ACTIVATION ENERGY, CATALYSTS, REACTION PROFILES

Doc Brown's Chemistry KS4 GCSE, IGCSE, O level & A level Revision Notes

PART B Exothermic and Endothermic Energy Changes - Chemical Energetics and Reaction Profiles and Catalysts

What is the activation energy of a chemical reaction? What is a chemical reaction profile? What is an energy profile for a chemical reaction? On this page activation energies, reaction profiles for exothermic reactions and endothermic reactions, catalysts are discussed. Reaction profiles for uncatalysed reactions and catalysed reactions are compared and explained. These revision notes on reaction profiles of chemical reactions, activation energies and effects of a catalyst should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (91, 9-5 & 5-1) science courses.


Sub-index for ENERGY CHANGES: 1. Heat changes in chemical/physical changes - exothermic and endothermic

2. Reversible reactions and energy changes  *  3. Activation energy and reaction profiles (this page)

4. Catalysts and activation energy  *  5. Introduction to bond energy/enthalpy calculations

6. Calorimeter methods of determining energy changes and examples of experiments

7. Energy transfer calculations from calorimeter results

See also Advanced Level Energetics: Thermochemistry Enthalpies of Reaction, Formation & Combustion

and Advanced Level Chemistry Kinetics - introduction to advanced theory of rates of reactions

3. Activation Energy and Reaction Profiles

3a. The significance of activation energy

  • When gases or liquids are heated the particles gain kinetic energy and move faster increasing the chance of collision between reactant molecules and therefore the increased chance of a fruitful collision (i.e. one resulting in product formation).
  • However! this is NOT the main reason for the increased reaction speed on increasing the temperature of reactant molecules because most molecular collisions do not result in chemical change.
  • Before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the activation energy (denoted by Ea).
    • Its a sort of 'threshold' energy required before any bonds can be broken i.e. before a reaction can take place.
  • Do not confuse activation energy with the overall energy change also shown in the energy profile diagrams below, that is the overall energy absorbed-taken in by the system (endothermic) or given out to the surroundings (exothermic). 
  • It does not matter whether the reaction is an exothermic or an endothermic energy change (see the pair of reaction profile diagrams below).
  • Higher temperature molecules in gases and liquids have a greater average kinetic energy and so a greater proportion of them will then have the required activation energy to react on collision.
  • The increased chance of higher energy collisions greatly increases the speed of the reaction because it greatly increases the chance of a fruitful collision forming the reaction products by bonds being broken in the reactants and new bonds formed in the reaction products.
  • The activation energy 'hump' can be related to the process of bond breaking and making (see section 5.).
    • Up the hump is endothermic, representing breaking bonds (energy absorbed, needed to pull atoms apart),
    • down the other side of the hump is exothermic, representing bond formation (energy released, as atoms become electronically more stable).
    • The 'reaction profile' diagrams below illustrate the course or progress of a reaction in terms of the energy changes taking place.
    • Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
    • Examples of an exothermic reaction profile and an endothermic reaction profile are shown below, with and without the activation energy - which must be reached on collision, before a reaction can occur.
    • Do NOT confuse the activation energy with overall energy change of the reaction (exothermic or endothermic) shown in the 2nd and 3rd set of diagrams below.

1. Simple energy level diagrams for exothermic and endothermic  reactions NOT showing the activation energy

(c) doc b (c) doc b

ENERGY PROFILES for chemical reactions

Left: Exothermic energy change profile, products have less energy than the reactants, heat released to the surroundings.

Left: Exothermic energy change profile, products have less energy than the reactants, heat released to the surroundings.

The bigger the difference in energy levels the greater the energy released or absorbed.


2. Energy level diagrams for exothermic and endothermic reactions showing the activation energy (Ea) 'hump' which must be overcome before a chemical reaction can take place

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ENERGY PROFILES for chemical reactions

Every reaction has an activation energy 'barrier' (the black line 'humps' of height Ea) that must be overcome before a particle collision can lead to a chemical change. The bigger the 'hump' the bigger the energy needed.

For advanced students only: How to derive activation energies from reaction rate data and the Arrhenius equation is explained in section 5. of the Advanced Level Notes on Kinetics section 5.

See also Rates of reaction - Effect of changing temperature


3. Note that the effect of a catalyst is to lower the activation energy Ea, enabling the reaction to go faster BUT it does NOT affect the overall energy change of the reaction.

Factors affecting the rates of Reaction - particle collision theory model (c) Doc Brown Factors affecting the rates of Reaction - particle collision theory model (c) Doc Brown

 A catalyst lowers the activation energy (the green humps of height Ea)

The catalyst provides a different pathway for the reaction that needs less energy to initiate the reaction it but it does NOT change the energy transfer value irrespective of whether it is an exothermic or an endothermic reaction. In other words the energy levels of the reactants and products do not change.

See Rates of reaction - Effect of using a catalyst for a full explanation

 

3b. Further examples of reaction progress profiles

Reaction profile diagram

Relative comments on these ENERGY PROFILES for chemical reactions
Very endothermic reaction with a big activation energy.
Very exothermic reaction with a small activation energy.
Moderately endothermic reaction with a moderately high activation energy.
Moderately exothermic reaction with a moderately high activation energy.
A small activation energy reaction with no net energy change. This is theoretically possible if the total energy absorbed by the reactants in bond breaking equals the energy released by bonds forming in the products (see section 5.).
Energy level diagram for an exothermic chemical reaction without showing the activation energy.

It could also be seen as quite exothermic with a highly unlikely zero activation energy, but reactions between two ions of opposite charge usually has a very low activation energy.

Energy level diagram for an endothermic chemical reaction without showing the activation energy.

It could also be seen as quite endothermic with a zero activation energy (highly unlikely, probably impossible?).


4. Catalysts and Activation Energy

  • Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules.
  • This effectively means the activation energy is reduced (see diagram 'humps' below).
  • Therefore at the same temperature, more reactant molecules have enough kinetic energy to react compared to the uncatalysed situation and so the reaction speeds up with the greater chance of a 'fruitful' collision.
    • Note that a catalyst does NOT change the energy of the molecules, it reduces the threshold kinetic energy needed for a molecules to react.
    • The overall energy change for a catalysed reaction is identical to the energy change for the same uncatalysed reaction.
  • Although a true catalyst does take part in the reaction, it does not get used up and can be reused with more reactants, it may change chemically on a temporary basis but would be reformed as the reaction products also form.
  • However a solid catalyst might change physically permanently by becoming more finely divided, especially if the reaction is exothermic.
  • Also note from the diagram that although the activation energy is reduced, the overall exothermic or endothermic energy change is the same for both the catalysed or uncatalysed reaction. The catalyst might help break the bonds BUT it cannot change the actual bond energies.

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ENERGY PROFILES for chemical reactions

Combined reaction profiles for an uncatalysed and catalysed reaction.


Sub-index for ENERGY CHANGES: 1. Heat changes in chemical/physical changes - exothermic and endothermic  *  2. Reversible reactions and energy changes  *  3. Activation energy and reaction profiles  *  4. Catalysts and activation energy  *  5. Introduction to bond energy/enthalpy calculations  *  6. Calorimeter methods of determining energy changes  *  7. Energy transfer calculations from calorimeter results


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