AS Chemistry Enthalpy Question

I am having trouble understanding when to put the plus or minus sign when you have to calculate the enthalpy change. I used minus because that's what they did in my textbook but the markscheme said it was plus! I am very confused, here's an example (it's question 3):

http://www.ocr.org.uk/Images/58197-mark-scheme-unit-f322-chains-energy-and-resources-january.pdf

Any help would be appreciated!

Reply 1

1144297

Original post by DedicatedWizard

It depends on the reaction - if it is exothermic then it is -ve, if it is endothermic it is +ve. The enthalpy refers to the heat content of a system - if it is lost and heats the surroundings it is -ve. Basically all enthalpy changes of combustion are exothermic and enthalpy changes of formation are endothermic, but for enthalpy change of reaction (as in that question) it depends.

Reply 2

Rome<3

I guess it means you don't know the difference between endothermic reactions and exit hermit reactions

Here's a clue
Endo means to absorb and Exo means to release
So when a reaction loses heatits an exothermic reaction and the products always have a -ve sign because energy is lost. (the solution feels warm)
When a reaction gains heat its an endothermic reaction and so the energy sign is always +ve. (the solution feel cold)
Ask me if you have anything you don't understand :smile: