Calorimetry and Enthalpy

Definition and Principles

Calorimetry is a laboratory technique used to measure the heat of chemical reactions, particularly under constant pressure conditions. The heat measured in such experiments is equivalent to the enthalpy change (ΔH) of the reaction. Most laboratory reactions occur at atmospheric pressure, making calorimetry a direct method for determining enthalpy changes.

• Enthalpy (ΔH): The heat content of a system at constant pressure; for reactions, it is the heat absorbed or released.

• Calorimeter: An apparatus consisting of an insulated container (often a Styrofoam cup) and a thermometer or temperature probe to minimize heat loss and accurately measure temperature changes.

• Heat of Reaction (q): The energy transferred as heat during a chemical reaction.

Example: Dissolving sodium hydroxide (NaOH) in water and measuring the temperature change to determine the enthalpy of dissolution.

Hess’s Law of Heat Summation

Concept and Application

Hess’s Law states that the total enthalpy change for a reaction is the same whether it occurs in one step or multiple steps. This principle allows chemists to calculate enthalpy changes for reactions that are difficult to measure directly by combining enthalpy changes from related reactions.

• Additive Property: Enthalpy is a state function; its change depends only on initial and final states, not the path taken.

• Mathematical Expression:

• Example: If Reaction 1 and Reaction 2 have known enthalpy changes, their sum gives the enthalpy change for Reaction 3, which is the net reaction.

Experimental Design: Measuring Enthalpy Changes

Reactions Studied

Two sets of reactions are performed to measure enthalpy changes using calorimetry and to confirm Hess’s Law:

• Set 1: Sodium hydroxide and hydrochloric acid

• Set 2: Sodium hydroxide and acetic acid

The reactions are:

• 1. NaOH(s) → NaOH(aq)   ΔH1

• 2. NaOH(s) + HCl(aq) → NaCl(aq) + H2O(l)   ΔH2

• 3. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)   ΔH3

• 4. NaOH(s) → NaOH(aq)   ΔH4 (identical to Reaction 1)

• 5. NaOH(s) + CH3COOH(aq) → NaCH3COO(aq) + H2O(l)   ΔH5

• 6. NaOH(aq) + CH3COOH(aq) → NaCH3COO(aq) + H2O(l)   ΔH6

Measured values of ΔH1 and ΔH2 are used to calculate ΔH3 via Hess’s Law, and similarly for the acetic acid reactions.

Lab Techniques and Procedures

Calorimetry Setup and Steps

The experiment uses a Vernier temperature probe, a Styrofoam cup, and a laptop for data collection. The procedure involves:

1. Preparing the calorimeter (Styrofoam cup in a beaker, temperature probe rinsed and dried).

2. Measuring and adding reactants (water, NaOH, HCl, or CH3COOH) in specified volumes and masses.

3. Recording initial and final temperatures (Ti and Tf), calculating ΔT.

4. Mixing reactants and monitoring temperature changes until equilibrium is reached.

5. Disposing of reaction mixtures and cleaning equipment between trials.

Safety Note: Sodium hydroxide is hygroscopic and corrosive; handle with care and clean spills immediately.

Data Collection and Table

Recording Experimental Data

For each reaction, record:

• Mass of solid sodium hydroxide (grams)

• Volumes of deionized water, HCl, NaOH, or CH3COOH (mL)

• Initial and final temperatures (Ti, Tf)

• Change in temperature (ΔT = Tf - Ti)

Additional info: Table structure inferred for clarity; original table was incomplete.

Calculations: Determining Enthalpy Change (ΔH)

Heat Flow and Enthalpy

The enthalpy change for each reaction is calculated in kJ/mol of NaOH. The heat flow (q) is determined using the mass of water, its specific heat, and the temperature change:

• Specific heat of water: 4.18 J/g·°C

• Density of water: 1.00 g/mL (assumed for all solutions)

• Assumptions: The calorimeter's heat capacity is negligible; total solution volume is the sum of individual volumes.

Key Equations:

• Heat absorbed by water:

• Heat of reaction:

• Enthalpy change per mole:

Example Calculation: If 100 g water, ΔT = 5°C, then:

Evaluating Hess’s Law

Comparing Measured and Calculated Enthalpy Changes

After calculating ΔH for each reaction, compare the measured value for Reaction 3 (or 6) with the value predicted by Hess’s Law:

• For Reaction 3:

• For Reaction 6:

• Percent error calculation:

Example: If measured ΔH3 = -55 kJ/mol, calculated ΔH3 = -53 kJ/mol:

Summary Table: Reactions and Enthalpy Relationships

Additional info: Table structure inferred for clarity; original table was incomplete.

Key Takeaways

• Calorimetry is a practical method for measuring enthalpy changes in chemical reactions.

• Hess’s Law allows calculation of enthalpy changes for reactions that are difficult to measure directly.

• Careful experimental design and data analysis are essential for accurate determination of ΔH.

• Comparing measured and calculated values tests the validity of Hess’s Law.