Energy Changes in Chemistry

• Exothermic and Endothermic Reactions

  Chemical reactions are either:

  • Exothermic - gives out heat
  • Endothermic - takes in heat
• Important exothermic reactions

  • Neutralisation
  • displacement
  • combustion
  • hydration of anhydrous salt
  • rusting
• Important Endothermic reactions

  • Thermal decomposition
  • electrolysis
  • dehydration of hydrated salt
• Reaction Profile Diagram 


  A reaction profile for a chemical reaction shows how the energy of chemicals changes as reaction proceeds.

  • Drawn as a graph sketch of energy on vertical axis against progress on the horizontal axis
  • Chemical energy of reactant is shown as one line labelled reactants and chemical energy of products is shown as a line further to the right labelled products
• Exothermic reaction profile 


  In exothermic reactions, chemical energy in reactants is converted into heat energy which is released. This means that chemical energy going from reactants to products has decreased

  • As products are at lower energy value than reactants, the change in energy ( energy of products - energy of reactants ) is a negative number
• Exothermic reaction profile diagram

  The reaction profile

  
• Endothermic reaction profile 


  Endothermic reaction, heat energy taken in is converted into chemical energy in the products. This means chemical energy going from reactants to products has increased

  • Products are higher energy value, change in energy in an endothermic reaction from reactants to products is a positive number
• Endothermic reaction profile diagram 


  The reaction profile is

  
• Understanding energy change values

  Energy is often measured in kJ. Energy changes is also measured in kJ.

  energy change = energy of products - energy of reactants
• Energy change example 


  A hypothetical exothermic reaction, assume that the energy of reactants is 150kJ and products energy is 120kJ

  • energy = 120-150 = -30kJ
  • it is an exothermic reaction
  • 30kJ of energy released as heat for the reaction
• Energy Change example 2

  Hypothetical endothermic reaction, assume energy of reactants is 100kJ and product energy is 140kJ

  • energy = 140 - 100 = 40kJ
  • endothermic reaction
  • 40kJ of energy taken in as heat fro the reaction
• Reversible Reactions 


  If the reaction is reversible, the reverse of the reaction has the opposite energy change. For a reversible reaction where the forward reaction is exothermic, the reverse reaction is endothermic

  • e.g forward reaction has energy change of -30kJ, the reverse reaction has an energy change of +30kJ
• Reversible reaction e.g 


  Example question

  
• Activation energy 


  Reacting particles collide and not all collisions cause a reaction to occur. Only those with activation energy or greater will cause a reaction

  • Activation energy is the minimum energy required for a reaction to occur
  • on a reaction profile the reaction pathway of products and reactants is shown. Energy difference between energy of reactants and top of pathway is activation energy. This applies to both endothermic and exothermic reactions
• Exothermic reaction 


  Reaction pathway

  
• Endothermic reaction 


  Reaction profile

  
• Catalyst 


  A catalyst works by providing an alternative reaction pathway of lower activation energy. Can also be shown on a reaction profile

  • The reaction profile shows that the catalysed reaction has a different or alternative reaction pathway and that this pathway has a lower activation energy
• Effect on catalyst on reaction pathway

  Reaction profile

  
• Catalyst effect example 


  Reaction profile example

  
• Explaining energy changes in terms of bonds

  All chemicals possess internal energy in their bond. Energy is required to break all types of bonds, and energy is released when all types of bonds are formed.

  This means that the bond breaking is endothermic and the bond making is exothermic
• Energy changes in bonds example 1


  Example

  
• Energy changes in bond example 2


  Example

  
• Calculating energy changes from bind energies

  A bond energy is energy required to break one mole of a covalent bond, measured in kJ
• General points for bond energies

  • Bond energy relates to the strength of a covalent bond. A higher bond energy value means a stronger covalent bond
  • Triple covalent bonds = higher bond energy value than double covalent bonds
  • Double covalent bonds = higher bond energy value than single covalent bonds

  
• Calculating energy changes 


  The best way to calculate these is to add up the bond energies of all the bonds broken in reaction and add up all the bond energies made in the reaction

  • calculated using
  • energy change = total of bond energies of all bonds broken - total of bond energies of all bonds made
• Energy change example 


  Example 1

  
• Energy change example


  Example 2