Rate Law and Stoichiometry

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. aA + bB cC + dD rate = k[A]a[B]b

Smog - reaction of ozone, O3(g) and nitrogen monoxide, NO(g) – is an elementary reaction: O3(g) + NO(g) → NO2(g) + O2(g) Determine the rate law for this reaction. rate = k[O3][NO]

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

Rate law matches the stoichiometry of the rate determining step. WHY? Reactants notfound in the RDS: - Do notaffect reaction rate significantly - would then be zero order reactants A + 2B C Rate = k[B]2

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

2 NO2 + F2 2 NO2F Reaction rate data has shown the rate law to be: Rate = k[NO2][F2] Suggested mechanism MUST have a RDS with NO2 and F2 – using coefficients of 1 (orders). RDS: NO2 + F2

2 NO2 + F2 2 NO2F The proposed mechanism is then: Rate = k[NO2][F2] Step 1:NO2 + F2NO2F + F-(RDS) Step 2: NO2 + F- NO2F Chemists cannot be positive about a mechanism, they can just suggest a possible mechanism which reflects experimental data.

NO2 + CO  NO + CO2 Reaction rate data has shown the rate law to be: Rate = k[NO2]2 The proposed mechanism is might be: Step 1: 2 NO2 NO3 + NO (RDS) Step 2: NO3 + CO  NO2 + CO2 NO2 + CO  NO + CO2

3 M + N P + 2 Q If the rate law for this reaction is: Rate = k[M]2 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 

Rate Laws describe affectof changes to reactantson reaction rate - determined experimentally. • Rate law does notusually correspond with reaction stoichiometry. • Rate law can be predicted using the coefficients of balanced elementaryreactions. • Determined rate law is the rate law for RDS. • Mechanisms are predicted from rate law / RDS.

CAN YOU / HAVE YOU? • Explainthe 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

Rate Law and Stoichiometry