S4 - Bond energy calculations
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How can you calculate the enthalpy change for a reaction?
Look at the bonds that are made and broken
What is bond energy?
The amount of energy in a bond
What does each type of chemical bond have?
A particular bond energy associated with it
What bonds do you add up?
The energy of the bonds that are broken
What bonds do you subtract?
The energy of the bonds that are made
Using the bond energies for the following reaction H2 + Cl2 —> 2HC H-H: +436 kj/mol Cl-Cl: +242 kj/mol H-Cl: +431 kj/mol Work out the energy needed for breaking the bonds
436 + 242 = 678 kj
Using the bond energies for the following reaction H2 + Cl2 —> 2HC H-H: +436 kj/mol Cl-Cl: +242 kj/mol H-Cl: +431 kj/mol Work out the energy needed to form the bonds
2 x 431 = 862kj
What formula can you use to calculate the differences in the energy calculated in the formation of HCl?
Enthalpy change = Total energy absorbed to break bonds - total energy released in making bonds
What is the enthalpy change in the formation of HCl (breaking=678 and forming=862)?
678 - 862 = -184 kj/mol
What does the enthalpy change of the formation of HCl show?
The value is negative so it in an exothermic reaction