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    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?
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    I AM SO SALTY. HESS DIDN'T EVEN COME UP IN MY EXAM.

    :cry:

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    Also, it would be easier to tell, if we could see your working out.
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    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 😊
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    (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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