Reaction Mechanics

OUTCOME QUESTION(S): C12-3-10 RATE MECHANICS Vocabulary & Concepts Explain the concept of a reaction mechanism. Include: rate determining step, intermediates, and catalysts Determine the reaction orders and rate law of a chemical reaction from experimental data. Explain the scientific process connecting a chemical reaction to its experimental rate law, and to the prediction of an appropriate reaction mechanism. Include: connecting the rate law to the RDS Vocabulary & Concepts elementary reaction complex reaction

Are chemical reactions really as simple as the equation suggests? 2 NO (g) + O2 (g) 2 NO2 (g) + Potential Energy Kinetic Energy Kinetic Energy Kinetic Energy

Three particle collisions are quite rare. 2 NO (g) + O2 (g) 2 NO2 (g) For this reaction to occur in one step: 3 particles must collide - at exactly the same time with the correct orientation and enough energy. Three particle collisions are quite rare. Consider how hard it would be to time the collision of three pool balls, rolled with different speeds, to strike each other at the same exact moment their numbers faced inward… Reactions typically proceed as a series of simpler steps each consisting of bimolecular collisions.

2 NO (g) + O2 (g) 2 NO2 (g) Elementary reaction: A simple reaction that takes place in one step. Complex reaction: A multi-step reaction made of elementary steps. The reaction mechanism is an outline of the series of elementary steps that make the overall reaction. In baking, the recipe (mechanism) explains how (steps) to make the cake (overall reactions) 2 NO (g) + O2 (g) 2 NO2 (g)

Sum of the steps of the mechanism equals the overall or net equation. 2 NO (g) + O2 (g) 2 NO2 (g) N2O2 - is called a reaction intermediate: A compound produced in one step that is required as a reactant of a subsequent step. Do not confuse intermediate with the activated complex that occurs during every collision as reactants form products

Neither intermediates nor catalysts appear in the Net Reaction The overall reaction is created by cancelling identical reactants and products found across the reaction arrow Decomposition of ozone, with chlorine catalyst. 1: Cl2 + O3 ClO + O2 2: O3 O2 + O 3: ClO + O Cl2 + O2 2 O3 (g) 3 O2 (g) ClO and O – intermediates Cl2 – catalyst Neither intermediates nor catalysts appear in the Net Reaction

NO2 + CO NO + CO2 Rate Determining Step Each elementary step has its own EA needs and proceeds at its own rate. The step with the slowest rate (greatest EA) is called the rate determining step (RDS): 1: NO2 + NO2 NO3 + NO (slow) 2: NO3 + CO NO2 + CO2 (fast) RDS NO2 + CO NO + CO2

NO2 + CO NO + CO2 Rate Determining Step 1: NO2 + NO2 NO3 + NO (slow) RDS has the greatest affect on the overall rate and so determines the overall “speed” of a complex reaction. Changes to the reactants in the other steps will have very little effect on the overall rate. 1: NO2 + NO2 NO3 + NO (slow) 2: NO3 + CO NO2 + CO2 (fast) RDS NO2 + CO NO + CO2 Consider  [CO]: increasing the rate of the second step has little effect overall, as the reaction still waits for the slow step

1 2 1: NO2 + NO2 NO3 + NO (slow) 2: NO3 + CO NO2 + CO2 (fast) endothermic exothermic -ΔH overall Creating a PE diagram for a complex reaction consists of combining the diagrams of each elementary step 1 2

A + 2B C 1. B + B E fast endothermic 2. E + A C slow endothermic ΔH 3 4 A + 2B C + ΔH 5 The diagram must contain all the information that is provided in the question – and be labeled 3 EA2 4 PE (kJ) products 1 2 ΔH 5 EA1 reactants Reaction Coordinates

a) What is the net reaction? P + Q X + T (slow, endo) X + P Y + R (fast, exo) Y + S T (moderate, exo) overall - ΔH a) What is the net reaction? a) 2P + Q + S  2T + R b) What are the reaction intermediates? b) X + Y c) First Step c) Which is the rate determining step? d) What would be the effect of increasing [P]? d) Increases the overall rate (it’s in the RDS) e) Decreases the overall rate (it’s in the RDS) e) What would be the effect of decreasing [Q]? f) What would be the effect of increasing [S]? f) No significant overall change in rate g) What would a PE graph look like for this reaction?

P + Q X + T (slow, endo) X + P Y + R (fast, exo) Y + S T (moderate, exo) overall - ΔH g) What would a PE graph look like for this reaction? EA2 EA1 PE (kJ) EA3 reactants ΔH products Reaction Coordinates

RECAP: Most reactions occur in steps - mechanism Each step is usually bimolecular Sum of these steps must equal the net equation Rate determining step (RDS) is the slowest step and has the greatest effect on the overall rate

CAN YOU / HAVE YOU? C12-3-10 RATE MECHANICS Vocabulary & Concepts Explain the concept of a reaction mechanism. Include: rate determining step, intermediates, and catalysts Determine the reaction orders and rate law of a chemical reaction from experimental data. Explain the scientific process connecting a chemical reaction to its experimental rate law, and to the prediction of an appropriate reaction mechanism. Include: connecting the rate law to the RDS Vocabulary & Concepts elementary reaction complex reaction