Sciences - Co-ordinated (Double) (0654) | Chemical energetics

1. Energy Transfer: The Two Main Types of Reactions

Chemical reactions involve rearranging atoms, which requires breaking existing bonds and forming new ones. This process always involves the transfer of thermal energy (heat). We classify reactions based on the direction of this energy transfer:

1.1 Exothermic Reactions (Energy Out!)

An exothermic reaction is a reaction that transfers thermal energy to the surroundings.

• Temperature Change: The temperature of the surroundings increases. The reaction vessel feels warm or hot.
• Energy Store: Chemical energy is converted into thermal energy, which is released.
• Definition (Core 1): State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings.

🔥 Real-World Examples:

1. Burning fuel (combustion): Burning methane gas releases huge amounts of heat energy.
2. Respiration (how your cells get energy): Glucose reacting with oxygen releases energy to keep you warm and power your body.
3. Mixing quicklime with water (neutralisation).
4. Hand warmers: These contain chemicals (often iron powder) that react slowly with oxygen, releasing heat to warm your hands.

1.2 Endothermic Reactions (Energy In!)

An endothermic reaction is a reaction that takes in thermal energy from the surroundings.

• Temperature Change: The temperature of the surroundings decreases. The reaction vessel feels cold.
• Energy Store: Thermal energy from the surroundings is converted into chemical energy (stored in the products).
• Definition (Core 2): State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings.

❄️ Real-World Examples:

1. Photosynthesis: Plants absorb light energy (thermal energy from the Sun) to make glucose.
2. Thermal decomposition: Heating limestone strongly to break it down.
3. Instant cold packs: These contain chemicals (like ammonium nitrate) that dissolve in water, absorbing heat from the immediate environment (making it feel cold).

2. Activation Energy (\(E_a\)) and the Energy Profile

Even exothermic reactions, which release energy overall, need a little boost to get started. Think of it like pushing a boulder downhill—you need an initial push up a small hill before it can roll down and release all its potential energy.

2.1 Defining Activation Energy (\(E_a\))

Activation Energy (\(E_a\)) is defined as the minimum energy that colliding particles must possess in order to react (Supplement 5).

• Particles must collide with the correct orientation AND with energy greater than or equal to $E_a$ for a reaction to occur.
• Without enough $E_a$, particles just bounce apart without reacting.

2.2 Reaction Pathway Diagrams (Energy Profiles)

These diagrams visually show how energy changes throughout a reaction, identifying the reactants, products, activation energy, and overall energy change.

A. Exothermic Reaction Pathway (Supplement 6)

In an exothermic reaction, the products have less energy than the reactants, so energy is released overall.

• The energy level drops from reactants to products.
• The arrow representing the overall energy change ($\Delta H$) points down.

B. Endothermic Reaction Pathway (Supplement 6)

In an endothermic reaction, the products have more energy than the reactants, so energy must be absorbed overall.

• The energy level rises from reactants to products.
• The initial hill ($E_a$) is usually much larger than the overall energy absorbed.

3. Enthalpy Change ($\Delta H$)

When measuring the energy transferred during a reaction at constant pressure, we use the term enthalpy change, symbolised by \(\Delta H\) (pronounced "delta aitch").

3.1 Defining $\Delta H$ (Supplement 4)

The transfer of thermal energy during a reaction is called the enthalpy change, $\Delta H$, of the reaction.

• The sign of \(\Delta H\) tells us whether the reaction is exo or endo:
  • For exothermic reactions, $\Delta H$ is negative. (Energy is lost from the chemicals).
  • For endothermic reactions, $\Delta H$ is positive. (Energy is gained by the chemicals).

4. The Source of Energy Change: Bond Breaking and Making

The overall thermal change (whether the reaction is exo or endo) is simply the difference between the energy needed to break old bonds and the energy released when forming new bonds.

4.1 Energy and Chemical Bonds (Supplement 7)

Every bond holds energy. To start a reaction, you must first pull the molecules apart (break the bonds).

1. Bond Breaking is an endothermic process. (It requires energy input).
2. Bond Making is an exothermic process. (It releases energy).

4.2 Determining the Overall Reaction Type

The reaction is the sum of these two opposing processes:

If the reaction is Exothermic:

The energy released during bond making is greater than the energy required for bond breaking.
(Overall, more energy comes out than was put in.)

If the reaction is Endothermic:

The energy required for bond breaking is greater than the energy released during bond making.
(Overall, more energy is consumed than released.)