Science, Tech, Math › Science What Is a Second Order Reaction in Chemistry? Share Flipboard Email Print Westend61 / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated August 12, 2019 A second order reaction is a type of chemical reaction that depends on the concentrations of one-second order reactant or two first-order reactants. This reaction proceeds at a rate proportional to the square of the concentration of one reactant, or the product of the concentrations of two reactants. How fast the reactants are consumed is called the reaction rate. Formulating General Chemical Reactions This reaction rate for a general chemical reaction aA + bB → cC + dD can be expressed in terms of the concentrations of the reactants by the equation: rate=k[A]x[B]yrate = k[A]x[B]yrate=k[A]x[B]y Here, k is a constant; [A] and [B] are the concentrations of the reactants; and x and y are the orders of the reactions determined by experimentation and not to be confused with the stoichiometric coefficients a and b. The order of a chemical reaction is the sum of the values x and y. A second order reaction is a reaction where x + y = 2. This can happen if one reactant is consumed at a rate proportional to the square of the reactant's concentration (rate = k[A]2) or both reactants are consumed linearly over time (rate = k[A][B]). The units of the rate constant, k, of a second-order reaction are M-1·s-1. In general, second-order reactions take the form: 2 A → productsorA + B → products. Examples of Second-Order Chemical Reactions This list of ten second-order chemical reactions features some reactions that are not balanced. This is because some reactions are intermediate reactions of other reactions. H+ + OH- → H2OHydrogen ions and hydroxy ions form water. 2 NO2 → 2 NO + O2Nitrogen dioxide decomposes into nitrogen monoxide and an oxygen molecule. 2 HI → I2 + H2Hydrogen Iodide decomposes into iodine gas and hydrogen gas. O + O3 → O2 + O2During combustion, oxygen atoms and ozone can form oxygen molecules. O2 + C → O + COAnother combustion reaction, oxygen molecules react with carbon to form oxygen atoms and carbon monoxide. O2 + CO → O + CO2This reaction often follows the previous reaction. Oxygen molecules react with carbon monoxide to form carbon dioxide and oxygen atoms. O + H2O → 2 OHOne common product of combustion is water. This, in turn, can react with all the loose oxygen atoms produced in the previous reactions to form hydroxides. 2 NOBr → 2 NO + Br2In the gas phase, nitrosyl bromide decomposes into nitrogen oxide and bromine gas. NH4CNO → H2NCONH2Ammonium cyanate in water isomerizes into urea. CH3COOC2H5 + NaOH → CH3COONa + C2H5OHIn this case, an example of the hydrolysis of an ester in the presence of a base, ethyl acetate in the presence of sodium hydroxide.