When particles are added pressure will do what and why

Because area of the container has increased , there will be fewer of collisions per unit area and the pressure will decrease . Volume is inversely proportional to pressure , if the number of  particles and temperature is constant
( V = 1/P) and number of particle is proportional to pressure if average Kinetic energy of the particle remain same , the average force particle will remain same too so at some places and there will be more collision and there is greater pressure

Exothermic,  physical change after it is already baked