My advice would be to avoid essay form, because it's unwieldy and long. The great thing about any responses for chemistry is that you can always use point form. It's much more concise and to the point that way. Remember that you are not out to prove a point or argue a case, as is usually necessary in essay form, merely to demonstrate that you have the knowledge. It's much more straightforward then, if you use the clearest and most concise way of putting things. I would go for point form and mini-headings...like a report form, extended on specific points you would like to clarify to more detail.