When designing a kinetics experiment, there’s a little trick you can do when trying to find the orders of reaction.
The trick is to make one of the reagents have a very high concentration. This is sometimes referred to as having the reagent in a “large excess“.
This ensures the concentration of the reagent at t=0 is almost the same as the concentration when the experiment is finished (for however long the experiment is allowed to go on for). Hence we take its concentration to be constant.
If the concentration is constant, i.e. it ‘doesn’t change’ then it can’t be responsible for the change in the reaction rate. Instead the other reagents whose concentration changes are responsible.
You’ve seen this a few times already in initial rates experiment where often (but not always!) in two experiments the concentration of one reagent is kept the same, allowing us to attribute the rate change to the change of conc. of the other reagent.
Obviously there would come a point where if we conducted experiments with successively lower concentration of this reagent (which was previously in ‘large excess’) then it’s concentration change would begin to become significant and the assumption it stays constant throughout would not be valid.
This automatically begs the question “When can we ignore it and when can we not”? Well, there is no absolute rule, but, if the concentration changes by 10% or less, then usually it’s influence on the rate is minimal (if it was zero order then it would never matter).
There is an answer with explanation to the George Facer question set I gave you which discusses this point. However the ‘explanation’ given in the answers is not immediately obvious to the inexperienced chemist.
Actually we have seen this ‘trick’ before when we did Kc expressions….
In aq. solution (i.e. water is in a large excess, having almost a huge 55 mol dm-3 concentration!) We said the [H2O] in aq solution was in effect constant (typical concs of other reagents would be something like 0.1 M, almost 500 times smaller) and so we combine the K constant with the ‘effectively constant’ [H2O] and used the symbol Kc.
We have also done it with Ka. The conc of water in a solution of a weak acid will also very large and so [H2O] was combined for the constant and we changed the name to Ka to reflect the incorporation of H2O into the K value.
Hope that makes sense.
One more thing to be aware of is the special case of carbox. acid(s) + alcohol(l) <–> ester(l) + water.
Here water is a product and wasn’t used as a solvent. This means its concentration changes drastically during the course of the reaction. So in the equilibrium expression where water is not used as a solvent, then H2O is included in the Kc equation.
Note this large excess ‘trick’ may come in handy if you get any kinetics experiments to do! or some other exam situation where you see the terms ‘large excess’ being used as it means you can ignore it when determining orders.
Here’s a question for you. Why ‘large excess’ and not just ‘excess’?
Hope you enjoy the lovely weekend 🙂
P.S. anyone like to play tennis?