Potential Energy Potential Energy DiagramsDiagrams

Chemical KineticsChemical Kinetics

Mrs. KayMrs. Kay

Review of ExothermicReview of Exothermic

► Reactants Ep is Reactants Ep is higher than Products higher than Products Ep.Ep.

► Now, we must Now, we must consider the consider the activation energy activation energy (the energy needed (the energy needed so that the reactants so that the reactants bonds will break and bonds will break and reform to make reform to make product)product)

Review of EndothermicReview of Endothermic

► Reactants Ep is lower Reactants Ep is lower than Products Ep.than Products Ep.

►Need to add more Need to add more energy to the system energy to the system for the forward for the forward reaction to take reaction to take place.place.

► Still need to consider Still need to consider activation energyactivation energy

Activated ComplexActivated Complex

► Is the short-lived, unstable structure Is the short-lived, unstable structure formed during a successful collision formed during a successful collision between reactant particles.between reactant particles.

►Old bonds of the reactants are in the Old bonds of the reactants are in the process of breaking, and new products process of breaking, and new products are formingare forming

►Ea is the minimum energy required for Ea is the minimum energy required for the activation complex to form and for the activation complex to form and for a successful reaction to occur.a successful reaction to occur.

Fast and slow reactionsFast and slow reactions

►The smaller the activation energy, the The smaller the activation energy, the faster the reaction will occur faster the reaction will occur regardless if exothermic or regardless if exothermic or endothermic.endothermic.

► If there is a large activation energy If there is a large activation energy needed, that means that more energy needed, that means that more energy (and therefore, time) is being used up (and therefore, time) is being used up for the successful collisions to take for the successful collisions to take place.place.

Sample ProblemSample Problem

►The following reaction has an activation The following reaction has an activation energy of 120kJ and a energy of 120kJ and a ΔΔH of 113kJ.H of 113kJ.

2NO2NO22 2NO + O 2NO + O22

Draw and label a potential energy (activation Draw and label a potential energy (activation energy) diagram for this forward reaction.energy) diagram for this forward reaction.

Calculate the activation energy for the reverse Calculate the activation energy for the reverse reaction (if the reaction went backwards)reaction (if the reaction went backwards)

Another ProblemAnother Problem

►The following hypothetical reaction The following hypothetical reaction has an activation energy of 70kJ and a has an activation energy of 70kJ and a ΔΔH of -130kJH of -130kJ

A + B A + B C + D C + D Draw and label a potential energy Draw and label a potential energy

diagram for the reactiondiagram for the reaction Calculate the activation energy for the Calculate the activation energy for the

reverse reaction.reverse reaction.

Watch the following FlashWatch the following Flash

►Review of what is occuring during a Review of what is occuring during a chemical reaction for both chemical reaction for both endothermic and exothermic.endothermic and exothermic.

►KNOW THIS!!KNOW THIS!!►http://mhhe.com/physsci/chemistry/es

sentialchemistry/flash/activa2.swf

Practice:Practice:

1.1. The following hypothetical reaction The following hypothetical reaction has an Ea of 120kJ and a has an Ea of 120kJ and a ΔΔH of 80kJH of 80kJ

2a + B 2a + B 2C + D 2C + D Draw and label a potential energy Draw and label a potential energy

diagram for this reaction.diagram for this reaction. What type of reaction is this?What type of reaction is this? Calculate the activation energy for the Calculate the activation energy for the

reverse reaction.reverse reaction. Calculate the Calculate the ΔΔH for the reverse H for the reverse

reaction.reaction.

2.2. Analyze the Analyze the activation energy activation energy diagram below.diagram below.

► What is the Ea for the What is the Ea for the forward reaction? For forward reaction? For the reverse reaction?the reverse reaction?

► What is the What is the ΔΔH for the H for the forward reaction? For forward reaction? For the reverse reaction?the reverse reaction?

► What is the energy of What is the energy of the activated the activated complex?complex?

Answer to #2Answer to #2► The activation energy (Ea) for the forward reaction is shown The activation energy (Ea) for the forward reaction is shown

byby (a) (a)::

Ea (forward) = H (activated complex) - H (reactants) = 400 - Ea (forward) = H (activated complex) - H (reactants) = 400 - 100 = 300 kJ mol-1 100 = 300 kJ mol-1

The activation energy (Ea) for the reverse reaction is shown The activation energy (Ea) for the reverse reaction is shown by by (b)(b)::

Ea (reverse) = H (activated complex) - H (products) = 400 - Ea (reverse) = H (activated complex) - H (products) = 400 - 300 = 100 kJ mol-1 300 = 100 kJ mol-1

The enthalpy change for the reaction is shown by The enthalpy change for the reaction is shown by (c)(c)::

H = H (products) - H (reactants) = 300 - 100 = +200 kJ mol- H = H (products) - H (reactants) = 300 - 100 = +200 kJ mol-1 1 for the forward and reverse reaction.for the forward and reverse reaction.