Rate Law and Stoichiometry A + 2B C

Explain the scientific process connecting a chemical reaction to its experimental rate law, and to the prediction of an appropriate reaction mechanism. Include: connect the rate law to the RDS Additional KEY Terms Elementary reaction

For reactions that occur in a single step (elementary reactions) Order of each reactant in the rate law is equal to the coefficient in the balanced equation. rate = k[A] a [B] b aA + bB cC + dD

Smog - reaction of ozone, O 3 (g) and nitrogen monoxide, NO(g) – is an elementary reaction: O 3 (g) + NO (g) NO 2 (g) + O 2 (g) Determine the rate law for this reaction. rate = k[O 3 ][NO]

Each step of a reaction mechanism is an elementary reaction. A + 2B C 1. 2 B E(slow) 2. E + A C(fast) [A] mol/L[B] mol/L Rate (mol/Lmin) Rate = k[B] 2 Rate = k[E][A] Rate = k[B] 2

WHY? Reactants not found in the RDS: - Do not affect reaction rate significantly - would then be zero order reactants Rate law matches the stoichiometry of the rate determining step. A + 2B C Rate = k[B] 2

Putting it all together: Chemists first determine the rate law experimentally... THEN deduce the mechanism of a reaction. The sum of all steps MUST result in the net equation to be acceptable.

2 NO 2 + F 2 2 NO 2 F Reaction rate data has shown the rate law to be: Rate = k[NO 2 ][F 2 ] Suggested mechanism MUST have a RDS with NO 2 and F 2 – using coefficients of 1 (orders). RDS: NO 2 + F 2

Chemists cannot be positive about a mechanism, they can just suggest a possible mechanism which reflects experimental data. The proposed mechanism is then: Step 2: NO 2 + F - NO 2 F Rate = k[NO 2 ][F 2 ] 2 NO 2 + F 2 2 NO 2 F Step 1: NO 2 + F 2 NO 2 F + F - (RDS)

Reaction rate data has shown the rate law to be: Rate = k[NO 2 ] 2 NO 2 + CO NO + CO 2 Step 1: 2 NO 2 NO 3 + NO (RDS) Step 2: NO 3 + CO NO 2 + CO 2 The proposed mechanism is might be: NO 2 + CO NO + CO 2

3 M + N P + 2 Q a) What would be the effect of doubling the [N]? b) What would be the effect of tripling the [M]? c) What is the stoichiometry of the RDS? 2 M If the rate law for this reaction is: Rate = k[M] 2

Rate Laws describe affect of changes to reactants on reaction rate - determined experimentally. Rate law does not usually correspond with reaction stoichiometry. Rate law can be predicted using the coefficients of balanced elementary reactions. Determined rate law is the rate law for RDS. Mechanisms are predicted from rate law / RDS.

CAN YOU / HAVE YOU? Explain the scientific process connecting a chemical reaction to its experimental rate law, and to the prediction of an appropriate reaction mechanism. Include: connect the rate law to the RDS Additional KEY Terms Elementary reaction