Energy Changes

Energy Changes

1. Endothermic reactions

   • A reaction where energy is transferred in from the surroundings
   • Used for reactions that need to reduce temperature such as instant cold packs and Thermal decompositon reactions
   • Bond Breaking

2. Exothermic reactions

   • A reaction where energy from reaction is transferred to the surrondings
   • Used for: Combustion, oxidation, neutralisation reactions and hand warmers
   • Bond Making

3. Reaction Profiles

   • Can show relative difference in energy reactants and products on reaction profiles
   • Exothermic
     • Energy released when new bonds are formed is greater thean energy required when bonds are broken
   • Endothermic
     • Activation energy needed to start reaction
     • Energy released when new bonds are formed is less than energy needed to when bonds are broken

4. Bond Energy calculations

   • Energy must be supplied to create or break bonds
   • Can calculate overall energy change through bond energies

5. Chemical Cells and batteries

   • Metals tend to lose electrons to form postive ions
   • When two metals are dipped into a salt solution and joined by a wire the more reactive metal will donate electrons to the less reactive metal
   • The Greater the difference in reactivity between metals the more voltage produced
   • Non renewable batteries stop working when one of the reactants is used up while Rechargeable batteries use reverse reactions to regenerate the product to its original form

6. Fuel cells

   • Most of world relies on fossil fuels
   • Hydrogen can be alternate (can be burnt in combustion engines or used to power verichles
   • Hydrogen gas can be oxidised to provide a source of electrons to the hydrogen fuel cell - waste product is water
   • Hydrogen is highly flammable, sometimes produced by unrenewable means and is difficult to store