Heat of Reaction Enthalpy change, calorimetry, Hess’s law applications

Heat of Reaction, Calorimetry, and Hess's Law

Welcome, SS2 Chemistry students! Today, we'll explore how energy changes during chemical reactions. Understanding these energy changes is crucial in many areas, from designing new fuels to understanding biological processes. We'll learn about enthalpy changes, how to measure them, and how to calculate them indirectly.

Core Concepts

1. Heat of Reaction (Enthalpy Change, ΔH)

Every chemical reaction involves either the absorption or release of energy, usually in the form of heat. This energy change, measured at constant pressure, is called the enthalpy change (ΔH) or heat of reaction. It represents the difference between the total energy of products and reactants. The unit for ΔH is typically kilojoules per mole (kJ/mol).

• Exothermic Reactions: Release heat energy into the surroundings. The products have lower energy than the reactants. ΔH is negative. Examples include combustion (burning) and neutralization reactions.
  • Example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ; ΔH = -890 kJ/mol (heat released)
• Endothermic Reactions: Absorb heat energy from the surroundings. The products have higher energy than the reactants. ΔH is positive. Examples include photosynthesis and the dissolving of certain salts like ammonium nitrate.
  • Example: N₂(g) + O₂(g) → 2NO(g) ; ΔH = +180 kJ/mol (heat absorbed)

This diagram shows how energy changes during exothermic (energy released, ΔH < 0) and endothermic (energy absorbed, ΔH > 0) reactions.

2. Calorimetry: Measuring Heat Changes

Calorimetry is the experimental technique used to measure the heat absorbed or released during a chemical or physical process. A device called a calorimeter is used for this purpose. A simple calorimeter can be a well-insulated container (like a coffee cup) holding a known amount of water.

The basic principle relies on the idea that the heat lost by the reaction equals the heat gained by the calorimeter and its contents (usually water), or vice versa. The formula used to calculate heat energy (q) is:

q = mcΔT

Where:

• q = heat energy (in Joules, J or kilojoules, kJ)
• m = mass of the substance (usually water) in the calorimeter (in grams, g)
• c = specific heat capacity of the substance (for water, c = 4.18 J/g°C or 4.18 J/gK)
• ΔT = change in temperature (final temperature - initial temperature, in °C or K)

By measuring the temperature change of a known mass of water, we can calculate the heat absorbed or released by the reaction, which then helps us determine ΔH.

A simple coffee cup calorimeter helps measure heat changes in solution by insulating the reaction from the surroundings.

3. Hess's Law of Constant Heat Summation

Sometimes, it's difficult or impossible to measure the heat of reaction directly for a particular process. Hess's Law provides a way to calculate enthalpy changes indirectly.

Hess's Law states that if a reaction can be expressed as the algebraic sum of two or more other reactions, then the enthalpy change for the overall reaction is the algebraic sum of the enthalpy changes of these individual reactions.

This means that the total enthalpy change for a reaction is independent of the pathway taken, as long as the initial and final conditions are the same. It's like taking different routes to climb a mountain – the change in altitude is the same regardless of the path you choose.

How to apply Hess's Law:

1. Identify the target reaction whose ΔH you want to find.
2. Manipulate (reverse, multiply) known reactions with known ΔH values so that when they are added together, they yield the target reaction.
3. Remember these rules when manipulating reactions:
   • If you reverse a reaction, change the sign of its ΔH.
   • If you multiply a reaction by a specific factor, multiply its ΔH by the same factor.
4. Add the manipulated ΔH values to get the ΔH for the target reaction.

This diagram illustrates Hess's Law: the enthalpy change from A to C is the same whether you go directly or via intermediate B.

Real-World Example

The burning of cooking gas (LPG, mainly propane and butane) in our homes is a highly exothermic reaction. The heat released is what we use to cook our food. Scientists use calorimetry to determine the exact heat released by different fuels, helping us choose efficient energy sources. Hess's law can be used to calculate the heat of formation of complex organic compounds, which might be difficult or dangerous to measure directly.

Practice/Reflection Question

Consider an instant cold pack, which gets cold when activated. Is the reaction inside an exothermic or endothermic process? How could you use the principle of calorimetry (even a simple coffee cup calorimeter) to prove this?