Back to Library

(AI-Powered Suggestions)

Related Topics

A topic from the subject of Thermodynamics in Chemistry.

  • 1 year ago
  • 6 min read
Thermochemical Equations in Chemistry
Introduction

Thermochemical equations are chemical equations that include the heat of reaction (ΔH). This allows for the determination of the enthalpy change of the reaction, which measures the energy released or absorbed during the reaction.

Basic Concepts
  • Heat of reaction (ΔH): The amount of heat released or absorbed during a chemical reaction, typically measured in kilojoules per mole (kJ/mol).
  • Enthalpy change (ΔH): A thermodynamic property representing the change in heat content of a system during a reaction, measured in kJ/mol. A positive ΔH indicates an endothermic reaction, and a negative ΔH indicates an exothermic reaction.
  • Exothermic reaction: A reaction that releases heat to its surroundings; ΔH is negative.
  • Endothermic reaction: A reaction that absorbs heat from its surroundings; ΔH is positive.
Equipment and Techniques

Several methods measure the heat of reaction. Common methods include:

  • Calorimetry: Measures the temperature change of a system undergoing a reaction. The heat of reaction is calculated using the temperature change and the system's heat capacity.
  • Bomb calorimetry: Measures the heat of reaction for combustion reactions in a sealed bomb. The heat of reaction is determined from the bomb's temperature change.
  • Solution calorimetry: Measures the heat of reaction for reactions in solution. The heat of reaction is calculated from the solution's temperature change.
Types of Experiments

Various thermochemical experiments can be performed. Common types include:

  • Enthalpy of combustion: Measures the heat released when a substance is completely burned (usually in a bomb calorimeter).
  • Enthalpy of solution: Measures the heat change when a substance dissolves in a solvent.
  • Enthalpy of neutralization: Measures the heat released when a strong acid and a strong base react.
Data Analysis

Data from thermochemical experiments determines the reaction's enthalpy change (ΔH). This enthalpy change can be used to calculate other thermodynamic properties, such as entropy change (ΔS) and Gibbs free energy change (ΔG).

Applications

Thermochemical equations have various applications, including:

  • Predicting reaction spontaneity: The Gibbs free energy change (ΔG) predicts spontaneity. A negative ΔG indicates a spontaneous reaction.
  • Designing chemical reactors: The heat of reaction helps determine the optimal temperature and pressure for a reaction.
  • Developing new materials: The enthalpy change helps determine the stability of a material.
Conclusion

Thermochemical equations are essential for understanding the energetics of chemical reactions. They are crucial for predicting reaction spontaneity, designing chemical reactors, and developing new materials.

Thermochemical Equations

Definition: A thermochemical equation is a chemical equation that shows the enthalpy change (heat) associated with a reaction. It includes the balanced chemical equation and the enthalpy change (ΔH).

Key Points:
  • The enthalpy change (ΔH) is expressed in kilojoules per mole (kJ/mol).
  • A positive ΔH indicates an endothermic reaction (heat is absorbed). The system gains heat from the surroundings.
  • A negative ΔH indicates an exothermic reaction (heat is released). The system releases heat to the surroundings.
  • The enthalpy change is a state function, meaning it depends only on the initial and final states of the system, not on the pathway taken.
  • Thermochemical equations can be used to calculate the heat flow in reactions and predict the feasibility of reactions. A highly negative ΔH suggests a spontaneous reaction under standard conditions.
Main Concepts:
  • Hess's Law: Enthalpy changes for a reaction are additive, regardless of the pathway taken. This allows for the calculation of ΔH for a reaction that cannot be measured directly by using a series of known reactions.
  • Standard Enthalpy of Formation (ΔHf°): ΔHf° of a compound is the enthalpy change when 1 mole of the compound is formed from its constituent elements in their standard states (usually 25°C and 1 atm).
  • Bond Energy: The energy required to break or form a bond can be used to estimate enthalpy changes. The difference between the energy required to break bonds in reactants and the energy released when forming bonds in products provides an estimate of ΔH.
Example:

Combustion of Methane:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)     ΔH = -890 kJ/mol

This equation shows that the combustion of 1 mole of methane releases 890 kJ of heat. This is an exothermic reaction.

Standard Conditions:

Thermochemical equations are often written under standard conditions (298 K and 1 atm pressure). This is indicated by a superscript ° symbol on ΔH (e.g., ΔH°).

Experiment: Investigating Thermochemical Equations
Materials:
  • 100 mL of 0.1 M acetic acid (CH3COOH)
  • 100 mL of 0.1 M sodium hydroxide (NaOH)
  • Thermometer
  • Insulated cup (e.g., Styrofoam cup)
  • Stopwatch
  • 2 x 100mL beakers
  • Stirring rod
Procedure:
Step 1: Prepare the solutions.
  1. Measure 100 mL of 0.1 M acetic acid and pour it into a 100mL beaker.
  2. Measure 100 mL of 0.1 M sodium hydroxide and pour it into a separate 100mL beaker.
Step 2: Measure the initial temperatures.
  1. Insert the thermometer into the acetic acid solution and record the initial temperature (T1). Allow the thermometer to equilibrate for about 30 seconds before recording.
  2. Remove the thermometer, rinse it briefly with distilled water, and dry it gently.
  3. Insert the thermometer into the sodium hydroxide solution and record the initial temperature (T2). Allow the thermometer to equilibrate for about 30 seconds before recording.
Step 3: Mix the solutions.
  1. Pour the sodium hydroxide solution into the insulated cup.
  2. Pour the acetic acid solution into the cup.
  3. Immediately begin stirring the mixture gently with the stirring rod.
Step 4: Measure the temperature change.
  1. Start the stopwatch immediately after adding the acetic acid.
  2. Monitor the temperature and record the highest temperature reached (Tmax) and the time taken to reach this temperature (Δt).
Observations:
  • The temperature of the mixture will increase after the solutions are mixed.
  • Record the highest temperature reached (Tmax).
  • Note the time taken to reach Tmax (Δt).
  • Observe any other changes (e.g., color change, precipitate formation, etc.)
Calculations:
  1. Calculate the heat released (Q):
    Q = mcpΔT
    where:
    m is the total mass of the solution (approximately 200 g, assuming the density is close to that of water)
    cp is the specific heat capacity of water (4.18 J/g°C)
    ΔT is the temperature change (Tmax - average of T1 and T2)
  2. Calculate the enthalpy change (ΔH):
    ΔH = -Q/n
    where n is the number of moles of the limiting reactant (in this case, either CH3COOH or NaOH, depending on which one is used in a lesser amount). Calculate the number of moles using the volume and concentration of the respective solution.
  3. Calculate the molar enthalpy change (ΔHm):
    ΔHm = ΔH/n (This gives ΔH per mole of limiting reactant)
Significance:
This experiment demonstrates the exothermic nature of the neutralization reaction between acetic acid (a weak acid) and sodium hydroxide (a strong base). The heat released during the reaction is measured and used to calculate the enthalpy change (ΔH), which is a measure of the energy change in the reaction. The calculated molar enthalpy change (ΔHm) provides a more meaningful result, expressing the enthalpy change per mole of the limiting reactant. This experiment helps illustrate the concepts of thermochemistry and energy changes in chemical reactions. Note that due to heat loss to the surroundings, the calculated ΔH will be an approximation.

Share on: