Calculating Equilibrium Concentrations

[Stevie Wisz]

When Prof. Lavelle was going over how to calculate the concentrations of products/reactants using ICE Box method I was confused as to how you (for the change in concentration) get -X or +2X, etc. Can someone explain this a little more in depth please? I realize you look at the molar ratio but I just need further clarification. Thank you!

[William Chwa 1E]

X is just a variable that you assign which represents the change in molar concentration for the reactant. The reactant will be used up during the reaction, so the change is represented by -X. Then, as you said, depending on the molar ratios of the products, the product(s)' molar concentration will increase by +nX, where n represents the ratio between the reactant and product.

[Lisa Leung 2H]

An example to illustrate this would be:

For the forward reaction of
(g)+(g)⇌2NO(g)


In the forward reaction, the change in molar concentration of each and would be -X amount since 1 mole of and 1 mole of is consumed(negative) for every 2 mole of NO produced. This would mean the change in molar concentration of NO would be +2X since 2 moles of NO is produced(positive) per mole of and . Like William said above, X is just a variable we assign to the unknown change in molar concentration.