Title EDX Topic 11 Energetics.pdf ✅

Topic ? Chemical Energetics 1
Topic ? 2 All Chemical reactions & changes of states of matter involve energy changes. Energy is either taken in or released. It can’t be in the form of light, sound but most probably energy change is in the form of heat. Accordingly, we classified chemical reactions into: Exothermic & Endothermic We can determine if a reaction is exothermic or endothermic by: 1. Calculating the enthalpy change of the reaction: 2. Practically by measuring the temp. change taking place during the reaction. energy needed in bond breaking - energy released in bond making final temperature - initial temperature Exothermic Endothermic Reactions that release thermal energy to the surrounding leading to an increase in the temperature of the surrounding. Reactions that take in thermal energy from the surrounding leading to a decrease in the temperature of the surrounding. Examples: Combustion (burning) of fuels Neutralisation Reaction of Alkali metals with water Respiration Examples: Thermal Decomposition Electrolysis Reaction of Ammonium Salt with water Photosynthesis Examples of Physical Changes: Condensation - Freezing Examples of Physical Changes: Melting - Evapouration - Boiling Energy Level Diagrams: Energy of the reactants is greater than the energy of the products Energy of the products is greater than energy of the reactants
3 In a chemical reaction, bonds between reactants are broken down, then new bonds are formed to make the products. Bond breaking needs energy, it’s an endothermic process Bond forming releases energy, it’s an exothermic process Calculating Enthalpy Changes of Chemical Reactions using Bond Energies energy needed in bond breaking - energy released in bond making Enthalpy Change Endothermic reactions: Energy needed for breaking bonds in reactants is greater than energy released in bond making in products. Exothermic reactions: Energy needed for breaking bonds in reactions is less than energy released in bond making in products. is positive for endothermic reactions & negative for exothermic ones. Activation Energy (Ea): The minimum energy needed by the colliding particles in order to react. Using a catalyst will lower the activation energy needed and speed up the reaction. Topic ?
4 Example 1 Bond Enthalpy (kJ/mol) H-H 436 Cl-Cl 242 H-Cl 431 Use the table of values of bond energies to calculate the energy change in this reaction Reactants Products Energy needed to break bonds Energy released when bonds are made Enthalpy Change Energy change is negative, so reaction is Exothermic energy needed in bond breaking - energy released in bond making Topic ?