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# Hess's Law- Enthalpy Change Watch

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.

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Updated: June 11, 2016
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