# How do we figure out whether reactions are endothermic or exothermic? [closed]

Are the following reactions exothermic or endothermic: $$\ce{H2 + I2 -> 2HI}$$

$$\ce{N2 + O2 -> 2NO}$$

So I know the second reaction is endothermic as we need to break triple bonds between nitrogen molecules. I am not sure about the first one. Actually, I want to know if there is some way we can decide whether a reaction is endothermic or exothermic just by looking at the equation?

I got a test coming up in a few months so this was one of the few topics I was having trouble with. Any help would be appreciated.

• Without looking up the thermodynamic data, or without background chemical knowledge, it cannot be said just from looking at the equation. – Poutnik Jun 16 at 7:10
• Question titles should be like book titles. They distinguish the book from other books, give a hint about content, but do not tell the full story of the book. The content then should elaborate the topic to full depth the author is able to do. – Poutnik Jun 16 at 7:10
• @Poutnik , i am sorry, this was my first question here and this usually how we ask questions on MSE so i thought CSE would be similar sorry. Do you know if the first reaction is endothermic or not? – Aditya_math Jun 16 at 7:12
• The first reaction is definitely endothermic and your reasoning is quite correct, Aditya. – Firefox1921 Jun 16 at 7:19

If you search for "standard enthalpy of formation hydrogen iodide", you get positive value $$\pu{26.5 kJ/mol}$$, what means the reaction is endothermic, as by convention, the values for elements in their standard state is zero.

The other one is strongly endothermic, what you realize, if you search the same for nitric oxide.

For a general reaction, search for standard enthalpy of formation of all reactants and products, multiply the values by stoichiometric coefficients and subtract the reactant values from the product values. If the result is negative, the reaction is exothermic. For the positive value, the reaction is endothermic.

For starters: enthalpy is defined as $$H = U + p . V$$, where $$U$$ is system internal energy, $$p$$ is pressure, $$V$$ is volume. Change of system enthalpy in constant pressure condition is change of system internal energy + volume work done by system: $$\Delta H = \Delta U + p . \Delta V$$. By other words, $$\Delta H$$ is the energy you have to provide to a system at constant pressure to increase its internal energy by $$\Delta U$$, if the system changes its volume by $$\Delta V$$.

For guessing the formation enthalpy patterns, there is no easy shortcut. For simple inorganic compounds, a lot of help in noticing patterns is the periodic element table.

All comes with experience and learnt expirical knowledge and patterns. Chemistry is in large extent empirical science, where many things cannot be derived. Or can, but it is much easier to just remember, unless one wants to jump in quantum chemistry. Even then not everything can be calculated.

• Thanks this is super helpful. I just got one more doubt though. I know that in exothermic reactions, energy on reactant side > energy on product side. So is this standard enthalpy of formation similar to energy levels? ( I haven't studied thermodynamics yet), Also is there some certain trend with which I can know the standard enthalpy formation value for compounds. If not then i'll happily learn those values but I think understanding it would be beneficial. Thank you so much though. – Aditya_math Jun 16 at 11:45
• See the answer update. – Poutnik Jun 16 at 12:05
• Thank you. This is really useful. After reading your answer i read a little bit about Thermodynamics and it seems really interesting, and apparently it's a really important topic too (where I live). Am going to have fun studying it, also thank you (this was my first question on the site) – Aditya_math Jun 16 at 12:19
• You are more than welcome. – Poutnik Jun 16 at 12:23