CHEM 104 1st Edition Lecture 16 Energy diagrams represent the energy involved in chemical reactions Graphs plot reaction progress on x axis vs energy on y axis Activation energy Ea minimum energy needed for a reaction to occur energy difference between starting point of reaction and highest energy point Transition state state corresponding to the highest energy Reaction is exothermic if the final energy state is lower than the initial state vice versa for endothermic reactions Temperature and reaction rate At higher temperatures a larger population of molecules has higher energy lower Ea is needed Arrhenius equation k Ae Ea RT o K rate constant A frequency factor constant Ea activation energy R gas constant 8 314 J mol K T temperature in Kelvin o Rearranged into lnk lnA Ea RT o Can be graphed with 1 T on x axis vs lnK on y axis Slope Ea R If k is known as 2 different temperatures Ea can be derived Reaction mechanisms a series of elementary steps that add up to the overall reaction equation Example 2I aq H2O2 aq H30 aq I2 aq 4H20 l Mechanism Step 1 H202 I HOI HO slow Step 2 HOI I I2 HO fast These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Step 3 2HO H3O 4H2O fast Rate k H2O2 I HOI and HO are intermediates The slowest step is always the rate determining rate limiting step since reaction can t go any faster than its slowest step and is the one used to experimentally determine the rate Intermediates are formed in one step but used up in a later step they don t show up in the overall equation Valid mechanism must satisfy the following conditions o Consist of elementary steps o Steps add up to the overall equation o Predict experimental rate law Reaction mechanisms can be seen in energy diagrams diagrams have a peak for each step
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