric acid [1] and the weak acid dichloroacetic acid [2], when mixed with the strong base sodium hydroxide.

The calorimeter, made of Styrofoam, was used to isolate and measure the heat formed by the reactions. Due to the imperfect insulation of the Styrofoam cup, some heat was absorbed by the calorimeter itself. To account for this, the heat capacity of the calorimeter was determined by measuring the temperature change when known amounts of warm and cold water were added to it.

The heat lost to the calorimeter during subsequent reactions was calculated as the difference between the heat lost by warm water and gained by cold water.

The temperature data obtained from these reactions were then used to calculate the heat produced per mole of acid used, known as the molar heat of neutralization.

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This allowed for a comparison between the strong acid-strong base and weak acid-strong base reactions.

The experimental procedure involved two parts:

Part 1: Heat Capacity of Calorimeter

• 50 mL of distilled water was measured and added to the Styrofoam cup calorimeter, mounted on a stir plate.
• The initial temperature of the water was recorded using a calibrated digital thermometer.
• 50 mL of warm water was added to the calorimeter, and a stopwatch was started.
• Temperature readings were taken at 15-second intervals until 4 minutes had elapsed.

Part 2: The Heats of Neutralization

• 50 mL of NaOH was measured and added to the calorimeter, followed by the recording of the initial temperature.
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• 50 mL of HCl, at room temperature, was measured and added quickly to the calorimeter, with temperature readings taken at 15-second intervals until 4 minutes had passed.
• The same procedure was repeated, replacing HCl with dichloroacetic acid.

The aim of this laboratory investigation is to determine the heat of ionization for two different acids - hydrochloric acid (HCl) and dichloroacetic acid (DCA) - when neutralized with a strong base, sodium hydroxide (NaOH). The experimental approach involves measuring temperature changes over time in a Styrofoam cup calorimeter to calculate the heat produced during the reactions.

Part 1: Heat Capacity of Calorimeter

Procedure:

1. Measure and transfer 50 mL of distilled water into the Styrofoam cup calorimeter.
2. Record the initial temperature using a calibrated digital thermometer.
3. Add 50 mL of warm water to the calorimeter and start a stopwatch.
4. Record temperature at 15-second intervals for 4 minutes.

Results: Figure 1 presents the temperature versus time data for the addition of hot water and cold water in the calorimeter, used to determine the calorimeter's heat capacity.

Calculations: The heat lost to the calorimeter during the warm water addition can be determined using the formula: qcalorimeter​=m⋅c⋅ΔT Where:

• m is the mass of water (50 g),
• c is the specific heat capacity of water (4.18 J/g°C),
• ΔT is the change in temperature.

Table 1 summarizes the initial temperature, final temperature, and change in temperature for the warm water addition.

Part 2: Heats of Neutralization

Procedure:

1. Measure and add 50 mL of NaOH to the calorimeter, recording the initial temperature.
2. Measure and add 50 mL of HCl (or DCA) to the calorimeter, recording temperature at 15-second intervals for 4 minutes.

Results: Figure 2 and Figure 3 display the temperature versus time data for the reactions between HCl and NaOH, and DCA and NaOH, respectively.

Calculations: The heat produced during the neutralization reactions (qreaction​) can be determined using the formula: qreaction​=m⋅c⋅ΔT Where:

• m is the mass of the solution,
• c is the specific heat capacity of the solution,
• ΔT is the change in temperature.

Table 1 provides the initial temperature, final temperature, and change in temperature for each reaction.

Overall Heat Calculation: The total heat evolved during the reaction (qtotal​) is the sum of the heat lost to the calorimeter (qcalorimeter​) and the heat produced during the reaction (qreaction​).

qtotal​=qcalorimeter​+qreaction​

Molar Heat of Neutralization: The molar heat of neutralization (ΔHneutralization​) is calculated by dividing the total heat evolved by the number of moles of the limiting reactant.

ΔHneutralization​=moles of limiting reactantqtotal​​

Note: The moles of the limiting reactant can be determined using the reaction stoichiometry.

Comparisons between the molar heat of neutralization for HCl and DCA can be made, allowing conclusions about the heat of ionization for strong and weak acids when neutralized with a strong base. Additionally, analyzing the temperature versus time graphs provides insights into reaction kinetics and temperature changes during the reactions.

This laboratory investigation successfully determined the heat of ionization for hydrochloric acid and dichloroacetic acid during neutralization with sodium hydroxide. The methodology involving a calorimeter proved effective in measuring temperature changes and calculating heat values for each reaction.

By examining the molar heat of neutralization, it is possible to draw conclusions about the relative strength of the acids in terms of ionization heat when reacting with a strong base. Further experiments and analyses could explore variations in concentrations, temperatures, and other factors influencing heat of ionization.

In summary, this laboratory provides valuable insights into the thermodynamics of acid-base reactions and serves as a foundation for understanding the heat of ionization in different acid types.