Enthalpy and Enthalpy Change

Introduction to Enthalpy

What is Enthalpy?

Enthalpy (H): represents the total energy content within a system. It is essential in identifying energy shifts during chemical reactions.

• State Function: Dependent only on the initial and final states of a system, and independent of the path taken.

Standard Enthalpy Change (ΔH)

• Standard Enthalpy Change (ΔH): Refers to the energy change under defined standard conditions:
  • Pressure: 1 atm
  • Temperature: 298 K

Types of Enthalpy Changes

Enthalpy of Reaction (ΔH₍ᵣₓₙ₎)

• Enthalpy of Reaction (ΔH₍ᵣₓₙ₎): Specifies the energy change occurring during a reaction.
• Example: The release of energy from wood combustion.

Enthalpy of Formation (ΔH₍f₎)

• Enthalpy of Formation (ΔH₍f₎): Energy required to form one mole of a compound from its constituent elements in their elemental states.

Enthalpy of Combustion (ΔH₍c₎)

• Enthalpy of Combustion (ΔH₍c₎): Energy released upon the combustion of one mole of a substance.
• Example:
  • Combustion of methane:
  • $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$

Visualising Exothermic and Endothermic Processes

• Exothermic Processes: Release heat with a negative ΔH, such as a campfire.
• Endothermic Processes: Absorb heat with a positive ΔH, such as ice melting.

This diagram illustrates the energy flow and heat exchange in different processes.

Connection between Enthalpy and Entropy

• Entropy: Represents the degree of disorder in a system.
• Both are crucial for predicting the spontaneity of reactions using Gibbs Free Energy.

Depicts factors influencing reaction spontaneity.

Introduction to Calorimetry

• Calorimetry: A method used to measure the heat exchanged during chemical reactions or physical transformations.

Types of Calorimeters

1. Simple Calorimeters

• Structure: An insulated container equipped with a thermometer.
• Usage: Ideal for straightforward educational experiments.
• Disadvantages: Substantial heat loss reduces accuracy.

2. Bomb Calorimeters

• Structure: Insulated, pressure-resistant apparatus.
• Usage: Provides accurate measurements in combustion energy analyses.
• Advantages: Enhanced precision due to reduced heat loss.

Using the Formula q = mCΔT

Importance of the Formula

The q = mCΔT formula is essential for determining heat changes, akin to calculating the energy required to boil water.

Explanation of the Formula

• q: Heat energy change, measured in joules (J).
• m: Mass of the substance, in grams (g) or kilograms (kg).
• C: Specific heat capacity, in J/g°C.
• ΔT: Temperature change.

Sample Calculations

Example 1: Basic

• Calculate the heat absorbed by 100 g of water, from 25°C to 75°C.

• Given information:

  • Mass (m) = 100 g
  • Specific heat capacity of water (C) = 4.18 J/g°C
  • Temperature change (ΔT) = 75°C - 25°C = 50°C

• Solution:

  • Using the formula q = mCΔT
  • q = 100 g × 4.18 J/g°C × 50°C
  • q = 20,900 J or 20.9 kJ

Analysing and Comparing Data

Comparing Experimental Data to Values

• Reliable Sources: Ensure accuracy by referencing academic journals, databases, and textbooks.
• Data Comparison Table: Provides a visual comparison between experimental and literature values.

Identifying and Discussing Discrepancies

• Common Discrepancies: Heat loss can affect results, and calibration errors may impact accuracy.

Case Studies and Examples

Exothermic Reaction Case Study

Combustion of Methane

• Formula: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Data Analysis:

• Evaluate heat loss and the completeness of combustion with examples of discrepancies.

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Endothermic Reaction Case Study

Dissolution of Ammonium Nitrate

• Formula: $NH_4NO_3 (s) + H_2O \rightarrow NH_4^+ + NO_3^-$

Ammonium nitrate is utilised in cold packs.

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These notes provide essential insights into understanding enthalpy changes, applications of calorimetry, and data analysis for conducting refined experiments and enhancing concept retention. Accurate interpretation and application facilitate problem-solving in real-world contexts, improving energy management, industrial processes, and academic performance.