> See also: > - [[Chemical Equilibria]] > - [[Chemical Reactions]] > - [[Enzyme Kinetics]] # Reaction Kinetics The rate law equation expresses the relationship of the rate of a reaction to the concentration of its reactants: $aA + bB \rightarrow cC + dD$ $\text{rate} = k[A]^x[B]^y$ At constant temperatures, the rate of an elementary reaction **Rate Constant (k):** Proportionally constant in the relationship between reaction rate and concentrations of reactants **Reaction Order:** An experimentally determined number defining the dependence of the reaction rate on the concentration of a reactant. - “The reaction is in the $x^{th}$ order with respect to A and $y^{th}$ order with respect to B.” - “The reaction is in the ($x^{th} + y^{th}$) order overall.” > [!important] **Relating Chemical Equilibria to Rates** > Right off the bat, be sure to recognize the difference between rate constants and equilibrium constants. > > $\frac{[PL]}{[P][L]}=K_a=\frac{k_a}{k_d}$ > > a > a > ## Factors Affecting Reaction Kinetics - **Concentrations:** the rate of a reaction is often directly proportional to the concentration of the reactants, increasing the concentration generally increases the frequency of collisions between reactant molecules (leading to an increased reaction rate as a result) - **Temperature:** heat increases the kinetic energy of molecules, leading to more frequent and energetic collisions - Surface Area (for Solids): - Catalysts - Presence of Inhibitors - Pressure (for Gases): - Nature of Reactants: - Presence of Light (for Photochemical Reactions) ## Rate Laws The reaction order of an elementary reaction corresponds to the **molecularity** of the reaction, or *the number of molecules that must simultaneously collide* to generate a product. ### Interpretation of Reaction Orders | Order | Relationship | | ------------ | -------------------------------------------------------------------------------------- | | *Zero Order* | The reaction rate is **independent** of the reactant concentration | | *First Order* | The reaction rate is **directly proportional** to the reactant concentration | | *Second Order* | The reaction rate is proportional to the **exponential** of the reactant concentration | Why would a reaction rate ever be independent of its reactants: - ### Differences Between Units Of Reaction Orders ![[Summary of Rate Laws.png|400]] Zero order: comparing concentration to time (ratio of M/s) The reaction rate depends on the concentrations of he reacting compounds in accordance with the rate law equation: