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GCSE Chemistry Notes: Reaction profiles & energy changes in chemical reactions

ACTIVATION ENERGY, CATALYSTS and 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


Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level & ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry



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 on the diagrams).
    • Its a sort of 'threshold' energy required before any bonds can be broken
    • i.e. before a reaction can begin to take place - bonds in reactants are broken - new bonds are formed as the atoms reorganise themselves into the product.
    • A successful collision is called a 'fruitful collision' and is illustrated in the 'colliding particle' picture below.
    • This is known as the collision theory of how chemical reactions 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). 
    • So look carefully at the reaction profile diagrams below.
  • 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.
  • (c) doc b (c) doc b

  • To the energy level diagrams above, I've added the 'humps' that represent the activation energies needed.
  • As I've mentioned above, the activation energy 'hump' can be related to the process of bond breaking and making (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 difference between the two energy changes determines the overall energy change for the reaction.
      • NOT the activation energy hump!
  • Exothermic reaction in terms of bonds broken and made
  • (c) doc b (c) doc b (c) doc b(c) doc b (c) doc b (c) doc b (c) doc b (c) doc b(c) doc b 'picture' formulae

  •  OR using displayed formulae

  • e.g. in the combustion of methane you must:

    • (i) for the reactants break four C-H bonds in a methane molecule and break two O=O bonds in each oxygen molecules (endothermic),

    • and (ii) for the products form two C=O bonds for the carbon dioxide molecule and form two O-H bonds in each of the two water molecules,

    • (ii) is much greater

  • Endothermic reaction in terms of bonds broken and made
    • The 'reaction profile' diagrams below illustrate the course or progress of a reaction in terms of the energy changes taking place.
    • Reaction profiles are used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
    • Do NOT confuse:
      • The activation energy - the height of the hump from the reactants energy level,
      • with the overall energy change of the reaction - the difference between the horizontal reactants and products potential energy levels,
      • and it doesn't matter if the reaction is exothermic or endothermic.

SUMMARY of reaction profile diagrams

 

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 NOT showing the activation energy see below

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

Right: Endothermic energy change profile, products have more energy than the reactants, heat absorbed from 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

(c) doc b (c) doc b

ENERGY PROFILES for chemical reactions showing the ACTIVATION ENERGY

The red arrow and blue arrows represent the exothermic and endothermic energy changes.

The purple arrows represent the activation energy and measured on the left side of the 'hump'.

DO NOT CONFUSE THE TWO!

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 activation 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.

and see also Rates of reaction - Effect of changing temperature


3b. Further examples of reaction progress profiles for chemical reactions

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


4. Catalysts and Activation Energy

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 - see diagrams below.

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

Combined reaction progress profiles for an uncatalysed and catalysed reaction.

ENERGY PROFILES for chemical reactions showing the activation energy with and without a catalyst

The red arrow and blue arrows represent the exothermic and endothermic energy changes.

The purple arrows represent the activation energy for the uncatalysed reaction and measured on the left side of the 'hump'.

The green arrows represent the activation energy for the catalysed reaction and measured on the lower 'hump'.

 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.

 

  • 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.

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


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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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LINKS to associated webpages

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

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


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