5.2Chemical Energetics
Activation energy and reaction pathway diagrams — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
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Lesson summary
5.2 Activation energy and reaction pathway diagrams — Notes — IGCSE Chemistry notes: Draw and interpret reaction pathway diagrams, and explain activation energy, enthalpy change and the effect of a catalyst.A reaction pathway diagram compares the energy of the reactants with the …
- Every substance stores chemical energy in its bonds
- The overall energy change is the height difference between reactants and products
- Labels to include: reactants, products, ΔH and Ea
- Activation energy Ea: the minimum energy that colliding particles must have for a reaction to take place
- It appears as the hump on the diagram
- Even exothermic reactions need an initial energy input, such as a spark to light methane
Section 1 of 9What a pathway diagram shows
1
A reaction pathway diagram compares the energy of the reactants with the energy of the products, with energy on the vertical axis and progress of reaction on the horizontal axis.
2
Every substance stores chemical energy in its bonds.
3
The overall energy change is the height difference between reactants and products.
4
Labels to include: reactants, products, ΔH and Ea.
Section 2 of 9Exothermic and endothermic diagrams
| Feature | Exothermic | Endothermic |
|---|---|---|
| Products compared with reactants | Lower energy | Higher energy |
| Sign of ΔH | Negative | Positive |
| ΔH arrow | Points down | Points up |
| Temperature of surroundings | Rises | Falls |
| Examples | Combustion, neutralisation, rusting, respiration | Thermal decomposition of CaCO3, photosynthesis, dissolving ammonium nitrate |
Section 3 of 9Activation energy
1
Activation energy Ea: the minimum energy that colliding particles must have for a reaction to take place.
2
It appears as the hump on the diagram.
3
There is a hump because bonds in the reactants must start to break before new bonds form, and breaking bonds needs energy.
4
Even exothermic reactions need an initial energy input, such as a spark to light methane.
5
A reaction with a higher activation energy needs more energy to start.
6
Ea = energy at the peak − energy of the reactants.
Section 4 of 9Enthalpy change, ΔH
1
Enthalpy change: the heat energy transferred during a reaction, measured in kJ/mol.
2
From a diagram, ΔH = energy of products − energy of reactants.
3
A negative ΔH means exothermic, for example ΔH = −890 kJ/mol for methane combustion.
4
A positive ΔH means endothermic, for example ΔH = +178 kJ/mol for thermal decomposition of calcium carbonate.
5
For the reverse reaction, Ea = energy at the peak − energy of the products.
Section 5 of 9Drawing the diagram
1
Label the vertical axis energy and the horizontal axis progress of reaction.
2
Draw a horizontal line on the left for the reactants and label it with their formulae.
3
Draw a curve rising to a peak and then falling to a horizontal line for the products.
4
Draw an arrow from the reactant line up to the peak and label it Ea.
5
Draw an arrow from the reactant line to the product line and label it ΔH.
Section 6 of 9Effect of a catalyst
1
A catalyst provides an alternative route with a lower activation energy.
2
On the diagram draw a second, lower hump between the same reactant and product lines and label it Ea with catalyst.
3
The energies of the reactants and products stay the same, so ΔH is unchanged.
4
More colliding particles have energy equal to or above Ea, so there are more successful collisions per second.
Section 7 of 9More from the specification
More from the specification
Enthalpy exchange with reaction system
- Enthalpy exchange with reaction system exothermic processes release thermal energy from the reacting system to surroundings, decreasing internal system enthalpy.
Exothermic reactions
- Exothermic reactions energy is released to the surroundings, so products are lower in energy and the enthalpy change is negative
Endothermic reactions
- Endothermic reactions energy is absorbed, so the temperature falls and the enthalpy change is positive
Bond energy
- Bond energy energy is taken in to break bonds and released on forming bonds, so the enthalpy change equals bonds broken minus bonds formed
Activation energy
- Activation energy the minimum energy needed to start a reaction, shown as the peak on a reaction pathway diagram;
- calculations use the amount of hydrogen and other molecules formed
Reaction profile
- A diagram showing the relative energies of reactants and products
Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Reaction pathway diagram | a diagram of the energy of the chemicals against the progress of the reaction. |
| Activation energy | the minimum energy colliding particles must have to react. |
| Enthalpy change | the heat energy transferred during a reaction, in kJ/mol. |
| Catalyst | a substance that lowers the activation energy and is not used up. |
| Reactants | the substances that react together at the start of a reaction. |
Section 9 of 9Exam technique
1
Saying a catalyst changes ΔHit only lowers the activation energy.
2
Drawing the ΔH arrow to the peak: ΔH is from the reactant level to the product level.
3
Starting the Ea arrow at the product level: it starts at the reactants.
4
Measuring activation energy from the product level: it is measured from the reactants to the peak.
5
Forgetting the sign of ΔHit is negative for exothermic and positive for endothermic reactions.
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