CHEM 104 1st Edition Lecture 16Energy 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/RTo K = rate constant, A = frequency factor (constant), Ea = activation energy, R = gas constant (8.314 J/mol*K), T = temperature in Kelvino Rearranged into: lnk = lnA – Ea/RTo 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 derivedReaction 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 stepso 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
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