[joseg707]

For a given decomposition reaction A --> products, if it starts with [A*] = 4.0 M, the first two half-lives are 34 and 68 minutes, respectively. Calculate the rate constant.

The answer is ~0.0074, but I don't know why it is a second order reaction. Could someone explain how I can differentiate between zero, first and second order please? Thanks in advance!

[sjb]

Have a look at http://en.wikipedia.org/w/index.php?title=Rate_equation&oldid=289187465 , and see if that helps. Also don't forget your units.

[Acid_Guru]

The two half lives differentiate with a factor of two, therefore it is a second order reaction.