# Is reactant concentration = product concentration at equilbrium?

When a reaction attains chemical equilibrium, it means that the forward reaction rate = backward reaction rate. Since rate is defined as the change in concentration per unit time,does it mean that at equilibrium,

1) concentration of reactants = concentration of products?

2) Also, is the time taken for the forward reaction = time taken for the backward reaction?

• Suppose you lose 100\$\$$. Suppose that Bill Gates also loses 100\$$. The change is the same. Does that mean the amount of money is also the same? – Ivan Neretin Jan 25 '18 at 9:48

## 2 Answers

1) Not necessarily. In chemical equilibriums Concentrations could be any of following variations after attaining equilibrium. $$\ce{[reactants] < [products]}$$ $$\ce{[reactants] = [products]}$$ $$\ce{[reactants] > [products]}$$ Although reaction rates of forward and backward are equal, something like this could also happen. Therefore concentrations are not ALWAYS equal at equilibrium of all reactions, but could be in some reactions. image source and more reading:-http://faculty.chem.queensu.ca/people/faculty/mombourquette/FirstYrChem/equilibrium/index.htm

2) Since the general idea of "Rate" means some change (say in this case reactants consumed or products produced) divided by time, thus AT EQUILIBRIUM, as you said, $$\ce{forward reaction rate = backward reaction rate}$$ But I don't see how it can strictly be said like their times are equal. I THINK it depends. Maybe someone can correct me.

I think that it depends on your definition of equilibrium. There is alwasy an equilibrium between reagents and products (if we are considering reversible reactions). It can be thermodinamically shifted in one of the two directions

Products

$$\ce{A + B <=>> C}$$

Reagents $$\ce{A + B <<=> C}$$

Same $$\ce{A + B <=> C}$$

it means that in the first case you will have a thermodinamical tendence to one of them or that in the latter case that there are not actually reagents and product but that for the two chemicals (it can be also one form of same chemical) is equal.

As an example C can be in the form of Graphite or Diamond. Only one of them is the thermodimically stable but it will take quite a lot of time to see them chance in to each other.. you can have a look here for a more detailed explaination here http://www.ch.ic.ac.uk/rzepa/mim/century/html/diamond.htm

If you reach the equilibrium in a reaction it means that even if the quantity of reagents and products are different the inteconversion (which is still present !) won't alterate their final ratio of concentration of each of them. It is important to notice that there is still an interconversion (that's why reversible equilibrium) . Also you have to keep in mind that changing the condition of your sistem (P,T,conc) can alterate your equilibrium so there is not ONE equilibrium. Just one caveat about concentration. As chemist we generally refer to molarity but it is important to define it before any reasoning ;) Hope this helps