#Chemical Kinetics: Elementary Reactions and Rate Laws

Let's dive into the heart of reaction speeds! This guide will help you master rate laws and elementary reactions, crucial for acing your AP Chemistry exam.

#What are Rate Laws?

At its core, a rate law is a mathematical expression that shows how the rate of a chemical reaction depends on the concentration of reactants. For a simple reaction like A → B, the rate law is given by:

$R = k[A]^n$

Where:

R is the reaction rate (how fast reactants turn into products).
k is the rate constant (a proportionality constant specific to the reaction at a given temperature).
[A] is the concentration of reactant A.
n is the order of the reaction with respect to reactant A (experimentally determined).

Key Concept

Rate laws are determined experimentally, not from the stoichiometric coefficients of a balanced equation.

#Experimental Determination of Rate Laws

#Why Experiments are Essential 🧪

You cannot determine the rate law by simply looking at the balanced chemical equation. The stoichiometric coefficients do not necessarily equal the reaction orders.
Rate laws must be found through experimentation.

#The Experimental Process

Run multiple trials of the reaction.
Vary the initial concentrations of reactants, typically doubling one reactant concentration at a time while keeping others constant.
Measure the initial rate of the reaction for each trial.
Analyze how changes in concentration affect the rate to determine the reaction order for each reactant.

Exam Tip

Always conduct experiments at the same temperature because the rate constant (k) is temperature-dependent.

#How Concentration Changes Affect Rate

Let's see how doubling concentration affects rate:

If doubling the concentration of a reactant doubles the rate, the reaction is first order with respect to that reactant (n=1).
If doubling the concentration of a reactant quadruples the rate, the reaction is second order with respect to that reactant (n=2).
If doubling the concentration of a reactant octuples the rate, the reaction is third order with respect to that reactant (n=3).

Quick Fact

It's rare to see reaction orders higher than 2.

Memory Aid

Think of it like this: - First order: rate changes linearly with concentration. - Second order: rate changes quadratically with concentration. - Third order: rate changes cubically with concentration.

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Elementary Reactions