If the rate law of a reaction A + B ® Product(s) is Rate = k[A]a[B]b, the overall order is a+b. You can find the overall order thus: start with equal amounts of the reactants. In a reaction where equal numbers of moles react, this means that at all times [A] = [B] = C, so the rate law is simply Rate = kC(a+b). It behaves as a simple (a+b) order reaction.
The other simplifying procedure is known as isolation of reagents. If you start the reaction with a huge excess of A, for example, then no matter how much B reacts it won't change the amount of A significantly. So the rate law becomes Rate = (k[A]a)[B]b, where the constant term k[A]a is a pseudo rate constant (it's constant, but it's not really the rate constant). The reaction then behaves as a b'th order reaction in B. We call it a pseudo-b order reaction.
The strategy, then, is to do successive reactions where all reactants are in large excess, and there is in each case a small amount of only one reactant. This reactant is said to be isolated, and the rate law follows that reactant alone.
This SIR simulates the reaction A + B ® Product(s). You may set the initial concentrations over two orders of magnitude. In the example [A], 1.0 M, has been set much bigger than [B], 0.01 M. When the reaction runs, the A and B concentrations are plotted; the A concentration hardly changes at all while the B concentration falls nearly to zero in about 50 seconds.
After the reaction run you have a number of plot options. You may graph concentration, its natural logarithm or its inverse for either reactant. A line-drawing facility is provided, so you may determine if the graph is linear, and measure its slope. Thus you may show how a reaction may be analyzed to reveal the overall order and the individual reactant orders, and how the pseudo rate constants of the individual reactants are related to the overall rate constant.
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Updated July 24, 2000