Rational and Rationale are two different words, especially in terms of meaning. However, these two words look very similar to the point that you might think they are the same. Rational is mostly used to perform the function of an adjective. It is used to describe anything that is capable of reasoning.
For example, a man is a rational creature. At times, rational is used to describe anything that sounds logical.
For example, John's answers to your questions were quite rational. It can also be used to describe someone that is balanced intellectually.
Rational is also used in mathematics to describe numbers that can be expressed as ratios. On the other hand, rationale means fundamental reasons or explanations of the basis. It means the reasons for something.
At times, questions like "what are the rationales for your actions" are usually asked. In this case, what you are expected to give as answers are your reasons for doing what you did.