• When bonds are broken or made?enthalpy changes?take place
  • A chemical bond is a force of attraction between two atoms
  • Breaking the bond requires the input of energy it is therefore an?endothermic?process

   

• The energy change required to break the bond depends on the atoms that form the bond
  • The energy required to break a particular bond is called the?bond dissociation enthalpy
  • This is usually just shortened to?bond enthalpy?or?bond energy

   

• Bond formation is the opposite of bond breaking and so energy is released when bonds are formed
  • It is therefore an?exothermic?process

   

To break bonds energy is required from the surroundings and to make new bonds energy is released from the reaction to the surroundings

• The amount of energy released when a particular bond is formed has the same magnitude as the energy taken in when the bond is broken but has the opposite sign

Overall enthalpy changes

• If more energy is released when new bonds are formed than energy is required to break bonds, the reaction is?exothermic
  • The products are?more stable?than the reactants

   

• If more energy is required to break bonds than energy is released when new bonds are formed, the reaction is?endothermic
  • The products are?less stable?than the reactants

   

• The relationship between bond breaking and bond making can be shown graphically like this:

Bond enthalpy profiles

Average bond energy

• Bond energies are affected by other atoms in the molecule (the environment)
• Therefore, an average of a number of the?same type of bond?but in different environments is calculated
• This bond energy is known as the?average bond energy and is defined as

'The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds'

Average bond enthalpy of C-H in methane

• The?average bond enthalpy?of C-H is found by taking the?bond dissociation enthalpy?for the whole molecule and dividing it by the number of C-H bonds
• The first C-H bond is easier to break than the second as the remaining hydrogens are pulled more closely to the carbon
• However, since it is impossible to measure the energy of each C-H bond an average is taken
• This value is also compared with a range of similar compounds to obtain an accepted value for the?average bond enthalpy

• Bond energies are used to find the ΔHr??of a reaction when this cannot be done experimentally
• The process is a step-by-step summation of the bond enthalpies of the all the molecules present finishing with this formula:

ΔrHθ?= enthalpy change for bonds broken + enthalpy change for bonds formed

• These two worked examples show how to lay out your calculation

Answer:

Step 1:?The chemical equation for the Haber process is:

N2?(g) + 3H2?(g) ? 2NH3?(g)

N≡N? ? ? ?3 H-H? ? ? ? ?6 N-H

Step 2:?Set out the calculation as a balance sheet as shown below:

Note! Values for bonds broken are positive (endothermic) and values for bonds formed are negative (exothermic)

Step 3:?Calculate the standard enthalpy of reaction

• • ΔrHθ?= enthalpy change for bonds broken + enthalpy change for bonds formed
  • ΔrHθ?= (+2253 kJ mol-1) + (-2346 kJ mol-1)
  • ΔrHθ?= -93 kJ mol-1

Answer:

Step 1:?The enthalpy of combustion is the enthalpy change when?one mole?of a substance reacts in excess oxygen to produce water and carbon dioxide

The chemical reaction should therefore be simplified such that only?one mole?of?ethyne?reacts in excess oxygen:

H-C≡C-H + 2 ? O=O → H-O-H + 2O=C=O

Step 2:?Set out the calculation as a balance sheet as shown below:

• • ΔrHθ?= enthalpy change for bonds broken + enthalpy change for bonds formed
  • ΔrHθ?= (+2912 kJ mol-1) + (- 4142 kJ mol-1)
  • ΔrHθ?= -1230 kJ mol-1