exothermic & endothermic

• different chemicals store different amounts of energy in their bonds, so when chemical reactions occur

  • there's a TRANSFER of energy when REACTANTS turn into products.
  • This can be where reactants either ABSORB energy from the surroundings or RELEASE it.
• Energy can NOT be CREATED or DESTROYED

  • so the TOTAL amount of ENERGY in the UNIVERSE always stays the same BEFORE & AFTER a chemical reaction.
• EXOTHERMIC REACTIONS

  reactions that RELEASE ENERGY to the surroundings

  • They show a RISE IN TEMPERATURE
• examples of exothermic reactions:

  • COMBUSTION (like burning fuels, usually in presence of oxygen)
  • NEUTRALISATION reactions
  • many OXIDATION REACTIONS like metals reacting with acids
• Practical uses of Exothermic Reactions

  • HAND WARMERS: Utilise exothermic oxidation of iron
  • SELF-HEATING CANS: Depend on exothermic reactions for heating beverages
• ENDOTHERMIC REACTIONS

  reactions that ABSORB ENERGY from the surroundings

  • They show a FALL IN TEMPERATURE
• endothermic reactions are less common but can be found in processes like:

  • the reaction between CITRIC ACID & SODIUM hydrogen carbonate
  • THERMAL DECOMPOSITION
• Practical uses of Exothermic Reactions:

  SPORTS INJURY PACKS: The chemical reaction in these packs absorbs heat, causing them to cool without freezing
• REACTION PROFILES

  graphs that show how ENERGY CHANGES throughout a reaction.

  • The Y-AXIS is the total ENERGY of the molecules
  • the X-AXIS is the PROGRESS OF REACTION

  The reactants come before the product

  
• REACTION PROFILES

  • The difference in HEIGHT between the reactants & products is the overall energy change of the reaction.
  • The curved lined between the reactants & products is how the energy changes as the reaction proceeds.
  • The difference in height between the REACTANTS & the PEAK of the graph is the ACTIVATION ENERGY.

  
• ACTIVATION ENERGY

  the MINIMUM ENERGY needed for reactant particles to COLLIDE with each other, in order to REACT.

  • The BIGGER the activation energy peak, the greater the activation energy, the more energy needed to start the reaction
• Chemical reactions can occur only when reacting particles collide with each other & with SUFFICIENT ENERGY.
• EXOTHERMIC REACTIONS:

  These RELEASE ENERGY, with products having a LOWER ENERGY than reactants.

  • ENERGY RELEASED is shown as a DROP in energy

  A) energy change

  B) activation energy

  
• ENDOTHERMIC REACTIONS:


  These absorb energy, resulting in products at a HIGHER ENERGY level than reactants.

  • ENERGY ABSORBED is shown by a RISE in energy

  A) activation energy

  B) energy change

  
• reaction profile shows the reaction is exothermic bc:

  • the level of products is below the level of reactants
  • the energy decreases overall
  • energy is transferred to the surroundings

  
• Methane + oxygen -> carbon dioxide + water 

  • CH4 + 2O2 -> CO2 + 2H2O 
  • Each of the different molecules would have different amounts of energy in their chemical energy stores

  
• Exothermic reactions

  These RELEASE ENERGY, with products having a LOWER ENERGY than reactants.

  • If the chemicals in the reaction have lost this much energy, then this much heat energy must been released to the surroundings
  • ENERGY RELEASED is shown as a DROP in energy
  • although it loses energy overall, some energy is still required to get the reaction going in the first place (activation energy)

  
• ENDOTHERMIC REACTIONS

  These absorb energy, resulting in products at a HIGHER ENERGY level than reactants.

  • Supply heat to Calcium carbonate to make calcium oxide + carbon
  • CaCO3 -> CaO + CO2
  • the products are higher than the reactants, they have more energy
  • ENERGY ABSORBED is the difference, shown by a RISE in energy

  
• • A: the energy change
  • B: a curve from the reactants to the products to show how the energy changes during the reaction
  • D: The increase in energy from the reactants energy level to the highest points on the curve is the activation energy

  
• If an exothermic reaction occurs in a closed container

  • the temp inside the container would increase as the reaction progresses & releases heat energy

  If an endothermic reaction occurs in a closed container, the temp inside the container would decrease

  
• • Same reactions with higher activations energy have higher curves
  • Same reactions with lower activations energy have lower curves

  
• A reaction profile for a specific reaction like CH4 + O2 -> CO2 + H2O

  • instead of writing reactants & products on the lines, its the chemicals from the equations