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GCSE Chemistry Notes: Reaction profiles & energy changes in chemical reactions
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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
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Index of GCSE level energetics notes This is a BIG website, you need to take time to explore it [SEARCH BOX] Use your mobile phone or ipad etc. in 'landscape' mode 3. Activation Energy and Reaction Profiles 3a. What is the activation energy of a chemical reaction and what is its significance?
3b. Activation energy and reaction profiles
3c. SUMMARY of reaction profile diagrams (i) Simple energy level diagrams for exothermic and endothermic reactions NOT showing the activation energy
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.
(ii) 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
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
4. The effect of catalysts on the 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.
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.
See also Rates of reaction - Effect of using a catalyst for a full explanation 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 See also Advanced A Level Energetics–Thermochemistry – Enthalpies of Reaction, Formation & Combustion and enthalpy calculations from calorimetry data for Advanced A Level chemistry students Advanced A Level Energetics INDEX of revision notes on thermochemistry, enthalpy, entropy etc.
LINKS to associated webpages
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 (9–1, 9-5 & 5-1) science courses |
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