Potential Energy Diagrams

Potential Energy Diagrams• For a given reaction, the Activation Energy and the

Enthalpy of Reaction can be shown graphically in a Potential Energy Diagram by plotting PE against a reaction coordinate which represents the progress of the reaction.

Reaction Coordinate

Pot

entia

l E

nerg

y→ Enthalpy of Reaction

Activation Energy

Potential Energy Diagrams• A catalyst is a substance that changes the speed of

reactions by changing the activation energy.• Catalysts that speed up reactions do so by ___________

the activation energy and changing the reaction pathway.

• Catalysts have no effect on a reaction’s heat of reaction.

Reaction Coordinate

Pot

entia

l E

nerg

y→

Activation Energy withno catalyst

Enthalpy of Reaction

Activation Energy witha catalyst

lowering

1

a) Endothermic Reactions:

N2(g) + O2(g) → 2 NO(g) HR = + 182.6 kJ

The activated complex is a temporary arrangement of atoms.

A - ________________________ B - ________________________C - ________________________1 - ________________________ 2 - ________________________3 - ________________________ 4 - ________________________ 5 - ________________________6 - ________________________ 7 - ________________________

Reaction Coordinate

Pot

entia

l E

nerg

y→

A

C

B

57

4

32

6

reactantsactivated complexproductsPE of reactantsactivation energycatalyzed reaction pathwayPE of activated complexheat of reactionreverse activation energyPE of products

Potential Energy Diagrams

Potential Energy Diagrams • N2(g) + O2(g) → 2 NO(g) HR = + 182.6 kJ

Ex.1) The “energy barrier” for the forward reaction is represented by arrow _____.

Ex.2) The net amount of energy absorbed is represented by arrow _____

Ex.3) When 4 moles of NO(g) are produced, the energy absorbed by the reactants is __________ kJ+ 365.2

2

5

Potential Energy Diagrams Ex.4) If a catalyst were introduced into the reaction, which

arrow(s) would NOT change? __________

Ex.5) A catalyst speeds up a reaction because it changes the reaction mechanism for the reaction and thus _____________ the activation energy.

Ex.6) Do the products above have a greater, lesser, or equal amount of stored energy as the reactants?

Explain. __________________________________

_________________________________________

_________________________________________

1, 5, & 7

lowers

The products have a greater amount of

stored energy than the reactants because the reaction is endothermic.

Potential Energy Diagrams Ex.7) Rewrite the reaction equation above putting the heat

into the equation.

__________________________________________

Ex.8) What is an activated complex?

__________________________________________

__________________________________________

__________________________________________

An activated complex is an unstable temporary

N2(g) + O2(g) + 182.6 kJ → 2 NO(g)

arrangement of atoms formed after the reactants

have begun to react.

b) Exothermic Reactions:

CO(g) + NO2(g) → CO2(g) + NO(g) + 226 kJ

The activated complex is a temporary arrangement of atoms.

1

X - ________________________ Y - ________________________Z - ________________________1 - ________________________ 2 - ________________________3 - ________________________ 4 - ________________________ 5 - ________________________6 - ________________________ 7 - ________________________

Reaction Coordinate

Pot

entia

l E

nerg

y→

X

Z

Y

5

7

4

3

2

6

reactantsactivated complexproducts

PE of reactants

activation energycatalyzed reaction pathway

PE of activated complexheat of reaction

reverse activation energyPE of products

Potential Energy Diagrams

Potential Energy Diagrams

• CO(g) + NO2(g) → CO2(g) + NO(g) + 226 kJ

Ex.1) The energy liberated when 0.5 mole of NO2(g) is consumed is __________ kJ.

Ex.2) The energy difference between the reactants andthe products is represented by arrow _____.

Ex.3) The energy of the activated complex is represented by arrow _____.

113

7

6

Potential Energy DiagramsEx.4) A catalyst speeds up a reaction because it changes

the _____________________swag__ for the reaction and thus lowers the activation energy.

Ex.5) Rewrite the reaction equation above putting the heat off to the side of the equation.

__________________________________________

Ex.6) What letter or number represents the temporary unstable grouping of atoms? _____

Ex.7) Rewrite the reaction equation above putting the heat off to the side of the equation.

_________________connorwashere_________________________

reaction mechanism

Y

CO(g) + NO2(g) → CO2(g) + NO(g) ΔH = -226 kJ

A + B → AB ΔH = +50 kJ

Potential Energy DiagramsEx.8) Given the reaction: A + B + 50 kJ → AB

If the activation energy is 55 kJ, then the reverse activation energy would be _______ kJ.

Reaction Coordinate

Po

ten

tial

En

erg

y →

55 kJ

50 kJ

? kJ

5

Potential Energy Diagrams

Ex.9) During an endothermic reaction, (1) the enthalpy is decreased (2) there is no activation energy (3) the enthalpy is increased(4) the reaction is slow

Ex.10) The net quantity of energy absorbed by an endothermic reaction (1) is greater than the activation energy (2) is less than the activation energy (3) is equal to the activation energy(4) can not be compared to the activation energy