Reaction Pathway Diagrams

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Reaction Pathway Diagrams

A reaction pathway diagram is a graph that shows energy changes during a chemical reaction, helping to identify whether energy is released or energy is absorbed.

What are reaction pathway diagrams?

A reaction pathway diagram is a graph that shows how energy changes during a chemical reaction.
The diagram has:
- Y-axis: Energy level
- X-axis: Progress of reaction
They compare the energy of reactants and products.
It helps to identify whether a reaction is exothermic or endothermic.

Exothermic reactions

Exo- means out, so an exothermic reaction releases energy into the surroundings (usually as heat/light).
- This means products have less energy than reactants.
The diagram shows:
- A decrease in energy from the start to the end of the reaction
- A downward arrow labelled “Energy released”
Examples:
- Burning fuels (combustion), neutralisation, respiration

Endothermic reactions

Endo- means in, so an endothermic reaction absorbs energy from the surroundings.
- This means products have more energy than the reactants.
The diagram shows:
- An increase in energy from the start to the end of the reaction
- An upward arrow labelled “Energy absorbed”
Examples:
- Melting ice, photosynthesis

Key parts of the diagram

Reactants: The starting substances in a reaction.
- Their energy is shown at the beginning of the curve.
Products: The substances formed at the end of the reaction.
- Their energy is shown at the end of the curve.
Activation Energy: The 'hump' or 'peak' of the curve.
Enthalpy Change (ΔH): The overall energy change in the reaction
Reaction pathway diagrams help us visualise energy changes.

Reaction Pathway Diagrams 2

Reaction pathway diagrams show activation energy, enthalpy change, and the balance between bond breaking and bond making to determine whether a reaction is exothermic or endothermic.

Activation energy (E_a)

Activation energy (E_a) is the minimum energy that reacting particles must have for a reaction to occur.
- This is shown as the 'hump' or 'peak' on the reaction pathway diagram.
This energy is required to break the initial bonds in the reactants before new bonds can form.
A high activation energy means a reaction is slower, as more energy is needed to start it.

Enthalpy change (ΔH)

ΔH is the difference in energy between the products and the reactants.
In an exothermic reaction:
- The products have a lower energy level than the reactants.
- The diagram shows a decrease in energy.
- The enthalpy change (ΔH) is negative.
In an endothermic reaction:
- The products have a higher energy level than the reactants.
- The diagram shows an increase in energy.
- The enthalpy change (ΔH) is positive.

Bond breaking & making

Breaking bonds in the reactants requires energy → endothermic process.
Making bonds in the products releases energy → exothermic process.

Bond breaking & making 2

The overall enthalpy change (ΔH) is calculated using bond energies:
- ΔH = (Energy absorbed to break bonds) - (Energy released to make bonds)
If more energy is released than absorbed → exothermic (ΔH is negative).
If more energy is absorbed than released → endothermic (ΔH is positive).