Are 9kg bricks too heavy to build an arch
I don't think weight - at 9 kg/brick - is an issue in this particular situation. Masonry is heavy by design. In another case I might consider weight to be an issue, just not the main thing here.
The key issue here is the shape. The way to do this is to use bricks that are designed for arches. Something like this Googled set of arch bricks for sale.
You can also cut rectangular bricks to turn them into wedges, but since this is a common problem, wedge-shaped bricks are readily available and a whole lot easier to work with.