Order For a particular reaction, aA+bB+cC→dDaA+bB+cC→dD, the rate law was experimentally determined to be rate=k[A]0[B]1[C]2=k[B][C]2rate=k[A ]0[B]1[C]2=k[B][C]2 This equation is zero order with respect to AA. Therefore, changing the concentration of AA has no effect on the rate because [A]0[A]0 will always equal 1. This equation is first order with respect to BB. This means that if the concentration of BB is doubled, the rate will double. If [B][B] is reduced by half, the rate will be halved. If [B][B] is tripled, the rate will triple, and so on. This equation is second order with respect to CC. This means that if the concentration of CC is doubled, the rate will quadruple. If [C][C] is tripled, the rate will increase by a factor of 9, and so on. . Overall reaction order and rate-constant units The sum of the individual orders gives the overall reaction order. The example equation above is third order overall because 0+1+2=30+1+2=3. For the units of rate to come out to be M/sM/s, the units of the rate constant for third-order reactions must be M−2⋅s−1M−2⋅s−1 since M/s=(M−2⋅s−1)(M3)M/s=(M−2⋅s−1)(M3) For a second-order reaction, the rate constant has units of M−1⋅s−1M−1⋅s−1 because M/s=(M−1⋅s−1)(M2)M/s=(M−1⋅s−1)(M2). In a first-order reaction, the rate constant has the units s−1s−1 because M/s=(s−1)(M1)M/s=(s−1)(M1). Analyzing a specific reaction Consider the following reaction: H2+2ICl→I2+2HClH2+2ICl→I2+2HCl, rate=k[H2][ICl]