[mjc123]

I'm sorry, but you're still  not giving us enough information to help you sensibly.

I got the same value

Same value as what, for what, doing what differently?

Or is it the fact that I can't compare 30 cm^3 with 25 cm^3?

There's no reason why you can't, as long as you calculate the concentrations correctly in each case. If you use the ln(1/Δt) method, it shouldn't matter, as the difference will show up in the intercept, not the slope.

I used the ln/1/t method, and yeah it seems that both the activation energies calculations are correct

Both of what? Do you mean you now get a "correct" value of Ea, i.e. about 50 kJ/mol? Or do you mean that you get, as expected, the same value of Ea from plots of ln k and ln (1/Δt) (even if this value is wildly off)? Or that you get "correct" values for both catalysed and uncatalysed? How do you know they are correct, especially as you think your result doesn't make sense?
Unless you show full calculations, graphs and results, we can't see where you may have gone wrong.

[Genie123]

Alright sorry, 5 cm^3 of catalyst is added:

These are the catalysed results:

205 - 283.15 K
65 - 293.15 K
46 - 303.15 K
37 -- 313.15 K
17 - 323.15 K
7 - - 333.15 K

I apply the same process but using 30 cm^3 instead of 25cm^3.

I get a higher activation energy for some reason.

[mjc123]

There's clearly something wrong with these data as they give a very poor straight line plot. You can see that just by looking - the rate more than doubles between 10 and 20°, between 40 and 50° and between 50 and 60°, but less than doubles between 20 and 40°. That suggests something amiss with the procedure. What was the catalyst? Could there have been an issue with mixing? Were all the solutions equilibrated at the reaction temperature before mixing? Did you add them all in the same order each time?

[Genie123]

Catalyst was CuSo4 (so copper ions). It was in solution, but there still could be an issue with mixing because a) i only stirred the beaker gently a few times, and I mixed the solutions outside the water bath, i.e not being at it's correct temperature so i'm assuming that's where I went wrong. Is it still possible to have a higher activation energy though, even with faster reaction times?

[mjc123]

Yes, but it's not what you'd expect for a catalysed reaction. You would expect a lower activation energy, and a faster reaction rate than the uncatalysed reaction at all temperatures, though the relative effect (catalysed/uncatalysed rate) would be greater at lower temperatures.
I don't think you can do anything sensible with those data. If possible, do the experiment again, and more carefully.