unit 4 pp q

the answer here is to keep the total volume constant.. why do we need to do that, we've never done this in any other experiment

Keeping the volume of each mixture the same ensures that the concentrations of reagents can be easily compared.

For example, having 10cm^3 of sulphuric acid in one mixture vs 20cm^3 of sulphuric acid in another mixture

If the two mixture have the same volume then it is obvious that the second mixture has twice the concentration of sulphuric acid!If instead the second mixture had some other volume then you'd have to worry about actually calculating concentrations in order to compare the two mixtures.

So keeping volume constant simply ensures that the concentration of reactants double/halve when we want them to

aha,, so does this mean that we always have to keep the volume constant when finding orders? sometimes the table is concentration to time not volume
so the volume should always be kept constant?
idk why im struggling to explain my point but yeah

Nope, we don't always need constant volumes to find order.

All you really need to find the order is two runs of the same reaction with different concentrations of the reagent of interest (keeping concentrations of all other reagents constant).

The easiest way is to have the first run with one concentration and the second run with double the concentration.

Here we simply add the water to make the volume the same for all the runs because that means the concentration of the reagents in each run is directly proportional to the volume of each reagent added!

imagine if (for some imaginary reaction) we had a mixture that was 20cm^3 of reagent A and had a total volume of 100cm^3 (80cm^3 of other reagents).
and a second mixture that has 10cm^3 of reagent A and a 90cm^3 total volume (again 80cm^3 of other reagents ie kept constant).
Ideally for an order experiment we would want the concentration of A to be twice as high in the first mixture than the second mixture.
At the moment this isn't true, because the first mixture has twice as much reagent but in a larger volume.
The way to fix it is to make them the same volume, by adding 10cm^3 of water to the second mixture.

So keeping the volume constant here is only done so that the concentrations of the reagents we're interested in can be kept in the right proportions without having to add more of other reagents!

Obviously in other cases, if you're given concentrations, then the volumes don't matter, because we're only worrying about the volume because it changes concentration!

Sorry for the long reply, hope that makes sense!