Reaction Calculations.

Quote: Originally posted by SmellNoEvil

I would like an explanation of how I would determine whether or not a reaction is possible. For instance: How would I determine whether or not the reaction: CO + H2O --> CO2 + H2 is possible? I know for certain that the reaction above will not occur, but what about determining whether or not other reactions will happen? I've heard of thermodynamic calculations to determine whether or not a reaction will proceed (Follow the link). Can somebody explain in more detail how to determine whether or not a certain reaction will happen(Without actually trying to carry it out)? Reference: http://www.sciencemadness.org/talk/viewthread.php?tid=29710&... [Edited on 4-3-2016 by SmellNoEvil]

This a complex, multi-faceted subject and I'm loathe to try and summarise it in a few lines but just for starters:

1. Thermodynamic considerations:

Take a simple reaction:

nA + mB ===> kC + lD

To determine whether such a reaction is thermodynamically favourable or not, we need to calculate the Change in Gibbs Free energy ΔG⁰ (where ⁰ stands for STP conditions) that it would cause (IF it proceeded).

This is calculated as:

ΔG⁰ = k ΔG⁰_C + l ΔG⁰_D - [n ΔG⁰_A + l ΔG⁰_B]

where the individual terms are the Gibbs Free Energy Changes of Formation of each reactant or reaction product. These values are usually obtained from various databases like NIST webbook and others.

If ΔG⁰ < 0, then the reaction is exo-energetic and thermodynamically favourable.

But if the reaction is carried out in conditions different from STP, corrections for temperature and/or pressure to ΔG⁰ need to be applied.

2. Kinetic considerations:

Even if ΔG < 0, this says precious little about reaction rate. For all we know, a reaction may be thermodynamically favourable in the calculated conditions, yet only proceed at an imperceptibly slow rate in those conditions.

But maybe there are other conditions of temperature and/or pressure that favour higher reaction rates? That part of the story requires introduction into various other concepts like Collision Theory and Activation Energy.

This resource is a quite a good low level entry one:

http://www.chemguide.co.uk/physmenu.html

So don't expect to learn all this overnight: most of us interested in it are perennial students of it!

[Edited on 4-3-2016 by blogfast25]

Quote: Originally posted by SmellNoEvil

For instance: How would I determine whether or not the reaction: CO + H2O --> CO2 + H2 is possible? I know for certain that the reaction above will not occur

Quote: Originally posted by Metacelsus

Basically, to determine whether a reaction occurs spontaneously, compute the Gibbs free energy change.

Nope. Plenty of reactions have highly negative ΔG, yet don't proceed at RT. And they don't necessarily proceed quickly at higher T either (see Activation Energy).

Most combustion reactions, e.g., don't proceed at RT. By your reasoning the world would be on fire all the time.

Look at the ΔG < 0 as a 'necessary but not sufficient condition' for a reaction to proceed.

Please don't try and simplify a complex subject: you end up spreading misinformation.

[Edited on 4-3-2016 by blogfast25]

Quote: Originally posted by Metacelsus

Quote: Originally posted by SmellNoEvil   For instance: How would I determine whether or not the reaction: CO + H2O --> CO2 + H2 is possible? I know for certain that the reaction above will not occur What do you mean it won't occur? It definitely occurs (under proper conditions), and is well-known: https://en.wikipedia.org/wiki/Water-gas_shift_reaction Basically, to determine whether a reaction occurs spontaneously, compute the Gibbs free energy change. Edit: Ninja'd [Edited on 3-4-2016 by Metacelsus]

Quote: Originally posted by SmellNoEvil

Blogfast,can you please give an example calculation of the reaction between sodium and chlorine gas,or any other simple reaction in a "For dummies" type of explanation?