Transcript Graphical Representation of Enthalpy Change

Graphical Representation of
Enthalpy Change
Chapter 5
Review:
 Endothermic Reaction: Addition of energy results
in the breaking of bonds (decomposition reaction)
 Molecule + ENERGY ---> Separated Atoms
 Exothermic Reaction: Bonds are formed and
energy is made available
 Separated Atoms ---> Molecule + Energy
 ACTIVATED COMPLEX: An unstable cluster of
colliding reactant atoms that is formed in the
partial conversion of reactants into products
 ACTIVATION ENERGY: the minimum quantity
of energy needed for a chemical reaction to occur
(both endo- and exothermic)
 ENERGY DIAGRAM: a graph used to visualize
the energy change of the substances involved in a
reaction
Enthalpy of a System
 The sum of all the types of potential and kinetic
energy within the system
 The energy contained within the chemical bonds
between the atoms of the molecules ==>
IMPORTANT POTENTIAL ENERGY
 In a chemical reaction there are many collisions
between reactant molecules- this causes bonds to
break and new ones to form and therefore new
molecules
 Because the bonds are different the potential
energy contained in these new molecules is also
different from the original (reactant) molecules
 This change in kinetic energy of the molecules is
exchanged with the surrounding environment as
heat energy
 The KINETIC energy of reactants = KINETIC
energy of products
 Therefore the difference in POTENTIAL energy of
reactants and products corresponds to the overall
enthalpy change of the reaction
Activated Complex
 In any chemical reaction there is a moment when
the different reactant molecules come in contact
with each other but they have not yet separated to
form the new compounds known as the products
 At this moment both reactant and product bonds
exist
 It is unstable and very brief; too brief to be timed
 The existence of this is therefore hypothetical but
experimentally confirmed
Activation Energy
 At the moment that the activation complex is
achieved a particular “spike” in potential energy
occurs
 This is due to the total reorganization of all the
atoms (reactants into products)
 This means that the activation complex has more
energy then either the reactants or the products
 If this amount of energy is not available the
reaction will not go to completion
Example
 Firewood:
 It is flammable
 It can however be safely stored inside your house
without bursting into flame when it contacts oxygen
 In order to burn you must light the wood, therefore
adding energy to the system in order for the activation
energy to be achieved and the reaction of burning begin
Relationship:
 The difference between the REACTANTS and the
ACTIVATED COMPLEX corresponds to the
ACTIVATION ENERGY (Ea)
Energy Diagrams
 A visualization of the change in potential energy
between reactants and products
Endothermic Chem. Rxn Diagram
Exothermic Chem. Rxn Diagram
Reading an Energy Diagram
 It is easy to confuse an ENERGY DIAGRAM with
the ENRGY BALANCE DIAGRAM used to
calculate the enthalpy change of a reaction
ENERGY DIAGRAM
ENERGY BALANCE
DIAGRAM
Y-Axis: Potential energy (Ep)
ONLY
Y-Axis: Enthalpy (Potential +
Kinetic Energy)
Plotting the Graph: Highest point
= Activated Complex
Plotting the Graph: Highest point
= free atoms
Highest Point: Hypothetical but
reflecting the real structure of
molecules
Highest Point: Not a true reality
as atoms are not actually
completely free at any point
(always some bonding)
Examples
1) Plot the energy diagram of a reaction between
methane (CH4) and sulphur (S2), which reacts
according to the following equation:
CH4(g) + 2 S2 (g) → 2 H2S(g)
The activation energy of this reaction is 140 kJ/mol
and its enthalpy change is 14 kJmol
2) Plot the energy diagram for the combustion of
methane if the activation energy is 240.0 kJ, the
enthalpy change is 802 kJ/mol, and the Ep of the
reactants is 1250.0 kJ.
CH4 (g) + 2 O2 (g) →CO2 (g) + 2 H2O (g)