You are Here: Home

# Hess's Law- Enthalpy Change

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
1. With Hess's Law how do i know when to add and when to take away enthalpy changes.

For example I have the question: Calculate the standard enthalpy change for the reaction of one mole of hydrazine, (N2H4) with oxygen, (O2) to form nitrogen, (N2) and water, (H2O).
I'm given the data of:
Formation of N2H4 is +51kJ mol-1
Formation of H2O is -286kJ mol-1

I know the equation of the reaction is: N2H4 + O2 = N2 + 2H2O. And its formed from the elements O2, 2H2 and N2. I also understand I need to do 2(-286) for the water because 2 moles are formed which is -572 kJ mol-1.

However I'm not sure if I add -572 and +51, or if i subtract 51 from -572?

How do I know when I have to add/subtract to find the enthalpy change?
2. I AM SO SALTY. HESS DIDN'T EVEN COME UP IN MY EXAM.

Spoiler:
Show
Also, it would be easier to tell, if we could see your working out.
3. The enthalpy change of reaction is defined as (sum of enthalpy of formation of products) - (sum of enthalpy of formation of reactants). In this case, the formation of hydrazine is endothermic so you require 2(-286) - (+51) = -623 kJ mol^-1. You would only add if the sum of the enthalpy of formation of the products is a negative number, in which case it's minus a negative number which is equivalent to addition 😊
4. (Original post by brwilding)
The enthalpy change of reaction is defined as (sum of enthalpy of formation of products) - (sum of enthalpy of formation of reactants). In this case, the formation of hydrazine is endothermic so you require 2(-286) - (+51) = -623 kJ mol^-1. You would only add if the sum of the enthalpy of formation of the products is a negative number, in which case it's minus a negative number which is equivalent to addition 😊
Thank You so much, this makes a lot of sense now.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: June 11, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

### Who would you like to thank?

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.