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Module 3 - Chapter 9 - Enthalpy changes - Coggle Diagram
Module 3 - Chapter 9 - Enthalpy changes
Average bond enthalpy
Energy requiredto break one mole of a specified type of bond in a gaseous molecules
Energy is required to break bonds
Bond enthalpies are endothermic
Bond enthalpies are positive
Limitations
Varies depending on chemical environment
Calculated from actual bond enthalpies in different environments
Bonds breaking
Energy is required to break bonds
Bond breaking is exothermic
Enthalpy change is positive
Bond making
Energy is released when bonds form
Bond making is exothermic
Enthalpy change is negative
Difference between energy required for bond breaking and energy released by bond making determins whether a reaction is exothermic/ endothermic
Exothermic
Energy released when making bonds in greater than energy required when breaking bonds
Endothermic
Energy required when breaking bonds is greater than energy released when making bonds
Calculating enthalpy changes
Enthalpy change of a reaction = bond enthalpies in reactant - bond enthalpies in products
As you are using average, the actual energy involved in breaking/ making individual bonds is slightly different
Bonds with lower bond enthalpy values would be expected to break first
Hess' law
If a reaction can take place by two routes, and starting/ finishing conditions are the same, the total enthalpy change is the same for each route
Comes from the idea of conservation of energy
Route 1: A + B
Route 2: C
A + B = C
Indirect determination of enthalpy changes
You can work out the standard enthalpy change of a reaction from the standard enthalpy changes of formation of the reactants and products
standard enthalpy changes of formation for elements is 0
Formation
Construct the enthalpy cycle between the reactant, products and their elements
Route 1: B + A
Route 2: C
Hess' law B + A = C, so A = C - B
Elemnts form the common link between the reactants and products
Reactants and products of the original reaction are formed from their elements and the arrow points upwards
Combustion
Enthalpy changes of combustion of the reactants and of the product can be measured directly.
Both the reactants and the products of the original reaction react to form combustion products, the arrow spoints downwards
Route 1: A + C
Route 2: B
Hess' law, A + C = B, so A = B-C
Summary
Enthalpy changes of formation =
Enthalpy changes of combustion =
