C5 - ENERGY CHANGES

• Exothermic reaction

  Reaction where energy is released to the surroundings, usually in the form of heat
• Endothermic reaction


  Reaction where energy is absorbed from the surroundings
• Representing reactions using reaction profiles

  1. Plot total energy of molecules on y-axis

  2. Plot progress of reaction on x-axis

  3. Place reactants on left, products on right

  4. Exothermic: products have less energy than reactants

  5. Endothermic: products have more energy than reactants
• Activation energy

  Minimum energy required for reactant particles to collide and react
• Higher activation energy

  More energy required to start the reaction
• Lower activation energy

  Less energy required to start the reaction
• Examples of exothermic reactions

  • Combustion reactions
  • Neutralization reactions between acids and bases
  • Most oxidation reactions
• Energy can't be created or destroyed, it can only be transferred from one place to another
• The most common way to exchange energy with the surroundings is in the form of heat
• Energy is stored in chemical bonds.
• Different chemical bonds hold different amounts of energy.
• If the reactant molecules in a reaction have more energy in their chemical bonds than the product molecules, will energy be absorbed or released overall?

  released
• endothermic reaction because the products have more energy than the reactants

  
• Exothermic reaction

  
• Bond energy

  The amount of energy needed to break one mole of a particular covalent bond
• Breaking bonds is an endothermic process because it requires energy from the surroundings
• Bond formation is an exothermic process as it releases energy to the surroundings
• Reaction of hydrogen and chlorine

  1. Break hydrogen-hydrogen bond and chlorine-chlorine bond (endothermic)

  2. Form two hydrogen-chloride bonds (exothermic)
• Determining if overall reaction is exothermic or endothermic

  Compare total energy required to break bonds vs total energy released when bonds form
• Reaction of nitrogen and hydrogen

  1. Break 1 nitrogen-nitrogen triple bond and 3 hydrogen-hydrogen bonds (endothermic)

  2. Form 6 nitrogen-hydrogen bonds (exothermic)
• Energy must be supplied to break bonds
• Bond breaking is an endothermic process
• Bond forming is an exothermic process
• In exothermic reactions the energy released by forming bonds is greater than the energy used to break them. In endothermic reactions the energy used to break bonds is greater than the energy released by forming them
• Calculating the overall energy change for a reaction using bond energies

  1. Find the energy required to break the original bonds

  2. Find the energy released by forming the new bonds

  3. Calculate the overall energy change using the equation: Overall energy change = energy required to break bonds - energy released by forming bonds
• You can't compare the overall energy changes of reactions unless you know the numerical differences in the bond energies
• Chlorine and bromine react with hydrogen in a similar way. Br-Br bonds are weaker than Cl-Cl bonds and H-Br bonds are weaker than H-Cl bonds. So less energy is needed to break the bonds in the reaction with bromine, but less energy is released when the new bonds form. So unless you know the exact difference, you can't say which reaction releases more energy
• To calculate the overall energy change of a reaction we have to use the following equation: energy of reactants - energy of products