lets just say that 5g of solid benzoic acid burns under gaseous oxygen atmoshphere. Temperature increases from 20c to 50c. So is this exothermic so the sign is negative? how is it not the same as 2 mol of solid iron (C = 25.1 J/(mol K)) is heated from 20C to 200C(endothermic) since both have to do with an increase in temperature

Answer 1

I see, Sir Eng, that you are back on the same kick. But that's good if you don't understand it. Here's the deal.
With the iron, you ADDED heat from an outside source to the iron. The iron ABSORBED the heat which makes q + for the iron portion of the reaction.
But conditions are not the same for benzoic acid burning. When benzoic acid burns, it PRODUCES heat, which ADDS heat to the calorimeter (the water in the calorimeter absorbs the heat) so heat for the calorimeter and water is + and the heat given off by the benzoic acid, (the heat of combustion) is negative. It's true that both have to do with an increase in temperature; however, the increase in temperature is CAUSED by the benzoic acid (and received by the calorimeter and water) and it is RECEIVED by the iron.[Said another way, the benzoic acid is GIVING OFF heat while the iron is ABSORBING heat. Just the reverse in the two instances and that's the basic reason the signs are different.] When you heated the iron, it had to be done with an outside source such as a burner of some kind. You COULD have heated it by burning benzoic acid. But try rubbing cold iron on benzoic acid and see if it will ignite the benzoic acid. Nope! Won't do it. This is important. Keep working at it until you get it. And don't be reluctant to post again if you still doubt the findings.

Answer 2

i see, thank you for clarifying