AQA A-Level Chemistry - Energetics Pt. 1 (Enthalpy & Bond Energies)

Summary

This video provides a detailed introduction to the energetics topic from the AQA AS Chemistry specification. The focus is on explaining enthalpy, including definitions, standard conditions, and key concepts like exothermic and endothermic reactions. The video further explores the concept of bond dissociation enthalpies to calculate enthalpy changes, emphasizing the differences in energy required to break and form bonds. The lecturer uses practical examples and energy level diagrams to clarify how energy changes dictate whether a reaction is exothermic or endothermic, offering a clear, methodical approach to understanding these fundamental concepts.

Takeaways

• 😀 Enthalpy change refers to the heat or energy change under constant pressure, and is symbolized by ΔH.
• 😀 A standard enthalpy change occurs under standard conditions: 298 K (25°C) and 100 kPa pressure.
• 😀 Exothermic reactions release heat, and their enthalpy change (ΔH) is negative, with reactants at a higher energy than products.
• 😀 Endothermic reactions absorb heat, and their enthalpy change (ΔH) is positive, with reactants at a lower energy than products.
• 😀 Activation energy (EA) is the energy required to initiate a reaction, often shown as a peak in energy level diagrams.
• 😀 Enthalpy change can be calculated using bond dissociation energies (the energy required to break bonds).
• 😀 In exothermic reactions, the energy released from bond formation is greater than the energy required to break bonds.
• 😀 For an exothermic reaction, energy level diagrams show reactants at a higher energy than products, and ΔH is negative.
• 😀 For an endothermic reaction, energy level diagrams show reactants at a lower energy than products, and ΔH is positive.
• 😀 The bond dissociation energy values can be used to calculate the overall enthalpy change for reactions like hydrogen and oxygen forming water.

Q & A

What is the definition of enthalpy change?

Enthalpy change (ΔH) is the heat or energy change that occurs under constant pressure during a chemical reaction.

What is meant by the term 'standard enthalpy change'?

The standard enthalpy change refers to the enthalpy change that occurs under standard conditions, which are 298 K (25°C) temperature and 100 kPa pressure.

What is the unit of enthalpy change?

The unit of enthalpy change is kilojoules per mole (kJ/mol).

What is the difference between exothermic and endothermic reactions?

In an exothermic reaction, energy is released (ΔH is negative), while in an endothermic reaction, energy is absorbed (ΔH is positive).

How can you remember the difference between exothermic and endothermic reactions?

Exothermic reactions release energy (heat is given out), and endothermic reactions absorb energy (heat is taken in).

What is the role of activation energy in a chemical reaction?

Activation energy is the minimum energy required to initiate a chemical reaction. It is often provided by a spark or heat to overcome the energy barrier for the reaction.

How does bond dissociation energy relate to enthalpy change?

Bond dissociation energy is the energy required to break bonds in the reactants. The enthalpy change of a reaction can be calculated by subtracting the energy required to break bonds (positive energy) from the energy released when new bonds are formed (negative energy).

What happens to the enthalpy change in an exothermic reaction?

In an exothermic reaction, the enthalpy change (ΔH) is negative because more energy is released from bond formation than is required to break the bonds in the reactants.

Why do we use bond dissociation energies to calculate enthalpy change?

Bond dissociation energies are used to calculate enthalpy change because they provide the energy needed to break bonds in reactants and the energy released when bonds are formed in products, allowing us to determine whether a reaction is exothermic or endothermic.

What is an example of an exothermic reaction, and how can its enthalpy change be calculated?

An example of an exothermic reaction is the combustion of hydrogen to form water. The enthalpy change can be calculated using bond dissociation energies by breaking bonds in the reactants (H2 and O2) and then calculating the energy released when bonds are formed in the products (H2O).