Chapter 12: Chemical Kinetics Introduction Presentation 12.1-12.3

12.1 Reaction Rates Its is not enough for us to know just the stoichiometry, or thermodynamics of a reaction We also need to know the factors that govern the rate of the reaction For any reaction to be useful to us, it must occur at a reasonable rate, and produce a reasonable amount of product

Chemical Kinetics Study of the speed that reactants are converted to products in a chemical reaction Factors such as, state of matter, particle size, temperature, concentration, and presence of a catalyst all have a role in the speed of a reaction and its effectiveness Spontaneity is not related to speed, but rather is a measurement of the possibility that the reaction will happen at the current conditions Stoichiometry ratio is usually maintained during the process

Reaction Rate

Reaction Rate Rate is based on a change in concentration over time Rates always decrease with time, so any expressions involving reactants will have negative values Calculated rates are always going to have positive values Rates do not have to be constant but must follow a pattern or predictable pathway

Rate Equation The Equation:

Calculation of Rate

12.2 Rate Laws: An Introduction We have an issue when looking at reversible reactions, and that is, where do we study them when looking to calculate a rate law? Answer: We avoid confusion by studying the rate of a reaction under conditions where the reverse reaction is negligible This means that for us: RATE DEPENDS ON REACTANTS ONLY!!!! Rates will be instantaneous rates measured at a time where the reverse reaction has not contributed to the overall rate

Rate Equation Rate = k[NO2]n Rate is reactant dependent K is the proportionality constant that we refer to as the rate constant n is the order of the reactant (integer value including 0 or even a fraction) The order of the reactant is determined experimentally via data and calculations.

Types of Rate Laws 1. Differential Rate Law – Expresses how the rate of a reaction is dependent on the concentration of the elements or compounds in the experiment. 2. Integrated Rate Law – Expresses how the concentration of a species (reactant or product) in a reaction changes with time Choice of rate law type depends on what data is easiest to collect

12.3 Determining the Form of the Rate Law See notes